Should finer solder particle sizes result in better process results? Our work suggests no.

In December we discussed a solder paste material evaluation for printing metric 0201 assembly. In summary, three solder pastes – two type 5 materials and one type 6 material – were analyzed by stencil printing them onto a PCB with an array of different patterns and two different pad designs of 100µm x 115µm and 125µm x 115µm, respectively, with three different component pitches of 100µm, 75µm and 50µm. In the end, the supplier A type 5 (T5) solder paste had the least variability, even with the 50µm interspace. The supplier B type 6 (T6) paste deposits appeared almost over-printed, with large deposit volumes and some wet bridging.

For component placement and reflow analysis, which were carried out on a Siplace TX placement machine and a Rehm nitrogen-capable reflow oven, the best-performing T5 paste from supplier A and the supplier B T6 paste were used. In addition to the original PCB design (PCB 1) containing discrete pad designs without traces, a second test PCB (PCB 2) integrated the same pad dimensions but with the addition of a conjoined trace between pads. Our observations were as follows:

Placement and reflow performance on PCB 1 (no traces). The transfer efficiency of supplier A T5 solder paste, as explained in the previous column, resulted in the volume of solder paste compatible with the metric 0201 process, though there were some slight variations between deposits, especially with the 50µm interspaces. During placement, there was 100% yield with no skewing, no missing components and precise seating of the components into the paste, so there was no material displacement for any of the 10,000 components placed on each board. Reflow in a nitrogen environment resulted in good wetting with no slumping or bridging. The story was the same for 75µm and 100µm pitch pads: placement and reflow were textbook.

The supplier B T6 solder material, as noted, produced very full deposits and some pad-to-pad variability during the printing phase. The expectation was that due to the larger deposits, significant wet bridging might be observed after placement, as the force – however light – may further displace some of the material. Surprisingly, the material came very close to post-placement bridging on the 50µm interspaced pads, but once the boards were reflowed, the material pulled back in, and the solder joints exhibited no post-reflow bridging. The 75µm and 100µm pitch pads did not reveal any challenges with placement or reflow.

Placement and reflow performance on PCB 2 (with conjoined traces). Moving to PCB 2, an end trace now connects the pads, which provides an avenue for material to flow with little resistance. This design further tests the process capability and is more representative of a real-world scenario. If the process is 50µm capable, which is clearly the most challenging, it logically follows it would then also be 75µm and 100µm capable. Starting with the Supplier A T5 solder paste and the 50µm interspace, the print deposits were excellent and reasonably consistent, with all passing visual inspection. Likewise, placement was again perfect – no displaced material or movement of solder paste to the traces, zero skewing and 100% placement yield. Because the traces did not have solder mask over them, they were tinned, which meant there were some complex forces to contend with during reflow. Even with these exceptionally small dimensions, the process resulted in 100% yield. Components barely moved – even on the locations with three deposits connected by a trace – and reflowed exactly where placed. As expected, it was the same story for the 75µm and 100µm interspaces.

With this more challenging board design, the process proved too much for the Supplier B T6 material on both the 50µm and 75µm pad pitches. Nice full deposits were on the pads, and at first glance one might assume this was good news. Once placement began, however, the volume of paste was too much, and some wet bridging was observed. Understanding the reflow process often corrects for this, our team wasn’t overly concerned. Unfortunately, after reflow, there was significant solder bridging. With the 75µm interspace, the deposit was good, and there was no bridging on placement. But, during reflow, the volume of solder paste still created bridging on conjoining tracks. This, while perhaps functional electrically, would fail IPC standards. There was no bridging on the 100µm gap design, but the solder deposits did appear oversized, though it assembled fine.

While current industry thinking and general rules imply finer solder particle sizes will result in better process results, our work suggests otherwise. In this case, the supplier A T5 material yielded better performance. That’s not to say the T5 material was without issues; it wasn’t. There was some variation between deposits, though this did not cause any bridging in printing, placement or reflow on the PCB 2 design. Once again, conventional wisdom is challenged!

Clive Ashmore is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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