Authored By:
Derrick Herron, Christopher Nash, Raymond Luo, Andy Wei
Indium Corporation
Clinton, New York, USA
Summary
One of the biggest challenges facing the electronics industry today is voiding in the solder joints that connect bottom terminated components to PCBs. This solder voiding, especially on thermal pads, can lead to hot spots and a reduction in the life of the component and the overall assembly. Limited improvement is gained from making modifications to the profile and solder paste alone. Without careful design considerations, it is also difficult to obtain precision in the level of voiding from one component to another. In the past, thermal pad design and the use of thin, custom-sized solder preforms placed into solder paste at the thermal pad have been studied. This paper looks at a new approach using readily available solder fortification preforms. These preforms are automatically placed into the paste of the thermal pad before the QFN component is placed, forcing the component to sit at an angle to allow for outgassing of flux during reflow before the solder fortification preform melts, then allowing the component to come to rest in its final desired orientation.
It has been determined that this approach can both decrease the level of voiding found in the solder joints beneath bottom terminated components and also decrease the amount of variation in this value. While this paper focuses on this technique when used with SAC305, it is versatile enough to be used with other alloys commonly available in paste and solder fortification preform forms. This method also has the added benefit of being fully compatible with current manufacturing processes, adding only extra placement steps without the need to change reflow profile, equipment, etc. This easy and inexpensive modification to current processes can have a big impact on the reliability of the final product.
Conclusions
It has been determined that the use of solder fortification preforms along with solder paste in the mounting of bottom terminated components to PCBs can aid in the reduction of solder voiding. The solder fortification preforms act to hold up one side of the bottom terminated component to allow for outgassing of the solder paste before the alloy melts and allows the component to come to rest on the paste surface.
Variability seen in the level of voiding from component to component is diminished with the use of solder fortification preforms as described in this work. Due to the observance of skewing when only one preform was placed off-center, it is recommended that two components placed along one edge of the pad be used. The use of 0402 preforms was found to be more effective at reducing voiding than when 0201 preforms were used.
This method of reducing voiding under bottom terminated components is desirable because solder fortification preforms are readily available for use in automated processes. Since placement equipment is most likely already being used, all that is needed is a reel of preforms and some additional placement steps in the current process. This easy addition to the process has been shown to provide very beneficial results.
Initially Published in the SMTA Proceedings
|
