Solder Joint Encapsulant Adhesive – POP Assembly Solution

1With the advancement of the electronic industry, package on package (POP) has become an increasingly popular IC package option for electronic devices, particularly in mobile devices. It offers benefits of miniaturization, design flexibility and cost efficiency. However, there are some issues that have been reported such as SIR drop due to small gap between top and bottom components, difficulty underfilling and rework due to stacked IC components, and process yield issues. It has been reported that using some methods such as dipping epoxy paste or epoxy flux have been used to address these issues; but so far, no successful case has been reported using these methods or materials in mass production. In order to address these issues for POP assembly, YINCAE has successfully developed a first individual solder joint encapsulant adhesive.

Solder joint encapsulant adhesives can be applied by printing or dipping process onto a substrate or component. They can remove metal oxide from pads and bumps to allow solder joints to form. It is then cured with the formation of a 3-D polymer network encapsulating each individual solder joint. There is no adhesive blocking outgassing channels between each solder joint, which ensures process yield. After using solder joint encapsulant adhesive for POP assembly, the pull strength of solder joint is increased five times, and the SIR issue is addressed with high process yield. All details such as assembly process, drop test performance, and thermal cycling test will be discussed in the full paper.

Introduction
With the advancements of the electronic industry, IC components become miniaturized, pitch size gets smaller, and I/O number increases. In addition to these factors, lead-free soldering process has to be implemented due to law requirements. As a result, there are some reliability issues such as poor process yield, weak mechanical strength of solder joints, and poor thermal cycling performance. YINCAE invented a world first solder joint encapsulant a few years ago, and billions of devices have been made with this solder joint encapsulant with approved performance and satisfaction in the customer field. Due to the benefits of miniaturization, design flexibility, and cost efficiency, package-on-package has become an increasingly popular IC package option for electronic devices. In order to address multi-core processors, higher data transfer rates, and wider bus memory architectures, POP with through-mold vias (TMV) has been used in mobile devices. Like CSP/BGA, POP also needs reliability enhancement to meet the end customer’s needs. However, the application process is very difficult (with processes like capillary underfill, corner bond, no-flow underfill and wafer-level underfill process) to address, particularly for POP with TMV, which has molded cavity surfaces. All the processes are encountered with unsatisfied process yield, reliability sacrifice, and lengthy application processes, among other issues. YINCAE solder joint encapsulant adhesive – SMT 256 and SMT 266 can enhance solder joint reliability, and eliminate underfill materials and the underfilling process, particularly for board-level underfills. SMT 256 and SMT 266 provide an excellent assembly solution for POP.

Process
The application process of solder joint encapsulant adhesive is shown in Figure 1 above. It should be noted that solder joint encapsulant adhesives can provide advantages of simple, short and high throughput manufacturing process over traditional solder paste plus underfilling process. SMT 256 has been designed for mass production, which can be applied by dipping, stencil printing, and brushing. SMT 266 is mainly focused on the rework process, which can be applied by micro-spraying, brushing, or dipping. The reflow process of solder joint encapsulant adhesive is fully compatible with typical industry solder paste reflow profiles. During reflow, solder joint encapsulant adhesives SMT 256 and SMT 266 can remove metal oxide from pads and bumps to allow solder joints to form. They are then cured with the formation of a 3-D polymer network encapsulating each individual solder joint leaving no adhesive between the solder joints. This ensures process yield since there is nothing to block outgassing channels. The schematic of SMT 256 or SMT 266 encapsulated solder joint is shown in Figure 2 below. Solder Joint Encapsulant Adhesive – POP Assembly Solution

2Reflow Profile
The SMT 256 and SMT 266 profile is fully compatible with all typical lead-free solder paste reflow profiles. The time from room temperature to peak temperature is 4 to 5 minutes and peak temperature is from 235 to 260 °C.

3Solder Wetting
Normally top IC of POP is reflow under nitrogen due to warpage issues. However, the top IC can be reflowed under air using solder joint encapsulant adhesive. Figure 3 below shows the stand-off height of solder joints after cross-section. It can be seen that the stand-off height of solder joints using solder joint encapsulant adhesive under air reflow is lower than that using traditional tacky flux under nitrogen reflow. It can also been seen that using solder joint encapsulant produces a smaller standard deviation than using traditional tacky flux. Fig. 3 Stand-off Height of solder joints: Tacky flux and Solder Joint Encapsulant (SMT256)

 4 5Solder Voids
It can be seen from figure 4 that the void percentage of solder joints was reduced to half (2.5%) using SMT 256 under air reflow compared to 5% using tacky flux under nitrogen reflow. These results are in agreement with solder wetting results. Excellent solder wetting normally leads to lower standoff height of solder joints and lower solder voids. Figure 5 below shows the pull strength of solder joints using SMT 256 and flux. From Figure 5, it can be seen that the solder joint strength using SMT 256 is up to 352N compared to 71N using Tacky flux. This indicates that the strength of solder joints can be improved by at least 5 times using solder joint encapsulant adhesive SMT 256. In addition, it should be noted that the standard deviation of solder joint strength using solder joint encapsulant adhesive is much smaller than the one using tacky flux. This shows that using solder joint encapsulant adhesive can reduce the standard deviation and lay down the foundation for stable quality electronic devices. Fig. 5 Pull Strength Changing with Dipping Height

6Head-in-Pillow
In SMT assembly of package on package (POP), it is very easy to observe head-in-pillow open solder joints due to the warpage effect if reflowed under air. Figure 6 below shows the crosssection of assembled POP obtained using solder paste and flux.
From Figure 6, it can clearly be seen that there is a head-in-pillow open joint. In order to eliminate the head-in-pillow effect in SMT assembly process of POP, nitrogen protection is added into reflow processes. It is very obvious that the addition of nitrogen protection adds a cost to the end product. It is well understood that head-in-pillow is normally caused by heavy oxidation of solder and warpage of substrates and components. POP encounters more warpage than regular BGA due to the stacked components.

