Solderability Issues

Solderability issues are probably the most common ones in the electronic assembly. A large variety of the problems can be divided into three categories. The largest one would represent the cases which can be solved by simply tweaking the process: adjusting the placement machine, cleaning the stencil, changing the reflow profile and so on.

Another category would include the cases where the components are barely solderable, due to their quality. Those ones are relatively easy to verify by the solderability tests (particularly wetting balance) and they can be caught by incoming inspection.

The third group is the most difficult to deal with, as each case may require a comprehensive analysis, in order to establish the root cause for the problem and so the corrective actions can be implemented. The case below is a typical example of the solderability issues from that group.

The Problem

A customer reported a solderability problem observed after assembly (SMT process) on the board side of an ENIG finished PCB. The majority of the assembled boards were affected. One of the assemblies was submitted to SENTEC for root case analysis. Typical appearance of the affected solder joints is shown in Fig. 1.

The Analysis

An area of the board with some of the component exhibiting the problem was cut out and inspected in a scanning electron microscope (SEM) equipped with an X-ray microanalysis system (EDS). The inspection revealed that areas of the solder pads not covered with solder exhibit the presence of an abnormally thin layer of electroless Ni. To shed more light on the root cause of the problem one of the components was cross-section. The results of the analysis confirmed that the layer of electroless Ni (E-Ni) is uneven and it is almost missing in some areas of the solder pad. The images in Figures 2 through 4 illustrate the findings:

Fig. 2 - An overall appearance of the joints - scanning electron microscopy (SEM);
Fig. 3 - An enlarged light microscopy image of the area of interest where the presence of a thinned layer of electroless Ni is evident;
Fig. 4 - A close up SEM image of the area where a typical structure of a good solder/solder pad interface was formed (a well defined layer of Ni-Sn intermetallics is evident in the right part of the image) and the area where no wetting between the solder and the pad (left part of the image) was evident.

The Root Cause

Looking at the images one may say that the layer of electroless Ni had been either defective on the original (bare) board or somehow got damaged during the SMT process. To validate the statement, one more cross-section was prepared. That time around an area of the board with traces covered by solder mask was chosen. One of the cross-sectioned traces is presented in Fig. 5. The image shows that the layer of electroless Ni is not solid and uniform, but it is rather splitting indicating a manufacturing problem with the board plating.

The Outcome

Once the defects in the layer of E-Ni were discovered, it became obvious why the customer had faced the problem. The issue was addressed with the board supplier. The company admitted that they got a problem with their electroless Ni plating process.

PFC Flexible Circuits Limited has utilized the services of SENTEC numerous times over last 2 years. We have availed ourselves of their experience in process troubleshooting, assembly problems, and reliability failures. In all cases their analysis and insight has been invaluable to out company and the analysis done by Sentec has been clearly superior to any work done at these much larger companies. Their response times is excellent as is their ability to interface with out customers at a very high professional level. It is not uncommon now for your Fortune 100 customers to use SENTEC for other problem solving in lieu of their own internal resources.end-quote

- Steve Kelly
PFC Flexible Circuits Limited


SENTEC Testing Laboratory Inc.
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