SMT BGA Ball Drop in Pb Free: A concern in Lead Free Assemblies
The Lead Free Directive by the European Union (RoHS - restriction of hazardous substances), have impacted everyone including the industries that are even exempt. Those of you who already have done some manufacturing of lead free products that use BGAs, you may have experienced problems in BGA assemblies. And if you have not, both you and your board and package suppliers are doing the right things or you are just lucky.
As we have learned in the IPC-7095 BGA committee (BGA Design and Assembly implementation), different companies are experiencing different types of problems in BGA assembly. It was discovered that many companies have some problems with the solder mask defined lands at the ball-BGA package interface.
In this section, we want to focus on a failure condition where the BGA ball separates from the package. In many cases, this failure can be very pronounced and will be easily visible after cross-sectioning and some cases the ball almost drops. In other cases, the failure will be seen if the package is mechanically pulled and all the separation will be at the package ball interface and the almost all the balls stay with PCB land. The fractured surface will be very flat indicative of brittle failure. I should note that this type of BGA ball drop has been seen even in tin lead process.
There are various causes of failure at BGA ball-package interface. One of the well-known phenomenon has been widely discussed in numerous papers and IPC-7095. As bigger BGAs are being used, we are starting to see failures at the ball-package interface and the problem is getting worse with lead free. In many cases the failure is related to solder mask defined lands on BGA package substrate where the solder mask acts as a crack initiator where BGA balls touches the solder mask.
This issue was tackled years ago and the recommendation in the standard has been to use non solder mask defined or metal defined lands on both the package and the PCB lands. While most users are using metal defined lands on the PCB, most of the packages continue to be supplied with solder mask defined lands. In some cases the package suppliers have no choice but to use solder mask defined lands such as when the land is too small or when the land is over a large ground plane. But no one has given a good reason for wide spread use of solder mask defined lands on the package substrate. But the good news is that users are staying away from solder mask defined lands.
But the solder mask defined land on the BGA package is not the only cause of failure at package- ball interface. One of the major causes for failure is the migration of copper from PCB to the top of the package to cause a failure at package-ball interface. The migration of copper from PCB land through the ball to the top of the package forms a brittle nickel-tin-copper ternary intermetallic with the nickel from the ENIG (electro-less nickel immersion gold) surface finish commonly used by package suppliers. The failure is generally seen at interface between nickel under plating on package substrate and the ternary intermetallic formed due to copper migration from the PCB land through the ball.
Widespread presence of copper in the ball and at the package interface has been seen through EDX analysis. Of course there is a slight amount of copper (about 0.5%) in most SAC (tin silver copper) solder balls used in lead free but that is not the source of the extensive presence of copper. Since this problem is also seen in tin-lead process where copper is present in solder ball as impurity confirms the observation that copper is migrating from the PCB land through the ball. So it should be no surprise to anyone that this problem will be compounded when the peak reflow temperature is raised from 220oC in tin-lead to 250oC in lead free process. In addition to higher peak temperature, the longer time above liquidus (TAL) compounds this problem.
So if copper is migrating from the PCB land through the ball to the top of the ball to form ternary intermetallic with the nickel in the package substrate, how can you mitigate (not prevent since there are other causes) this problem? A thermal profile with lower peak reflow temperature and shorter time especially shorter TAL does help. However, plating nickel over copper as a barrier layer (ENIG Surface finish) is one of the ways to mitigate this problem.
As noted earlier, migration of copper is not the only cause of the problem. Otherwise the failure would be much more wide spread since not everyone sees this problem when using non-ENIG surface finishes. So the plating problem in the BGA package substrate should also be the focus of the investigation.