Authored By:
M. Barnes, D.W. Lee, D. Heller
Heller Industries
T. Cucu ,M. Holtzer
MI. Alpha Assembly Solutions
J. Fudala, J. Renda
MacDermid Enthone Electronic Solutions
Summary
Pockets of gas, or voids, trapped in the solder interface between discrete power management devices and circuit assemblies are, unfortunately, excellent insulators, or barriers to thermal conductivity. This resistance to heat flow reduces the electrical efficiency of these devices, reducing battery life and expected functional life time of electronic assemblies. There is also a corresponding increase in current density (as the area for current conduction is reduced) that generates additional heat, further leading to performance degradation.
This paper will describe the results of a series of experiments performed in an in-line convection reflow oven, using a typical lead free reflow profile, with three types of bottom terminated components commonly used in power management applications. A solder paste flux and alloy with a known high level of voiding was used as the control. This solder alloy is of unique interest, despite its voiding in ambient reflow conditions, as it has shown superior resistance to failure under automotive thermal cycling conditions (-40C to +125C) and vibration.
The experimental design was comprised of two levels of vacuum (5 and 20 torr) applied at two levels of time (30 seconds and 60 seconds) while the test assemblies were at or above the liquidus temperature of the lead free solder alloy. Each 2 x 2 factorial was performed on identical printed circuit boards with four (4) different substrate surface finishes, including Immersion silver, Immersion tin, ENIG (Electroless nickel, Immersion gold) and an Organic Solder Preservative (OSP) finish used. Each condition was repeated three times and three controls with no vacuum were also processed for each surface finish. Therefore, a total of 60 component/substrate samples were processed and subsequently examined for voiding using X-ray analysis.
The results of this study indicate that the vacuum pressure, time under vacuum and the surface finish have little effect on the results when vacuum reflow is utilized. The use of a low pressure vacuum when the solder alloy is in liquidus conclusively results in a significant reduction of observable voids in each combination of surface finish and reflow process condition.
Conclusions
The use of a vacuum chamber in an in-line convection reflow oven has the potential to reduce voiding significantly in large area pads designed to draw heat away from power semiconductors. This allows for more effective heat dissipation, less heat generation, longer battery life and more reliable assemblies.
Using 20 torr of vacuum and 30 seconds of dwell time was sufficient to cause this significant reduction in voiding. Sixty second dwell in the vacuum showed no additional (significant) decrease in the level of voiding observed. Likewise, reducing the atmospheric pressure from 20 torr to 5 torr did not show a measureable improvement in void reduction. Finally, the four surface finishes used in this study all showed the same level of void reduction using the vacuum process while the solder alloy was in the molten phase.
Initially Published in the IPC Proceedings
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