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Epoxy Flux         A Low Cost High Reliability Approach for PoP Assembly
                                            Dr. Ning-Cheng Lee
                                      Indium Corporation of America
                                            Clinton, NY 13323
                         Tel: 315-853-4900, Fax: 315-853-1000, askus@indium.com

                                                       ABSTRACT
   Package on package (PoP) is a packaging that rapidly prevails in mobile devices of the electronic industry, due to
   its flexibility in combining memory and processor into one component with a reasonably low profile. However,
   similar to BGA, the solder joints of assembled PoP are prone to cracking upon dropping onto the floor, thus
   needing reinforcement by underfilling. The underfilling process needs an underfill material plus additional
   dispensing equipment, dispensing and flowing steps, and subsequent time-consuming curing. Furthermore,
   underfilled PoP suffers solder extrusion upon rework, particularly when reworking at the opposite side of the board
   right underneath the PoP. Epoxy flux is a new material developed to address the issues described above. It is a
   liquid epoxy with fluxing power, and it is compatible with solder paste. Applied by a dipping process, epoxy flux can
   be used at the mounting of bottom package and top package. First, the bottom package is dipped in a film of epoxy
   flux, and then placed onto the footprint pad on a PCB with or without solder paste. Then, the top package is dipped
   in epoxy flux and placed on top of the bottom package. The PCB with a stacked PoP is subsequently reflowed in the
   oven together with other surface mount components placed on printed solder paste. Epoxy flux combines soldering
   and reinforcement into one single process. With a controlled pick up flux volume, a venting channel is formed,
   allowing outgassing at reflow. The low stress characteristics of epoxy flux prevents the formation of solder
   extrusion. Overall, epoxy flux provides a low cost and high reliability solution for PoP assembly.


   Key Words: Epoxy flux, Package on Package, PoP, assembly, reliability, soldering

   INTRODUCTION                                                  assembled PoP are prone to cracking upon dropping
   Package on package (PoP) is a packaging that rapidly          onto the floor, thus needing reinforcement by
   prevails in mobile devices of the electronic industry,        underfilling. The conventional SMT one-pass
   due to its flexibility in combining memory and                assembly process for PoP is shown in Fig. 1. Here the
   processor into one component with a reasonably low            bottom package is placed onto solder paste printed on
   profile. However, similar to BGA, the solder joints of        PCB, followed by placement of a fluxed top package.




Figure 1. Comparison of various PoP assembly processes
The stacked devices are then reflowed, followed by
   underfilling. The underfilling process needs an
   underfill material plus additional dispensing
   equipment, dispensing and flowing steps, and
   subsequent time-consuming curing. Furthermore,
   underfilled PoP suffers solder extrusion upon rework,
   particularly when reworking at the opposite side of
   the board right underneath the PoP. Epoxy flux is a
   new material developed to address the issues
   described above [1, 2]. The concept of material
   design, methods of deposition of the materials, and
   performance are presented and discussed in this
   work.
                                                                   Figure 2. Schematic of one jetting device.
   MATERIAL DESIGN
   Epoxy flux is a thermoset flux system. It provides              Table 1. Feature of several commercial Jetting
   fluxing power upon reflow at temperature above the              devices.
   melting temperature of solder, thus allowing the                 Jetting Design                Drop size
   formation of solder joints. It can also cure to form                    A          30-35 pL (nanograms)
   thermoset flux residue around the solder joint thus                     B          20-25 µL (micrograms)
   providing mechanical reinforcement. When epoxy
                                                                           C          Largest (~100 micrograms)
   flux is applied to fill the gap between packages of
   PoP or between bottom package and PCB, it                       process, epoxy flux is first jetted onto the pads with
   practically functions as an underfill in terms of               solder paste already printed, followed by placing the
   reinforcement.                                                  bottom package. Again, epoxy flux is jetted onto the
                                                                   topside of the pads of the bottom package, followed
   PROCESS DESIGN                                                  by placing on the top package. This step can be
   There are several ways of applying epoxy flux for               repeated if there is more than one top package. After
   PoP assembly, including jetting, dipping, as well as            all of the packages are stacked sequentially, the board
   printing. Jetting and dipping are more flexible, with           is then sent through a reflow oven to complete the
   processes shown in Fig. 1.                                      soldering and curing simultaneously.

