SMT BGA Ball Drop in Pb Free: A concern in Lead Free Assemblies: Part 1
No matter where you stand on the wisdom of lead free directive by the European Union and their numerous followers across the globe including China, Mexico and the state of California, the impact of this directive is going to be felt by everyone including the industries that are even exempt. And just because the August 13, 2005 deadline for implementation of WEEE (waste electrical and electronic equipment) has come and gone and is now moved to the end of 2005 in most European countries, it is highly unlikely that July 1, 2006 deadline for RoHS (restriction of hazardous substances) is going to be moved. 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 our IPC-7095 BGA committee (BGA Design and Assembly implementation), different companies are experiencing different types of problems in BGA assembly. We discovered through our survey for the revision of IPC 7095 that many companies are starting to have some problems with the solder mask defined lands at the ball-BGA package interface.
In this column I 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 phenomenons 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. In the current revision of IPC-7095 we are looking at this issue again. If you have any input, please feel free to contact me or IPC.
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.
This failure mechanism has been confirmed by me and many others. 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. This is the subject of my next column. Stay tuned. Speaking of ENIG, whether at package or PCB surfaces, be aware of black pad problem in ENIG. Black pad is one of those mysterious mechanisms that are not well understood at this time but phosphorous is understood to be the cause of the problem.
Ray P. Prasad is the author of the textbook Surface Mount Technology: Principles and Practice. Additionally, he is president of BeamWorks Inc. and founder of the Ray Prasad Consultancy Group. Contact him at 15375 SW Beaverton Creek Court, Beaverton, OR 97006; (503) 297-5898 or (503) 646-3224; Fax: (503) 646-1654; E-mail: firstname.lastname@example.org; Web site: www.rayprasad.com.
Ray Prasad will be teaching in depth SMT and BGA Design and Manufacturing and Lead Free Implementation courses in Portland, Ore. See www.rayprasad.com for course and registration details.
Pull Quote: “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”
Ray P. Prasad (www.rayprasad.com)
Ray Prasad held key technology positions at Boeing and Intel for 15 years before starting his consulting practice in 1994 on SMT, BGA, fine pitch and Lead Free implementation. He also joined BeamWorks, a laser selective assembly equipment manufacturer for Lead Free and tin lead products in 2000. He was the SMT Program Manager at Intel responsible for developing and implementing SMT in Intel products and developed and taught in-house and design and manufacturing courses to Intel engineers. He also managed Intel PentiumProTM package program for Intel and introduced SMT into Boeing airplanes as the lead engineer at that company. For establishing the SMT infrastructure at Intel he was recognized by Intel CEO Andy Grove and received Intel’s highest award – Intel Achievement Award.
Author of the text book Surface Mount Technology: Principles and Practice published by Walter Kluwer Academic Publishers, now also translated into Chinese, and numerous papers, Ray is a popular workshop leader at national and international conferences. A long time member of IPC, he is currently the chairman of BGA committee IPC-7095 “Design and Assembly Process Implementation for BGA”. He is the past chairman of the Surface Mount Land Pattern (IPC-SM-782) and Package Cracking (J-STD-020) committees.
Ray is the recipient of SMTA’s Member of Distinction Award and IPC President's award for his contribution to SMTA and the advancement of electronics industry. He is a columnist for the SMT magazine and also serves on its advisory board. Ray Prasad received his BS in Metallurgical Engineering from the Regional Institute of Technology, Jamshedpur in India and MS in Materials Science and Engineering and MBA from the University of California at Berkeley. He is a registered Professional Metallurgical Engineer.
Those of you who already have done some manufacturing of lead free products that use BGAs, you may have experienced problems in BGA assemblies.