87Figure 7 above shows a cross-section picture of assembled
POP using YINCAE solder joint encapsulant. There were no head-in-pillow open solder joints found in the assembled POP and every solder joint proved to be high quality from X-ray and cross-section results. The mechanism of eliminating head-inpillow phenomenon is proposed: Solder joint encapsulant covers solder bump and pad after dipping; the solder joint encapsulant functions as an oxidation barrier preventing the solder bump and pad from oxidizing during reflow. In addition, the solder joint encapsulant starts to cure more and more with increasing temperature, resulting in shrinkage stress to pull the bump in contact with the corresponding pads so that the head-in-pillow 9phenomenon is eliminated.

Dendrite in POP
Figure 8 below shows the dendrite in assembled POP using solder paste and flux, which is causing the current leakage problems experienced after sales. Normally the dendrite cannot be found immediately after manufacturing. After sale to the end user, the dendrite starts to form, resulting in higher RMA (Returning Materials Authorization), which could lead to millions of dollars lost. It is particularly easy for dendrites to form in assembled POP TMV. This is because the flux residue will accumulate into the cavity of POP TMV after reflow using traditional flux or solder paste. It has been proven that solder joint encapsulant adhesive can be cured to form a 3D polymer network and encapsulate 10solder joint, which can easily prevent dendrite formation and electromigration. Fig. 8 Dendrite in Assembled POP Drop Test The drop test has been conducted for assembled devices with solder joint encapsulant adhesive (SMT 256), no flow under fill (NF), capillary underfill (CUF), and solder paste only. The results are shown in Figure 9 below. Fig. 9 Drop Test Performance Using SMT 256, NF (no-flow underfill), CUF (capillary underfill) and Solder Paste. Drop test conditions are: six feet height, concrete floor, free fall. From Figure 9, we can see that the drop times is up to 200 times using SMT 256 solder joint encapsulant, which is same as that obtained using no-flow underfill, but much better than that obtained using solder paste. The drop test performance is in agreement with the results of the pull test in Figure 5. Figure 10 below shows the thermal cycling performance using different approaches for enhancement. Thermal cycling conditions are: one hour per cycle; temperature from –55°C to 125°C ,and 15 minutes dwell time at two extreme temperatures. It is very interesting to note that traditional capillary approach could decrease reliability resulting in thermal cycle sacrifice. Using solder joint encapsulant SMT 256 or SMT 266, the first failure cycles is as high as 6000 cycles – at least 4000 to 5000 cycles higher than other processes. This is because solder joint encapsulant adhesive enhances only the strength of the solder joints by 5 to 10x times but won’t cause extra stress due to the air space in between solder joints.11 12 13

 

Reworkability
14Using the standard autoprofile, the solder joints achieved a maximum temperature of 237oC and were above melting point for 67 seconds, followed by BGA removal for site analysis. One issue we examined was difficulty of removal. Underfilled parts can be difficult to remove during rework. Either the underfill does not sufficiently soften, or the shear volume is just too much to overcome the pull force required for removal. However, the solder joint encapsulant proved to provide easy removal after reheating to reflow temperature. Pickup tube flex seals were not required to provide additional removal force to remove the BGAs. Removed parts showed rings of encapsulant surrounding the solder joint, seen in Figure 11 below. Fig. 11 Substrate after BGA Removal Removal of cured encapsulants from the site was quite easy. Vigorous scrubbing with MEK was not required. Light dabbing of the site with a q-tip soaked in MEK was adequate. RossTech 119EC flux remover also proved equally effective in removing the cured encapsulant. From Figure 12 below, we can see there is no damage to the reworked substrate. Fig. 12 Substrate after Rework (a) (b) Fig. 13 X-ray Images of Assembled BGA with Fresh (a) and (b) Reworked Substrates Figure 13 above shows X-ray images of assembled BGA with fresh and reworked substrates. It is very obvious to see that there are no voids in the solder joints and all solder joints are in very good shape. There is no difference between the fresh and reworked parts after SMT assembly process.

Conclusion
A new solder joint encapsulant has been successfully developed, which can not only increase solder joint strength by 5 times, but also provide excellent reliability for advanced IC components. It provides the total assembly solution for advanced POP, such as POP with TMV. Using solder joint encapsulant for POP assembly would have the following benefits: • Eliminate head-in-pillow issues; • Prevent dendrite formation. • Eliminate difficult under filling process, particularly for POP with TMV; • Enhance reliability; • Easy rework.

REFERENCES
1. Mary Liu and Wusheng Yin presented “ A First Room Temperature Stable and Jettable Solder Joint Encapsulant Adhesive” in IMAPS New England – 38 the Symposium & Expo, May 3, 2011
2. Mary Liu and Wusheng Yin had technical poster” A First Individual Solder Joint Encapsulant Adhesives” in Raleigh NC, IMAPS, November 3, 2010
3. Mary Liu and Wusheng Yin Presented “ World First Solder Joint Encapsulant Adhesives” in Shenzhen (China), NEPCON, August 31, 2010
4. Mary Liu and Wusheng Yin presented “ A First Individual Solder Joint Encapsulant Adhesives” in San Francisco, SEMICON West/IMAPS, July 14, 2010

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