   Epoxy Flux Jetting                                              Epoxy Flux Dipping
   The schematic of one jetting device is shown in Fig.            In the dipping process, the packages are predipped
   2, with the jetting dot size information of several             into an epoxy flux film prior to placement. The depth
   commercial machines shown in Table 1. In the jetting            of the epoxy flux film can be regulated by a doctor




Figure 3. Flux dipping process using rotary device with doctor blade for controlling the flux film thickness.
SAC305 solder bump and SAC305 solder paste. Fig.
                                                          5 shows the X-ray picture of the joints, with 100%
                                                          full wetting observed for all solder joints. The shear
                                                          strength of this BGA is 143lbs, compared with only
                                                          113lbs using a conventional flux. Upon shearing,
                                                          failure of the epoxy fluxed samples occurred on the
                                                          board side, with copper pads peeled off for some of
                                                          the joints, as shown in Fig. 6. Apparently, the shear
Figure 4. Schematic of linear fluxer. The plate           strength of the epoxy flux sample should be greater
slides open to expose cavity [3].                         than 143lbs if pad lifting is avoided.

                                                          Cross-Section
                                                          The Cross-sectional view of assembled 0.5 mm pitch
                                                          BGA (7 x 7 mm) assembled with SAC305 ball,
                                                          SAC305 solder paste, and epoxy flux is shown in Fig.
                                                          7. The close-up views of this device are shown in
                                                          Fig. 8 to Fig. 10. The fillet shown in Fig. 8 indicates
                                                          the space under the package is well filled. Solder
                                                          joints formed with the BGA exhibiting full wetting,
                                                          as shown in Fig. 9. Fig. 10 shows a vacancy in epoxy
                                                          flux between two solder joints. This vacancy is
                                                          characteristic of epoxy flux, and is designed as a
                                                           venting channel . Without this venting channel,
                                                          outgassing from the PCB substrate and packages
                                                          caused by Pb-free reflow temperature often causes
                                                          bubbling within the fully sealed liquid epoxy layer
Figure 5. X-ray picture of 0.8 mm BGA (10.5 x             under the package, and consequently resulting in
13 mm) with SAC305 solder bump, SAC305                    drifting or tilting of packages. This venting channel is
solder paste, and epoxy flux.                             typically designed to be less than 5% of the interface
                                                          area, and is distributed randomly.




                                                          Figure 7. Cross-section of 0.5 mm pitch BGA (7x7
                                                          mm) assembled with epoxy flux.




Figure 6. Board surface after BGA in Fig. 5
being sheared off.
blade rotary device, with process illustrated by Fig. 3
or a linear fluxer, as shown in Fig. 4. Typically, the
film thickness is set at 80-100% of the solder bump
height.

PERFORMANCE OF EPOXY FLUX
Solder Wetting
Solder wetting performance of epoxy flux is
demonstrated on 0.8 mm BGA (10.5 x 13 mm) with            Figure 8. Fillet of epoxy fluxed sample shown in Fig.
Reliability
                                                                Reliability of the epoxy flux system can be
                                                                demonstrated with the data shown in Table 2 for 0.3
                                                                mm pitch CSP, size: 2.67 mm x 2.67 mm, I/O: 81,
                                                                using SAC305 ball and SAC305 paste. Epoxy flux
                                                                shows more than 60% higher shear strength than
                                                                conventional flux, and are several times higher in
                                                                drop numbers to failure. The thermal cycling test
                                                                results show all epoxy fluxes last more than 1000
                                                                cycles for the cycling condition -55 to 125°C.

                                                                Compatibility with Solder Paste
    Figure 9. Cross-section of solder joint for BGA             At assembly of the PoP, since the bottom package
    shown in Fig. 7.                                            has solder bumps, printing solder paste on PCB pads
                                                                is not critical for the successful assembly of the
                                                                bottom package. However, at times, open between
                                                                bottom package and PCB may be the result of
                                                                excessive thermal warpage. Under this condition,
                                                                printing solder paste becomes essential for
                                                                minimizing opens.
                                                                As all solder pastes contain significant amounts of
                                                                solvents, burn off of the volatiles of solder paste
                                                                embedded in the liquid epoxy flux under the bottom
                                                                package becomes a very challenging task.
                                                                Furthermore, the powder of solder paste soaked in
                                                                liquid epoxy is also prone to ooze out and form
                                                                bridging and numerous solder balls.
   Figure 10. Venting channel between solder joints.            Fig. 11 shows a simulated test for solder paste under
                                                                the BGA and embedded in liquid epoxy flux. Here
                                                                the SAC305 solder paste was printed onto PCB pads,
Table 2. Reliability of epoxy fluxes versus conventional flux   and then covered by liquid epoxy flux A. A glass
for 0.3 mm pitch CSP, size: 2.67 mm x 2.67 mm, I/O: 81          slide was placed on top of the solder paste/epoxy flux
                Conventional      Epoxy       Epoxy             to mimic the body of a BGA. This sandwiched PCB
Tests           Solder Paste      Flux A      Flux B            was then placed on a hot plate with a surface
                                                                temperature of 250°C. Fig. 11(a), (b), and (c)
Chip shear      19.2 ± 0.1        24.0 ±      36.5 ±
test, lbs.                        4.3         1.3               represent the view at 0 second, 50 seconds, and 120
                                                                seconds on hot plate. The solder paste immensely
                                              (+80%
                                                                diffused away from the printed dots at 50 seconds,
                                              typical)
                                                                and never coalesced back to form proper solder
Drop test       80, 80, 90,       150,        220,    220,      bumps after 120 seconds.
(drop # to      90                180,        230,    240,
fail)                             210         250,    290,      Fig. 12 shows the same experiment using the same
                                              300,    320       SAC305 solder paste but embedded in epoxy flux B.
Thermal         168, 168,         719,        870,              Here the solder paste remained as one integral body
cycling test    168               897,        1089,             the entire time for each printed dot.
(-55 ~                            897,        1498,
125 °C)                           1098        1843              Results above show that a properly formulated epoxy
                                                                flux can allow for a satisfactory coalescence of solder
                                                                paste without suffering solder balling or bridging.
Figure 12. SAC305 solder paste with epoxy flux B
                                                         under glass slide placed on hot plate at 250°C.



