Up to now, as far as manufacturers involved in electronics assembly concerned with BGA components used at a medium or large scale, soldering defects of BGA components are exposed through the implementation of electrical tests. Other methods to control assembly process quality and identify defects on BGA solder joints include sample test on paste screening, X-ray inspection and electrical test analysis.
It’s an extremely challenging task to meet assessment requirement for electrical test on BGA components since it’s quite difficult to pick up test points under BGA components. Thus, electrical tests can never be relied on BGA defects inspection and assessment, which increases the cost of defect removal and rework to some extent. Electrical tests can only judge whether current is on or off when BGA components get connected. With non-physical solder joint test assisted, it’ll be beneficial for assembly process improvement and SPC (Statistical Process Control).
BGA SMT assembly is just a fundamentally physical connecting technology. To be able to determine and control the quality of such a type of process, physical elements should be known and inspected affecting its reliability on long-term operation such as solder paste amount, alignment between leads and pad and wettability.
What are the defects of BGA solder joints?
Open Circuit in Non-collapsible BGA Solder Joints
Open circuits at non-collapsible BGA solder joints usually result from pad pollution. Because solder fails to get pad on PCB wetted, it’ll climb onto solder balls and then on component surface. As is discussed in previous part, electrical test can determine the existence of open circuits but fails to determine the real cause of defects.
With cross-section X-ray inspection technology applied, image slices obtained between pad and component layer can be used to distinguish open circuits occurring as a result of pollution. Open circuits as a result of pollution tend to generate small pad radius and large component radius, distinctions between component radius and pad radius can be used to determine whether open circuits take place as a result of pollution. If open circuits take place due to insufficient solder, difference between component radius and pad radius will be very small and this type of difference can be only indicated by cross-section X-ray inspection devices.
Voiding in Collapsible BGA Solder Joints
Voiding in collapsible BGA solder joints takes place because flowing steam is stopped at solder connections with low eutectic point, which is a leading defect held by collapsible BGA components. During the process of reflow soldering, since the influence of floatage generated by voiding concentrates at component surface, majority of solder joint failures take place there as well.
Voiding can be eliminated through preheating during reflow soldering process and short preheating time adding and relatively low preheating temperature. If voiding goes beyond regulated size, number or density, the reliability of components will be dramatically reduced. Voiding in collapsible BGA components can be clearly indicated by cross-section X-ray image slice. Some voiding can be determined and measured within those images or indirectly indicated by the obvious image at BGA solder joints.