Quality Control Solder Joint Inspection

Problem: A device manufacturer had a product that involved a few different boards with varying components.  There were circumstances that caused the need for a change in manufacturers of one of the boards.  Due to this change, the device production at this facility would be shut down until the boards from this new vendor were validated.  The Quality Control department required that an inspection of the solder joints of some of the components on the board be analyzed in order to determine whether this new board manufacturer met their specifications.  If the desired specifications were met and no issues were found during the inspection of the joints then production of the devices could resume.

Approach: MicroVision Laboratories’ analytical experts were able to meet with the QC Engineers and develop an analytical plan as to which solder joints to cross section and inspect.  The client not only wanted to determine if there were any significant issues with the solder joint but also determine that there was a good intermetallic bond between the tin/lead solder and the copper wires of the chip packages.

 

Analysis and Results: The desired chip packages were sectioned from the larger board, and placed in an epoxy mounting cup.  The epoxy was mixed and allowed to harden. The resulting epoxy puck was cross sectioned and polished.  The epoxy mounting and cross sectioning process gave precise, perfectly preserved cross sectional surfaces through the desired components and their solder bonds.

The profile of the flow of the solder at these bonds was documented using the SEM with backscatter imaging, which correlates brightness in the image with atomic density. Some voids were found in the solder as shown the SEM image.  An EDS spectrum of the solder was acquired which showed that the solder was a tin/lead (80/20) solder. 
 
The EDS map clearly shows the copper wire and copper pad (red) with the tin lead solder (light blue) that appears to have flowed well and made a good bond between the copper elements.  This map also shows the fiberglass bundles that add structural integrity to the board. 

Close examination of any possible defects or voids was undertaken at higher magnification.  The voids did not appear to create any structural or conductivity issues.  Additionally, the formation and contiguity of intermetallic bonds between the contacts and solder were shown using a combination of EDS line scan elemental spectroscopy and elemental mapping.  The SEM image and the EDS map to the left show the intermetallic layer between the copper wire and the tin/lead solder via the mixture of the red copper and the blue tin. 
 
The EDS line scan below shows the relative concentrations of the elements involved in the intermetallic bond with relation to the

position along the line.  The scan from left to right shows a high tin concentration (green line) while areas of higher lead concentrations (blue line) were not intersected by the line scan.  At the interface between the tin/lead solder and copper (red line), there is a mixture of the solder and copper which is the intermetallic layer.  The EDS Map provides a nice visual mixture of colors which shows the intermetallic layer while the line scan clearly shows the intermetallic with the elemental graph.

Conclusion:  The client was able to obtain a comprehensive sampling of the state of the soldering process from the new manufacturer at a very reasonable cost.  Rather than acquiring a mass of images, electrical test print outs, or low resolution optical pictures, the client received a comprehensive report detailing the relevant findings of the analysis on the boards from the new manufacturer.  The solder joints in this study showed good bond integrity as well as excellent intermetallic layers.  These findings allowed for the validation of the new manufacturer and their process which allowed for the clients device production to resume.  The QC Engineers were very pleased with the customer service, analysis, pricing and quick turnaround time of this project.

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