Figure 11. SAC305 solder paste with epoxy flux A under
glass slide placed on hot plate at 250°C.
 SIR Performance
 Being a thermoset system, epoxy flux is destined as a
 no-clean material, hence has to meet surface
 insulation resistance requirements without cleaning.
 Fig. 13 demonstrates that the IPC SIR requirement,
 greater than 100 mega ohms at 168 hours per J-STD-
 004, is easily met by one epoxy flux material.
                                                         Figure 13. SIR performance of one epoxy flux per
 Reflow Window                                           J-STD-004 procedure.
 Epoxy fluxes can be designed to be very robust in
 terms of reflow windows. Table 3 shows the
 electrical continuity for daisy-chained BGAs            epoxy fluxes. Successful solder joint formation was
 assembled under various reflow profiles with two        achieved at a very broad reflow profile range.
Table 3. Electrical continuity of BGA assembled
 using two epoxy fluxes reflowed at various profiles.
  Profile #


              temperature, min
              Time to peak

                                 temperature, °C
                                 Peak


                                                   TAL, sec


                                                              w/Epoxy Flux C
                                                              Continuity

                                                                               w/Epoxy Flux D
                                                                               Continuity
                                                                                                Figure 15. Schematic of shock wave passing through
                                                                                                the board under BGA component.
       1              4              230             59        Good             Good
       2              4              237             70        Good             Good
       3              4              245             80        Good             Good
       4              6              237             nd        Good             Good
       5              3              237             nd        Good             Good
   6     2.5    237      nd                                    Good             Good
 Note: nd not detectable




                                                                                                Figure 16. Schematic of corner bonding process [8]
                                                                                                as shown in Fig. 14. When the wave passes through
                                                                                                the area under a rigid component such as BGA or
                                                                                                PoP, each solder joint experiences compression and
                                                                                                tension sequentially, as shown in Fig. 15, and it is the
                                                                                                tension which results in joint fracture [5]. The failure
                                                                                                modes reported [6] include pad cratering (83.3%),
                                                                                                I/O trace fracture (73.3%), board side solder crack
Figure 14. Printed circuit board experiencing shock                                             (33.3%), daisy chain trace fracture (3.3%), and chip
wave traveling through upon drop [4].                                                           side solder crack (1.7%) in terms of percentage of
                                                                                                analyzed components. Study of Srinivas shows the
                                                                                                failure rate of bottom joints is higher than top joints
 Physical Properties                                                                            for PoP upon drop [7]. Accordingly, the most critical
 The physical properties of a representative epoxy flux                                         reinforcement desired would be a bonding force
 are listed below. The low Tg property is considered                                            holding the board surface and the bottom package
 crucial for avoiding the solder extrusion phenomenon
                                                                                                body together.
 at reflow or thermal cycling.
           Tg - 56 °C
                                                                                                POLYMERIC REINFORCEMENT
           CTE (<Tg) - 64 ppm
                                                                                                APPROACHES
           Typical Viscosity - 5-7 kcps
                                                                                                There are several polymeric reinforcement
           Shelf life (at -20°C) - 3 month
           Pot life (at room temp.) - 24 hours                                                  approaches attempted by the industry as introduced
           SIR (Ohms) - >1.0 E+8 (pass)                                                         schematically below.
           Reworkable
           Flux activation temperature - 150 oC                                                 Corner Bond
                                                                                                   Pro
 FAILURE MODES UPON DROP                                                                           Cure at solder paste reflow process. No additional
 Upon drop of portable devices, the printed circuit                                                curing step & oven needed.
 board experiences a shock wave traveling through it,                                              Reinforce BGA to some extent.
For precision deposition, premium dispensing
                                                                equipment is needed, & significant increase in
                                                                cycle time becomes inevitable.
                                                                Cannot reinforce bonding between top and
                                                                bottom packages.

