Qualitative Model Describing Hot Tear Above VIPPO and Other Design Elements
The mechanism for the formation of Hot Tears is discussed and applied to other design elements that can be found on Printed Circuit Board Assemblies.
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Authored By:
Gรผnter Gera, Udo Welzel, Yin Jizhe, Harald Feufel
Robert-Bosch GmbH
Schwieberdingen, Germany
Summary
Over the last couples of years there have been numerous reports of a unique soldering failure resulting in a separation of BGA solder joints from the intermetallic compound at the interposer during reflow. In most cases, the failures were correlated with the use of Via-In-Pad-Plated-Over-Technology (VIPPO). Since the separation could be proven to occur during the Phase transition from solid to liquid [1] it was called Hot Tear. Since the Hot Tear results in a very thin separation it is usually not inspectable neither by means of X-Ray inspection nor by electrical testing, but results in very early field failures.
In this paper, the general mechanism for the formation of Hot Tears will be discussed and applied to numerous other design elements that can be found on Printed Circuit Board Assemblies (PCBA). We will show that due to several industry trends e.g. VIPPO, heavy copper PCBs, buried vias, non-eutectic alloys, thinner components, thicker boards, via in pad, etc. the probability of Hot Tears is steadily increasing.
Conclusions
The understanding of the failure mode Hot Tear has been extended from cohesive tearing during the solidification of liquid material to all possible forms of tearing - cohesive and adhesive โ during the phase transition both from solid to liquid and from liquid to solid. It could be shown that for the Hot Tears a thin liquid film needs to be present. Adhesive Hot Tears occur if the liquid film forms near one of the interfaces by means of directional melting or solidification. The temperature difference โ๐ and the thickness of the solder joint ๐ could be identified as the main influencing factors on the directional phase transition. The tensile stress necessary for the formation of Hot Tears can be originated either by the Component, the solder material itself or by the PCB. With respect to PCB induced stress, it could be shown that local variations of the copper content have a strong impact on the thermal expansion perpendicular to the PCB surface. With this qualitative model, it is possible to completely understand the formation of Hot Tears of BGA balls on mixed VIPPO/non-VIPPO designs during the second Reflow.
Initially Published in the SMTA Proceedings
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