As package I/O’s get smaller and denser, characterizing the effects of different stencil underside wiping strategies grow in importance. An experiment was devised using the SMTA Miniaturization test vehicle to gain insight into the effects of different underwipe chemistries. The tests examined the effects of wiping on different sized packages, different wipe frequencies, and different wipe chemistries.
The experiment used wipe frequencies of 2, 4, 6, 8 and 10 prints per wipe. Data collected by the SPI machine was analyzed in two ways: the overall performance within the wipe frequency group, and the pre-wipe and post-wipe print performance at each wipe interval. Two different chemistries and dry (vacuum) wipe were evaluated, along with an uncoated stencil and one with commercially available adhesion-resistant coating.
The results indicated the following conclusions:
- The process that was studied was very well controlled; far better than many that have used the same test PCB
- At the 0.5 BGA level and below, adhesion-resistant coating is very impactful to achieving high quality print results
- Wet wipe is better than dry; with one chemistry proving better on shorter wipe intervals, and one proving more appropriate for longer wipe intervals.
The findings lead to a hypothesis that the formulation and drying time of the underwipe chemistry is a considerable factor in solvent selection, which can be based on stencil aperture density, print area length in the Y-axis and production throughput rates.
The print quality on these tests was remarkable compared to other production processes. This leads to conclusions that:
- Best practices are deployed throughout the process
- Results from this study are not necessarily predictive of other production processes that are not as tightly controlled
- Results from this study are comparable to those performed in laboratory environment rather than production environments
- With the proper process controls and best practices in place, production environments can perform at the same quality level as laboratories
