Authored By:
Steven Perng, Weidong Xie, Tae-Kyu Lee, and Cherif Guirguis
Cisco Systems, Inc.
San Jose, CA, USA
Summary
There have been extensive studies on large ASIC BGA warpage due to CTE mismatch. The resulting defect modes include solder bridging around corner pins, and Head-In-Pillow (HIP) joints around corner pins or under die shadow area.
As the increasing trend on higher routing density and transfer speed, VIPPO and back drill are becoming common features on high-end products.
The interactions among these features and/or other features, i.e. the traditional dog-bone pad design, could induce a new failure mode that is different from what was abovementioned. Instead of the CTE mismatch on package material set, this new failure mode is induced by the CTE mismatch among the neighboring material, stack-up, as well as pad designs.
Recent studies on fine pitch memory devices showed, in some extreme cases, the interaction between PCB, solder joint, and package may introduce solder detachment on package side in the area of VIPPO and non-VIPPO mixed design. Since the detachment can be transient during the reflow process, the traditional post-reflow continuity testing will not be able to detect the defects.
To detect the existence of solder detachment defect with transient nature, an innovative in-situ, real-time monitoring methodology is introduced. Utilizing specially design daisy chain test vehicles with data logger, the proposed methodology can effectively monitor the continuity of solder joints in real time during reflow or rework process. The methodology with proto type apparatus has been verified on fine pitch memory devices with VIPPO and non-VIPPO mixed design, and all VIPPO design.
This paper summarized the development of this innovative in-situ, real time methodology, which is capable of detecting BGA transient discontinuity. The potential of expanding this detection methodology for other applications is also touched briefly.
Conclusions
An innovative detection method for BGA transient discontinuity using daisy chain circuit is developed. The detection method has been tested on a 0.8mm pitch memory device test vehicle with two pad designs, including VIPPO/non-VIPPO mixed pad design and All VIPPO pad design.
All four memory devices with mixed pad design exhibits solder joint discontinuity during the ramping up stage. The joints reconnected after few seconds consistently among the four devices. The devices were put into dye-and-pry analysis. Dye penetration was found in multiple locations. Which suggests, after detachment, the solder joint either reconnect partially or has contact without forming a joint.
These solder joints may pass electric test. However, it is not a reliable joint and will eventually fails in the field. Since the solder joint discontinuity is a transient phenomenon, any post reflow electric test won't be able to detect the unreliable joint effectively. This category of solder joint is considered as having high-risk latent defect. More effort is needed for product level non-destructive detection method.
Initially Published in the SMTA Proceedings
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