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Trouble in Your Tank: Is it Black Pad or Something Else? Case Study No. 2
January 9, 2013 |Estimated reading time: 4 minutes
Editor's Note: This article originally appeared in the June 2012 issue of The PCB Magazine.
Introduction
A few columns back, I gave readers an opportunity to test their troubleshooting skills when presented with a real-life case study. This month, I present another case study, one in which the problem at hand relates to a solderability defect.
Solderability Issue
An assembly company determined that several PCBs exhibited dewetting on certain features after assembly. Optical inspection of three of the boards indicated numerous dewetting locations. These circuit boards were plated with electroless nickel/immersion gold (ENIG) as the final finish. Dewetting on ENIG substrates is observed as dissolution of the Au, followed by exposure of the Ni base metal. The assembly process is lead-free and based on SAC 305 alloy. The solder paste after reflow did not completely wet the surface mount pads. Since the assembly company’s Q.C. manager noted that there appeared to be exposed nickel, the assumption was that the ENIG process was a contributor to the defect.
Figure 1: PCB after assembly. Note arrow points to dewetting.
After further observation, several other issues were noted:
- Severe dewetting on large pads (Figure 2)
- Dark pads in some areas (Figure2)
- Misaligned components, as well as some components not remaining connected to the circuit feature (Figure 3)
Figure 2: Arrows point to severe dewetting; there are other areas as well.
In Figure 3, several pads appear dark or black. The assembly staff felt that black pad or other interfacial defects could be present. This statement was made based on what appeared to be some dark areas on surface mount pads and areas where components failed to adhere to the surface. Often, interfacial defects and brittle fractures are attributed to the ENIG final finish process and black pad. A more detailed analysis is warranted and will be detailed elsewhere in this paper.
Figure 3: Additional issues.
How Would You Approach the Problem?
There are a number of methods one could use to get to the root cause of this problem. However, I caution against concluding that the defects are black pad related.
So, beside the obvious visual examination of the assembled PCBs, what other techniques and methods may be used to understand and solve this issue?
The methods employed for this study were:
- XRF (measure thickness of the plated layers)
- Micro-section analysis
- SEM/EDAX
- Standard IPC solderability tests
XRF X-Ray Fluorescence
There was a concern that plating of the electroless nickel could in some way impact solder joint formation. Indeed, the results of the plating thicknesses (on non-assembled boards of the lot in question as well as non-soldered pads of the assembled boards) indicated that nickel plating thickness was insufficient and far below the IPC ENIG specification. Per the customer’s supplied information, the specified/designed plating thicknesses for these samples are 3-4 µ of gold and 150-200 µ of nickel.
In comparison to this specification, the Au-plating thickness appears to be slightly on the high side (3-5 µ), while the Ni-plating thicknesses were alarming. XRF measurements showed nickel thicknesses severely below the requirement. Nickel thicknesses measured were 45-55 µ—clearly a major concern at the board fabricator level.
Microsection Analysis
The sectional analysis showed tin-nickel intermetallic formation even on the pads and surface features that exhibited dewetting. What is clearly evident is the non-uniformity of the intermetallic that could have easily led to poor component adhesion and overall less than optimum solderability. However, there was none of the spikes along the grain boundaries in the nickel deposit that are often attributed to hyper-corrosion of the nickel after the immersion gold process step (Figure 4).
Figure 4: Tin-nickel intermetallic formation—intermetallic structure in brackets.
While the intermetallic formed is clearly visible, it should be noted that the growth was non-uniform and could have contributed to poor adhesion of the components. It was then decided to view the surface of the problem boards where there was exposed gold, i.e., where the solder did not completely wet the pad (Figure5).
Figure 5: View of the surface; gold deposit looks normal. Striations most likely due to scrubbing marks in panel before soldermask, and where the electroless nickel deposit was simply too thin.
Additional examinations did not show any evidence of mud-cracked appearance and darkened grain boundaries typically present in the nickel deposit that is associated with black pad (Figure 6). Gold deposit has been stripped, exposing the underlying nickel.
Figure 6: Example of mud-cracked appearance in the nickel deposit with large, darkened grain boundaries. This was not present in the circuit boards presented for this study.
Elemental Analysis
So what would explain the areas of dewetting and poor solder joint formation? Again, the major concern was the poor solder joint formation seen with many of the surface mount components (Figure 7).
Figure 7: Poor wetting of surface mount pad.
As cooler heads began to prevail, it was clear that the very low nickel thickness was impacting solder joint formation.
However, there is one other point of contention and that is the reflow profile. Was the proper reflow peak temperature reached? Was there sufficient dwell time? These are questions that the end user could not or would not answer. One thing is for sure, the end user could not blame the problem on black pad.
Again, there is no substitute for proper process control.
Michael Carano is with OMG Electronic Chemicals (formerly Electrochemicals), a developer and provider of processes and materials for the electronics industry supply chain. He has been involved in the PWB, general metal finishing photovoltaic industries for nearly 30 years. Carano holds nine U.S. patents in topics including plating, metallization processes and PWB fabrication techniques.