                                                             Place-N-Bond Thermoplastic Underfilm
                                                                Pro
                                                                No dispensing equipment needed.
                                                                Cure at solder paste reflow process. No additional
                                                                curing step & oven needed.
                                                                Reinforce BGA to some extent.
                                                                Less promising for preventing crater failure.
 Figure 17. Schematic of Place-N-Bond Underfilm                 No prebaking needed.
 process [9].                                                   Often reworkable.
                                                                Con
                                                                 Need dummy pads designed in on PCB, more
                                                                 reserved space required around BGA.
                                                                Film placement may disturb components placed
                                                                earlier & cause defects.
                                                                Epoxy wicking may interfere solder wetting.
                                                                Cannot reinforce bonding between top and
                                                                bottom packages.

                                                             Edge Bond Epoxy Liquid Process
                                                                Pro
                                                                Fast UV cure, no heat cure needed.
                                                                Epoxy won t interfere with soldering.
 Figure 18. Schematic of edge-bond, epoxy liquid                Easy inspection.
 process.                                                       Con
                                                                One more step dispensing needed. Cycle time is
                                                                unacceptable for high volume, high throughput
                                                                production.
                                                                For high performance, premium dispensing
                                                                equipment needed.
                                                                Less promising in preventing crater failure.
                                                                Cannot reinforce bonding between top and
                                                                bottom packages.

                                                             Capillary Flow Underfill
                                                                Pro
                                                                Mature technology.




Figure 19. Schematic of capillary flow underfill process
[8].
       Less promising for preventing crater failure.
      No prebaking needed.
      Often reworkable.
      Con
      Premature curing due to altered reflow profile can
      result in difficulty of BGA collapse upon reflow.
      Polymer wicking or smear at placement can
      interfere with solder paste reflow.                  Figure 20. Schematic of no-flow underfill process [8, 10].
Potential with the highest reinforcement.            enhancement achieved includes mechanical, thermal,
   Promise reduction in both joint & crater failures.   and electrical properties.
   Con
   Requires post reflow underfill dispense, capillary   REFERENCE
   flow & cure. Costs more time & equipment.            1. Wusheng Yin, Geoffrey Beckwith, Hong-Sik
   May require prebake to avoid voiding if there is         Hwang, Lee Kresge, and Ning-Cheng Lee,
   delay prior to underfilling.                              Epoxy Flux An Answer For Low Cost No-
   Reworkability can be an issue, including                 Clean     Flip    Chip     Assembly ,     Nepcon
   components around BGA which were flooded by              West/Fiberoptic Automation Expo, San Jose,
   underfill.                                               CA, December 3-6, 2002
                                                        2. Wusheng Yin and Ning-Cheng Lee, A Novel
No-Flow Underfill                                           Epoxy Flux For Lead-Free Soldering ,
   Pro                                                      International     Brazing     and     Soldering
   Simple dispense process, no flow needed.                 Conference, San Diego, CA, Feb. 16-21, 2003
   Cure during solder paste reflow.                     3. Gerry Padnos (Juki), PCB Assembly System
   Significant reinforcement.                               Set-Up for Pop , Apex, S19-03, April 6-9,
   Promise reduction in both joint & crater failures.       2010, Las Vegas, NV.
   Some are reworkable.                                 4. R. Wayne Johnson, Yueli Liu, Guoyun Tian,
   Con                                                      Shyam Gale, & Pradeep Lall (Auburn
   Placement may introduce voids.                           University),     Assembly and Drop Test
   PCB prebaking may be needed.                             Reliability of Lead Free CSPs , 2003.
   For large BGA, open & chip drifting or lifting       5. Tz-Cheng Chiu, Kejun Zeng, Roger Stierman
   may be the result due to earlier gelling of              and Darvin Edwards, Kazuaki Ano, " Effect of
   underfill at the hotter perimeter.                       Thermal Aging on Board Level Drop
   Solder wetting may be hampered due to                    Reliability for Pb-free BGA Packages", 54th
   premature gelling upon alternation in reflow             ECTC, P.1256-1262, June 1-4, 2004, Las
   profile.                                                 Vegas, Nevada.
   No filler allowed.                                   6. Andrew Farris, Jianbiao Pan, Albert Liddicoat,
                                                            Michael Krist, Nicholas Vickers, Brian J.
Epoxy Flux                                                  Toleno, Dan Maslyk, Dongkai Shangguan,
The processes of epoxy flux are shown in Fig. 1 to 4.       Jasbir Bath, Dennis Willie, David A. Geiger,
   Pro                                                       Drop test reliability of edge-bonded lead-free
   Only one extra dipping step needed. The simplest         chip scale packages , Flextronic, Henkel, and
   process among all polymer reinforcement                  Cal Poly State University report, 2009.
   approaches.                                          7. Vikram Srinivas, Center for Advance Life
   No solder wetting interference concern.                  Cycle Engineering, Reliability Evaluation of
   No PCB prebaking needed due to designed-in               One-Pass and Two-Pass Techniques of
   venting channel.                                         Assembly for Package-on-Packages under
   Significant reinforcement.                               Torsion Loads , APEX, S34_03, April 6-9,
   Promising reduction in both joint & crater               2010, Las Vegas, NV
   failures.                                            8. R. Wayne Johnson, Yueli Liu, Guoyun Tian,
   Reworkable.                                              Shyam Gale, & Pradeep Lall (Auburn
   Compatible with assembly of BGA & PoP,                   University),     Assembly and Drop Test
   including PoP stacking.                                  Reliability of Lead Free CSPs , 2003.
   Compatible with solder paste, thus can tolerate      9. Data sheet of Place-N-Bond Underfilm,
   warpage at soldering.                                    Alltemated Inc.; www.Alltemated.com
   Con                                                  10. Wusheng Yin, Geoffrey Beckwith, Hong-Sik
   Need epoxy flux bed.                                     Hwang, and Ning-Cheng Lee, Low Cost No-
   Larger nozzle size may be needed for pick &              Flow Underfilling Being A Reality For
   place at dipping step.                                   Manufacturing , Nepcon West/Fiberoptic
                                                            Automation Expo, San Jose, CA, December 3-
SUMMARY                                                     6, 2002
Among existing polymeric approaches for assembly
and reinforcement for PoP, epoxy flux turns out to be
the most versatile low cost option. The reliability

