New Light on a BGA Soldering Problem

BGA/CSP/WSP array solder joint failures can be very vexing for several reasons.  First, BGA solder joint opens are difficult to diagnose in the assembly because BGA electrical problems are always difficult to diagnose due to the lack of access to electrical test points and the high complexity of the circuitry.  The second troublesome aspect is that reworking a BGA is difficult and third, there is the possibility of the assembly passing functional test and having latent field failures.  Therefore very high solder joint assembly yields are required. High solder joint yields were the original promise when BGA packages were introduced many years ago but based on the numerous publications of head-in-pillow failures there is widespread inconsistency on achieving those high yields (see my 2010 blog posting on BGAs). Now there is another failure mechanism which is causing problems - non-wet opens.

Non-wet opens (NWO) are a problem in BGA/CSP soldering where the solder paste does not wet the PCB pad.  These failures can be misdiagnosed as 1) head-in-pillow failures, 2) failures due to a clogged stencil or 3) a PCB solderability problem.  Possibly because of  misdiagnoses there has not been any published research into the various factors that result in this failure.  That is until the recent SMTA International conference in October.  The team at Intel presented an excellent paper [1] that is a valuable resource in learning about this problem.  Here are some of the important findings of this research.
1.   One of the key findings is that solder paste has a considerable influence on NWO open failures.  Both the wetting activity and the paste tackiness are important.  Intel was able to turn NWO on and off just by switching the solder paste.  
2.   Paste volume also was shown to add some margin to prevent NWOs.  This factor is not as large as the paste formulation.
3.   The soak profile during reflow is a factor.  Longer soak times produce fewer NWOs.
4.   Inert atmosphere soldering did not have a significant influence.
5.   PCB surface finish showed some effect.  Interestingly, lead-free HASL and OSP were the poorest finishes compared with immersion silver.
6.   Component warpage is also a factor in a more complicated way.  Concave or convex warpage can change the failure mode from NWO to HiP.

In summary, this paper is important since it is the first paper to shed more light on the nature of NWOs.  The influence of paste formulation is especially significant.   This paper should prompt further investigation especially by the solder paste suppliers.  In fact, several solder paste suppliers are actively and quietly researching how to mitigate this risk.  It is possible that the development of head-in-pillow resistant solder paste has resulted in the side effect of more NWOs. These types of unintended side effects are very common as technology advances. Due to the high solder joint yields required of today’s complex electronic assemblies, it is urgent that progress be made to eliminate this problem.

References
1.  D. Amir, et al.  The Challenges of Non Wet Open BGA Solder Defect, SMTAI, October 2012