Authored By:
Ed Briggs and Ron Lasky
Indium Corporation
Summary
The explosive growth of personal electronic devices, such as mobile phones and personal music devices, has driven the need for smaller and smaller active and passive electrical components. Not too long ago, 0401 (40 x 10 mils) passives were seen as the ultimate in miniaturization, yet the introduction of 0201's and most recently 01005 passives has occurred.
For active components , area array packages with 0.4mm lead spacing have become virtually a requirement for enabling the many features in modern portable electronic devices, with 0.3mm packages already on the way. This miniaturization trend, occurring at the same time as the conversion to RoHS compliant lead-free assembly, has put a considerable strain on the electronic assembly industry.
This paper will discuss the specific challenge of the Graping Effect and the work that has been performed to mitigate this phenomenon. Discussed are the effects of the solder paste material attributes, consistent stencil printing of the small solder paste deposits required, and minimizing oxidation of the small solder paste deposit during reflow. All of these steps are necessary to assure a good finished solder joint.
Conclusions
To reduce the graping effect, it is vital to ensure an optimal printing and reflow process. Using the guidelines provided for the area ratio and good process/equipment set-up will ensure good transfer efficiency. From a reflow standpoint, decreasing the total heat input will decrease the likelihood of the effect. Using a RTP type profile with a ramp rate of ~1C/second is suggested.
Material factors also influence the outcome, with an increase in the observance of graping as the solder paste particle size decreases and the area of surface oxides increase. Water-soluble solder paste chemistries do not provide the oxidation barrier that resins do for no-clean chemistries and are more prone to the graping effect.
Though the area ratio for circular and square aperture designs may be equal, the potential for the graping phenomenon increases with circular aperture designs due to decreased paste volume and decreased transfer efficiency. Though not performed in this experiment, but observed with customer evaluations, the use of nitrogen does diminish or
eliminate this effect
Initially Published in the SMTA Proceedings
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