The incredible popularity of compact, handheld products brings a number of new challenges for designers and manufacturers. The demand for high density is increasing the number of terminations and driving lead spacing to smaller dimensions. The miniaturization of these products has altered the reliability consideration as designers have to consider the shock that comes when products are inevitably dropped.
Ball grid arrays are a dominant interconnection technology for the semiconductors used in many of the handheld portable products. As manufacturers and OEMs grapple with the challenges faced using BGAs with high lead counts and fine pitches, IPC is providing guidance for dealing with this high density.
IPC-7095C, the latest iteration of the popular guideline entitled Design and Assembly Process Implementation for BGAs, is now ready for use. The base document has undergone an extensive revision during a lengthy development by the committee that included a wide range of representatives from OEMs to fabricators to EMS companies. Ray Prasad of Prasad Consultancy Group spearheaded the effort.
“Many of the surveys conducted during the last few years show that large BGAs are one of the component packages that give users concerns for having good processes and good process control,” said Dieter Bergman, IPC director of technology transfer. “We went through the older B revision of IPC-7095 and analyzed what things have changed since the initial release. When the committee was working on the new C revision, we reviewed the B rev paragraph by paragraph.”
These revisions were driven in large part by customer concerns. Their focus on end product reliability prompted the creation of new information that focuses on mechanical as well as thermal stresses. The prevalence of handhelds prompted committee members to focus on the problems that come when small products are dropped. The shock often causes BGA connections to separate.
Originally, many thought that these open circuits were in the solder joint or were caused by peeling copper. Further analysis revealed a fault known as cratering, in which the resin under the copper starts to fracture.
There are many reasons for this phenomenon, which stems from new laminate formulations to address the higher lead-free temperatures and thinner layers at the surface creating less resin over the glass reinforcement. This problem is accentuated when large BGA packages have slight bowing at the corners and the stress is produced across those joints.
The combination of all the issues drove the development of the C revision to the BGA implementation standard. The package interposer curvature, the smaller features, the board laminate conditions, the mixture of lead-free and tin/lead processing all combined to add to the concern for mechanical stress due to shock. Several solutions are recommended, such as putting glue dots or underfilling solder mask defined lands, as well as using excess solder paste at the corners.
A range of processes have been examined so the standard can give users the latest information on best practices. Techniques for controlling temperatures and improving solder reflow are also covered. Specific solder alloy requirements are addressed for users who are attaching dissimilar package materials.
Another change in the latest iteration of IPC-7095 is that voids in solder joints, although still a concern based on their location, have become more of a process indicator. The recommendations for process improvement have been retained and are now in the Appendix of the document.
The standard takes another look at X-ray inspection techniques, which have vastly improved and are often used to examine solder connections that are under the package. “X-ray is an important tool,” Bergman said.
Another issue that is covered in detail is head on pillow. It is a condition where reflowed solder ball and reflowed solder paste do not coalesce forming an incomplete bond that is unreliable. Common causes include contamination, ball or solder paste oxidation, and board warpage.