Despite the fact that the production of assemblies has technically advanced, assemblies, especially those that have been assembled with BGA components, often have soldering defects that require rework. The following example shows a typical case for today’s inspection and rework requirements: on a relatively small assembly of 15 x 15 cm (5.9 x 5.9 inches) a BGA shows several defects after manufacturing. In order to detect these defects, in addition to an X-ray system, an Ersascope should be used to look below the BGA.
Before the inspection with the Ersascope, the assembly was x-rayed to provide the customer with a comparison of the x-ray and Ersascope inspection and to offer the possibility to use both technologies in combination for defect analysis. The results showed that the same type of errors consisting of multiple bridging and variations in sphere size occurred on different assemblies. The applied X-ray inspection offered a clear defect detection. However, the reasons for the detected defects could not be fully explained using X-ray. It is possible that the reasons for the defects were caused by the PCB layout (vias), the pad shape and / or the paste printing and reflow process (paste quantity, thermal profile).
The Ersascope inspection performed in addition to the X-ray inspection partially revealed the reasons for the errors. Due to the chip population in the vicinity of the BGA, an inspection was not possible at all locations, but three conclusions were drawn nevertheless:
1. The solder bridges were clearly visible in the outer corners of the BGA. The balls were convex instead of the regular concave shape.
2. At least one ball diameter was clearly enlarged and was about bridged to the neighboring ball. The amount of solder contained in these two balls was different.
3. Although the X-ray examination did not show any internal bridges, the back light of the Ersascope was unable to pass through. Based on these results and to fully understand the possible reasons for the failure, the BGA component was replaced.
The procedure was performed as follows:
1. the BGA was desoldered without any problems, although the PCB had some internal ground layers
2. the solder residues were removed with a soldering iron. The pads were cleaned with a flux remover.
3. the same reflow profile was chosen for the installation of the BGA as for desoldering. Using a flux stencil, flux was applied to the component. No additional solder paste was added. Afterwards the component was automatically placed and soldered at the correct position. The newly soldered component did not show any of the previously detected anomalies, thus confirming the theory that the amount of solder paste in the manufacturing process was too large.
The following conclusion resulted from this procedure:
1. in BGAs, X-ray and Ersascope inspections can reveal the same soldering defects.
2. a combination of both inspection technologies can help to find the reason for the defects.
3. rework on BGA applications with a rework system can lead to production-like or even better results.
Do you have questions regarding this matter?
Your contact person: Jörg Nolte
company: Ersa GmbH