Via-in-Pad Plated over Design Considerations to Mitigate Solder Separation Failure

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Fig5-Cisco-29Nov17.jpgSince the separated solder ball shape is rounded near the open or partially open interface, this indicates that the solder joint underwent reflow subsequent to the separation. Furthermore, since the separation is between the bulk solder and the IMC and does not reflect a brittle fracture, it is suspected that the separation occurred after the solder has softened and is nearly molten. Figure 6 illustrates a brittle solder joint failure in which the fracture occurs within the IMC itself and exhibits more of a flat surface indicative of crack propagation.

Figure 5: Partial solder separation.

Failure Mechanism

There seem to be two primary effects that are occurring which contribute to this failure mechanism. First, there is a CTE mismatch between the VIPPO structure and the non-VIPPO, or deep-backdrill VIPPO, structure, that results in a greater expansion of the PCB beneath the non-VIPPO BGA pad, or the deep-backdrill VIPPO pad, as compared with the VIPPO BGA pad. Secondly, the higher thermal conductivity of the VIPPO structure as compared with the non-VIPPO, or deep-backdrill VIPPO structure, allows the VIPPO solder joints to reach liquidus before the adjacent solder joints having a non-VIPPO, or deep-backdrill VIPPO pad. Therefore, during a secondary reflow process, when the adjacent non-VIPPO solder joints are still solid, tensile stresses are induced in the VIPPO solder joints as the adjacent non-VIPPO solder joints are pushed up due to the greater out-of-plane PCB expansion beneath those pads.

Subsequently, when the VIPPO solder joints become molten, these high stresses are relieved as the bulk solder “tears” or separates from the IMC. This solder separation can occur at either the package interface or the PCB interface of the solder joint, depending on whichever is the weaker interface. Since the PCB pad design is generally a non-soldermask-defined pad (NSMD) and the BGA package typically uses soldermask-defined pads (SMD), the separation will more likely occur at the package interface.

Fig6-Cisco-29Nov17.jpgFigure 6: Complete solder separation.

Alternatively, a 100% VIPPO BGA footprint without deep backdrill does not introduce the additional stresses that are exhibited with the CTE mismatch between adjacent VIPPO and non-VIPPO pad designs. Additionally, a 100% VIPPO BGA footprint without deep backdrill does not create the high thermal gradients between adjacent solder joints that the mixed VIPPO/non-VIPPO BGA footprints achieve. Therefore, this type of failure mode has not been identified with 100% VIPPO BGA footprints with no deep backdrill.

Evaluation Plan

In order to better understand the influence of various PCB and packaging design parameters on this failure mode, three different test vehicles have been designed to assess the following:

1. Influence of drill hole size (DHS) for the VIPPO structures: 9.8 mils vs. 7.9 mils DHS

2. Influence of BGA package body size and BGA pitch

3. Influence of varying backdrill (BD) depths and BGA package body size

Each test vehicle is assembled through a primary and secondary SMT attach process, followed by inspection and physical analysis to validate the solder joint integrity after each reflow. The printed circuit assembly equipment, process parameters, tooling (e.g., stencil design and technology), assembly materials (e.g., solder paste) and inspection equipment and methodologies utilized for these builds are consistent with Cisco’s standard production processes in order to minimize the number of variables introduced in this study.

To read the full version of this article, which appeared in the November 2017 issue of SMT Magazine, click here.



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