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Epoxy flux a low cost high reliability approach for pop assembly-imaps 2011

  • 1. Epoxy Flux A Low Cost High Reliability Approach for PoP Assembly Dr. Ning-Cheng Lee Indium Corporation of America Clinton, NY 13323 Tel: 315-853-4900, Fax: 315-853-1000, askus@indium.com ABSTRACT Package on package (PoP) is a packaging that rapidly prevails in mobile devices of the electronic industry, due to its flexibility in combining memory and processor into one component with a reasonably low profile. However, similar to BGA, the solder joints of assembled PoP are prone to cracking upon dropping onto the floor, thus needing reinforcement by underfilling. The underfilling process needs an underfill material plus additional dispensing equipment, dispensing and flowing steps, and subsequent time-consuming curing. Furthermore, underfilled PoP suffers solder extrusion upon rework, particularly when reworking at the opposite side of the board right underneath the PoP. Epoxy flux is a new material developed to address the issues described above. It is a liquid epoxy with fluxing power, and it is compatible with solder paste. Applied by a dipping process, epoxy flux can be used at the mounting of bottom package and top package. First, the bottom package is dipped in a film of epoxy flux, and then placed onto the footprint pad on a PCB with or without solder paste. Then, the top package is dipped in epoxy flux and placed on top of the bottom package. The PCB with a stacked PoP is subsequently reflowed in the oven together with other surface mount components placed on printed solder paste. Epoxy flux combines soldering and reinforcement into one single process. With a controlled pick up flux volume, a venting channel is formed, allowing outgassing at reflow. The low stress characteristics of epoxy flux prevents the formation of solder extrusion. Overall, epoxy flux provides a low cost and high reliability solution for PoP assembly. Key Words: Epoxy flux, Package on Package, PoP, assembly, reliability, soldering INTRODUCTION assembled PoP are prone to cracking upon dropping Package on package (PoP) is a packaging that rapidly onto the floor, thus needing reinforcement by prevails in mobile devices of the electronic industry, underfilling. The conventional SMT one-pass due to its flexibility in combining memory and assembly process for PoP is shown in Fig. 1. Here the processor into one component with a reasonably low bottom package is placed onto solder paste printed on profile. However, similar to BGA, the solder joints of PCB, followed by placement of a fluxed top package. Figure 1. Comparison of various PoP assembly processes
  • 2. The stacked devices are then reflowed, followed by underfilling. The underfilling process needs an underfill material plus additional dispensing equipment, dispensing and flowing steps, and subsequent time-consuming curing. Furthermore, underfilled PoP suffers solder extrusion upon rework, particularly when reworking at the opposite side of the board right underneath the PoP. Epoxy flux is a new material developed to address the issues described above [1, 2]. The concept of material design, methods of deposition of the materials, and performance are presented and discussed in this work. Figure 2. Schematic of one jetting device. MATERIAL DESIGN Epoxy flux is a thermoset flux system. It provides Table 1. Feature of several commercial Jetting fluxing power upon reflow at temperature above the devices. melting temperature of solder, thus allowing the Jetting Design Drop size formation of solder joints. It can also cure to form A 30-35 pL (nanograms) thermoset flux residue around the solder joint thus B 20-25 µL (micrograms) providing mechanical reinforcement. When epoxy C Largest (~100 micrograms) flux is applied to fill the gap between packages of PoP or between bottom package and PCB, it process, epoxy flux is first jetted onto the pads with practically functions as an underfill in terms of solder paste already printed, followed by placing the reinforcement. bottom package. Again, epoxy flux is jetted onto the topside of the pads of the bottom package, followed PROCESS DESIGN by placing on the top package. This step can be There are several ways of applying epoxy flux for repeated if there is more than one top package. After PoP assembly, including jetting, dipping, as well as all of the packages are stacked sequentially, the board printing. Jetting and dipping are more flexible, with is then sent through a reflow oven to complete the processes shown in Fig. 1. soldering and curing simultaneously. Epoxy Flux Jetting Epoxy Flux Dipping The schematic of one jetting device is shown in Fig. In the dipping process, the packages are predipped 2, with the jetting dot size information of several into an epoxy flux film prior to placement. The depth commercial machines shown in Table 1. In the jetting of the epoxy flux film can be regulated by a doctor Figure 3. Flux dipping process using rotary device with doctor blade for controlling the flux film thickness.
