Failure Analysis In Depth

BGA Analysis and Cross-Section

Problem:

Determine the cause of voids that appear in BGA balls that contain a microvia beneath them.

Analysis:

X-ray is used to identify the quality of the solder joints and key areas of interest for the cross section analysis. During the x-ray analysis, other anomalies were noticed as well.

After X-ray, a cross section of the part was made and the scanning electron microscope (SEM) was used to obtain key high-resolution images. By using the SEM, we were able to determine that a small air pocket and residual board fabrication volatiles were present in the via prior to reflow causing a large void at the bottom of the solder joint. This proved that the void was due to via in the pad and board fabrication, not the processing of the component.

Problem Solved:

The optimum solution is a conductively filled via, a copper plugged via or a solid microvia in pad. This allows the solder to fully wet the bond pad and ensures that no voids escape from the microvia either because of insufficient filling of the via or outgassing of the via contaminants into the solder joint.

Assembly Failure Mode Analysis

Problem:

Determine the preliminary failure mode for a manufacturer based on the attachment process being used.

Analysis:

X-ray is used to identify the quality of the solder joints and other possible areas of interest for the cross section analysis.

Cross sections of three key areas were made for further analysis. After careful review of the optical micrographs, one of the three samples warranted further imaging. SEM was used along with energy dispersive spectroscopy (EDS) to further analyze the solder joints determining that the quality base intermetallics formed during processing.

During the EDS of the solder joints some large areas of Copper-Tin intermetallics were noticed.

Problem Solved:

In the figure above, you can see that there are some large copper-tin intermetallic layers just above the nickel layer. Thus, the conclusion is that there are pinholes in the nickel layer that lead to copper-tin intermetallic areas. These large intermetallic regions will cause the joints to be significantly more brittle and lead to component failures.