Authored By:
Ronald Frosch
Guenther Mayr, Manfred Riedler
AT&S
Shanghai, China
Summary
In light of the recent technological trends within PCB manufacturing industry, there is an increasing degree of interest in understanding the influence factors of mechanical stress on the durability of mobile devices. In the past, many papers focused on PCB reliability and the influence factors during drop shock test. In most cases, the potential influence factors in regards to underfill have not been fully investigated. Additionally, there is no clear direction on the influence of the interaction between solder mask inks and underfill systems.
The intent of this paper is to identify an accurate method to predict drop test behavior by understanding the surface tension of both, the solder mask ink and the underfill material. This could become a significant advantage for improving the reliability of the entire electronic construct. In this paper a method has been examined that can be used to subsequently analyze the reliability of the latest mobile device related materials and design.
The prescribed test has been constructed using a cross comparison of pad design, surface finish, solder mask and underfill, measured by drop testing. Based on the resulting data, a method was evaluated to predict and optimize drop test reliability by understanding the surface tension of solder mask and underfill (adhesion).
We are now able to identify specific advantages and limitations for different material combinations, without the need of expensive and time intensive drop tests. In an effort to achieve a broader understanding of the entire process and product scope, the participants in these trials were an HDI PCB manufacturer (AT&S) and it's material suppliers.
Conclusions
The first part of this paper was a drop test DOE focusing on four factors and their influence to the drop shock reliability of assembled PCBs. The second part has focused on the main influence factor, the interaction between solder mask and underfilling system, including a method to predict the efficiency of such additional production step without the need of time & cost intensive drop tests.
It was proven that the drop shock performance of two different solder mask inks without an underfilling step are quite comparable, independent of pad type or surface finishing. The use of underfill provided a reliability improvement for both solder mask types, but the efficiency strongly depended on the specific solder mask. Consequently, underfill / solder mask interaction (adhesion) has a major impact to the final drop shock reliability.
The different ink and underfill types were measured by contact angle. Based on these results, surface energy, work of dhesion and the interfacial tension of each sample was calculated. The calculated adhesion fits quite well with the drop test results - the better the adhesion, the better the shock resistance.
Finally, it should be mentioned that contact angle measurements strongly depend on factors like contamination, retreatment or environment, therefore, comparison tests should only be carried out by knowing exact experimental setup.
Initially Published in the IPC Proceedings
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