  • 3. SAC305 solder bump and SAC305 solder paste. Fig. 5 shows the X-ray picture of the joints, with 100% full wetting observed for all solder joints. The shear strength of this BGA is 143lbs, compared with only 113lbs using a conventional flux. Upon shearing, failure of the epoxy fluxed samples occurred on the board side, with copper pads peeled off for some of the joints, as shown in Fig. 6. Apparently, the shear Figure 4. Schematic of linear fluxer. The plate strength of the epoxy flux sample should be greater slides open to expose cavity [3]. than 143lbs if pad lifting is avoided. Cross-Section The Cross-sectional view of assembled 0.5 mm pitch BGA (7 x 7 mm) assembled with SAC305 ball, SAC305 solder paste, and epoxy flux is shown in Fig. 7. The close-up views of this device are shown in Fig. 8 to Fig. 10. The fillet shown in Fig. 8 indicates the space under the package is well filled. Solder joints formed with the BGA exhibiting full wetting, as shown in Fig. 9. Fig. 10 shows a vacancy in epoxy flux between two solder joints. This vacancy is characteristic of epoxy flux, and is designed as a venting channel . Without this venting channel, outgassing from the PCB substrate and packages caused by Pb-free reflow temperature often causes bubbling within the fully sealed liquid epoxy layer Figure 5. X-ray picture of 0.8 mm BGA (10.5 x under the package, and consequently resulting in 13 mm) with SAC305 solder bump, SAC305 drifting or tilting of packages. This venting channel is solder paste, and epoxy flux. typically designed to be less than 5% of the interface area, and is distributed randomly. Figure 7. Cross-section of 0.5 mm pitch BGA (7x7 mm) assembled with epoxy flux. Figure 6. Board surface after BGA in Fig. 5 being sheared off. blade rotary device, with process illustrated by Fig. 3 or a linear fluxer, as shown in Fig. 4. Typically, the film thickness is set at 80-100% of the solder bump height. PERFORMANCE OF EPOXY FLUX Solder Wetting Solder wetting performance of epoxy flux is demonstrated on 0.8 mm BGA (10.5 x 13 mm) with Figure 8. Fillet of epoxy fluxed sample shown in Fig.
  • 4. Reliability Reliability of the epoxy flux system can be demonstrated with the data shown in Table 2 for 0.3 mm pitch CSP, size: 2.67 mm x 2.67 mm, I/O: 81, using SAC305 ball and SAC305 paste. Epoxy flux shows more than 60% higher shear strength than conventional flux, and are several times higher in drop numbers to failure. The thermal cycling test results show all epoxy fluxes last more than 1000 cycles for the cycling condition -55 to 125°C. Compatibility with Solder Paste Figure 9. Cross-section of solder joint for BGA At assembly of the PoP, since the bottom package shown in Fig. 7. has solder bumps, printing solder paste on PCB pads is not critical for the successful assembly of the bottom package. However, at times, open between bottom package and PCB may be the result of excessive thermal warpage. Under this condition, printing solder paste becomes essential for minimizing opens. As all solder pastes contain significant amounts of solvents, burn off of the volatiles of solder paste embedded in the liquid epoxy flux under the bottom package becomes a very challenging task. Furthermore, the powder of solder paste soaked in liquid epoxy is also prone to ooze out and form bridging and numerous solder balls. Figure 10. Venting channel between solder joints. Fig. 11 shows a simulated test for solder paste under the BGA and embedded in liquid epoxy flux. Here the SAC305 solder paste was printed onto PCB pads, Table 2. Reliability of epoxy fluxes versus conventional flux and then covered by liquid epoxy flux A. A glass for 0.3 mm pitch CSP, size: 2.67 mm x 2.67 mm, I/O: 81 slide was placed on top of the solder paste/epoxy flux Conventional Epoxy Epoxy to mimic the body of a BGA. This sandwiched PCB Tests Solder Paste Flux A Flux B was then placed on a hot plate with a surface temperature of 250°C. Fig. 11(a), (b), and (c) Chip shear 19.2 ± 0.1 24.0 ± 36.5 ± test, lbs. 4.3 1.3 represent the view at 0 second, 50 seconds, and 120 seconds on hot plate. The solder paste immensely (+80% diffused away from the printed dots at 50 seconds, typical) and never coalesced back to form proper solder Drop test 80, 80, 90, 150, 220, 220, bumps after 120 seconds. (drop # to 90 180, 230, 240, fail) 210 250, 290, Fig. 12 shows the same experiment using the same 300, 320 SAC305 solder paste but embedded in epoxy flux B. Thermal 168, 168, 719, 870, Here the solder paste remained as one integral body cycling test 168 897, 1089, the entire time for each printed dot. (-55 ~ 897, 1498, 125 °C) 1098 1843 Results above show that a properly formulated epoxy flux can allow for a satisfactory coalescence of solder paste without suffering solder balling or bridging.
  • 5. Figure 12. SAC305 solder paste with epoxy flux B under glass slide placed on hot plate at 250°C. Figure 11. SAC305 solder paste with epoxy flux A under glass slide placed on hot plate at 250°C. SIR Performance Being a thermoset system, epoxy flux is destined as a no-clean material, hence has to meet surface insulation resistance requirements without cleaning. Fig. 13 demonstrates that the IPC SIR requirement, greater than 100 mega ohms at 168 hours per J-STD- 004, is easily met by one epoxy flux material. Figure 13. SIR performance of one epoxy flux per Reflow Window J-STD-004 procedure. Epoxy fluxes can be designed to be very robust in terms of reflow windows. Table 3 shows the electrical continuity for daisy-chained BGAs epoxy fluxes. Successful solder joint formation was assembled under various reflow profiles with two achieved at a very broad reflow profile range.
  • 6. Table 3. Electrical continuity of BGA assembled using two epoxy fluxes reflowed at various profiles. Profile # temperature, min Time to peak temperature, °C Peak TAL, sec w/Epoxy Flux C Continuity w/Epoxy Flux D Continuity Figure 15. Schematic of shock wave passing through the board under BGA component. 1 4 230 59 Good Good 2 4 237 70 Good Good 3 4 245 80 Good Good 4 6 237 nd Good Good 5 3 237 nd Good Good 6 2.5 237 nd Good Good Note: nd not detectable Figure 16. Schematic of corner bonding process [8] as shown in Fig. 14. When the wave passes through the area under a rigid component such as BGA or PoP, each solder joint experiences compression and tension sequentially, as shown in Fig. 15, and it is the tension which results in joint fracture [5]. The failure modes reported [6] include pad cratering (83.3%), I/O trace fracture (73.3%), board side solder crack Figure 14. Printed circuit board experiencing shock (33.3%), daisy chain trace fracture (3.3%), and chip wave traveling through upon drop [4]. side solder crack (1.7%) in terms of percentage of analyzed components. Study of Srinivas shows the failure rate of bottom joints is higher than top joints Physical Properties for PoP upon drop [7]. Accordingly, the most critical The physical properties of a representative epoxy flux reinforcement desired would be a bonding force are listed below. The low Tg property is considered holding the board surface and the bottom package crucial for avoiding the solder extrusion phenomenon body together. at reflow or thermal cycling. Tg - 56 °C POLYMERIC REINFORCEMENT CTE (<Tg) - 64 ppm APPROACHES Typical Viscosity - 5-7 kcps There are several polymeric reinforcement Shelf life (at -20°C) - 3 month Pot life (at room temp.) - 24 hours approaches attempted by the industry as introduced SIR (Ohms) - >1.0 E+8 (pass) schematically below. Reworkable Flux activation temperature - 150 oC Corner Bond Pro FAILURE MODES UPON DROP Cure at solder paste reflow process. No additional Upon drop of portable devices, the printed circuit curing step & oven needed. board experiences a shock wave traveling through it, Reinforce BGA to some extent.
  • 7. For precision deposition, premium dispensing equipment is needed, & significant increase in cycle time becomes inevitable. Cannot reinforce bonding between top and bottom packages. Place-N-Bond Thermoplastic Underfilm Pro No dispensing equipment needed. Cure at solder paste reflow process. No additional curing step & oven needed. Reinforce BGA to some extent. Less promising for preventing crater failure. Figure 17. Schematic of Place-N-Bond Underfilm No prebaking needed. process [9]. Often reworkable. Con Need dummy pads designed in on PCB, more reserved space required around BGA. Film placement may disturb components placed earlier & cause defects. Epoxy wicking may interfere solder wetting. Cannot reinforce bonding between top and bottom packages. Edge Bond Epoxy Liquid Process Pro Fast UV cure, no heat cure needed. Epoxy won t interfere with soldering. Figure 18. Schematic of edge-bond, epoxy liquid Easy inspection. process. Con One more step dispensing needed. Cycle time is unacceptable for high volume, high throughput production. For high performance, premium dispensing equipment needed. Less promising in preventing crater failure. Cannot reinforce bonding between top and bottom packages. Capillary Flow Underfill Pro Mature technology. Figure 19. Schematic of capillary flow underfill process [8]. Less promising for preventing crater failure. No prebaking needed. Often reworkable. Con Premature curing due to altered reflow profile can result in difficulty of BGA collapse upon reflow. Polymer wicking or smear at placement can interfere with solder paste reflow. Figure 20. Schematic of no-flow underfill process [8, 10].
  • 8. Potential with the highest reinforcement. enhancement achieved includes mechanical, thermal, Promise reduction in both joint & crater failures. and electrical properties. Con Requires post reflow underfill dispense, capillary REFERENCE flow & cure. Costs more time & equipment. 1. Wusheng Yin, Geoffrey Beckwith, Hong-Sik May require prebake to avoid voiding if there is Hwang, Lee Kresge, and Ning-Cheng Lee, delay prior to underfilling. Epoxy Flux An Answer For Low Cost No- Reworkability can be an issue, including Clean Flip Chip Assembly , Nepcon components around BGA which were flooded by West/Fiberoptic Automation Expo, San Jose, underfill. CA, December 3-6, 2002 2. Wusheng Yin and Ning-Cheng Lee, A Novel No-Flow Underfill Epoxy Flux For Lead-Free Soldering , Pro International Brazing and Soldering Simple dispense process, no flow needed. Conference, San Diego, CA, Feb. 16-21, 2003 Cure during solder paste reflow. 3. Gerry Padnos (Juki), PCB Assembly System Significant reinforcement. Set-Up for Pop , Apex, S19-03, April 6-9, Promise reduction in both joint & crater failures. 2010, Las Vegas, NV. Some are reworkable. 4. R. Wayne Johnson, Yueli Liu, Guoyun Tian, Con Shyam Gale, & Pradeep Lall (Auburn Placement may introduce voids. University), Assembly and Drop Test PCB prebaking may be needed. Reliability of Lead Free CSPs , 2003. For large BGA, open & chip drifting or lifting 5. Tz-Cheng Chiu, Kejun Zeng, Roger Stierman may be the result due to earlier gelling of and Darvin Edwards, Kazuaki Ano, " Effect of underfill at the hotter perimeter. Thermal Aging on Board Level Drop Solder wetting may be hampered due to Reliability for Pb-free BGA Packages", 54th premature gelling upon alternation in reflow ECTC, P.1256-1262, June 1-4, 2004, Las profile. Vegas, Nevada. No filler allowed. 6. Andrew Farris, Jianbiao Pan, Albert Liddicoat, Michael Krist, Nicholas Vickers, Brian J. Epoxy Flux Toleno, Dan Maslyk, Dongkai Shangguan, The processes of epoxy flux are shown in Fig. 1 to 4. Jasbir Bath, Dennis Willie, David A. Geiger, Pro Drop test reliability of edge-bonded lead-free Only one extra dipping step needed. The simplest chip scale packages , Flextronic, Henkel, and process among all polymer reinforcement Cal Poly State University report, 2009. approaches. 7. Vikram Srinivas, Center for Advance Life No solder wetting interference concern. Cycle Engineering, Reliability Evaluation of No PCB prebaking needed due to designed-in One-Pass and Two-Pass Techniques of venting channel. Assembly for Package-on-Packages under Significant reinforcement. Torsion Loads , APEX, S34_03, April 6-9, Promising reduction in both joint & crater 2010, Las Vegas, NV failures. 8. R. Wayne Johnson, Yueli Liu, Guoyun Tian, Reworkable. Shyam Gale, & Pradeep Lall (Auburn Compatible with assembly of BGA & PoP, University), Assembly and Drop Test including PoP stacking. Reliability of Lead Free CSPs , 2003. Compatible with solder paste, thus can tolerate 9. Data sheet of Place-N-Bond Underfilm, warpage at soldering. Alltemated Inc.; www.Alltemated.com Con 10. Wusheng Yin, Geoffrey Beckwith, Hong-Sik Need epoxy flux bed. Hwang, and Ning-Cheng Lee, Low Cost No- Larger nozzle size may be needed for pick & Flow Underfilling Being A Reality For place at dipping step. Manufacturing , Nepcon West/Fiberoptic Automation Expo, San Jose, CA, December 3- SUMMARY 6, 2002 Among existing polymeric approaches for assembly and reinforcement for PoP, epoxy flux turns out to be the most versatile low cost option. The reliability