Clive Ashmore

Could 0.3mm CSPs spur mass implementation of Type 5 powders?

It doesn’t seem long ago I wrote a piece on Type 4.5 solder pastes and their printing advantages for miniaturized geometries. A quick search of my archives, however, revealed  it’s been nearly four years since that column appeared!

In that time, much has changed in terms of manufacturers’ movement toward the integration of devices like 01005s and 0.3mm CSPs, as well as the number of evaluations of metric 0201s and metric 03015s taking place. More and more, our printing applications experts are being asked about 150µm deposit sizes and the process implications of such small material volumes. This, indeed, pushes the process limits and leads me back to the basics of what I refer to as the “five particle rule.”

In short, this rule I was taught when I began in SMT states that if the particle size of the solder paste is not such that five particles will fit shoulder to shoulder on the narrowest aperture, it’s likely an indication of potential printing problems. And, these issues are regardless of speeds, squeegee pressures, stencil thickness, board quality and even the printer; they are solely dictated by particle count per aperture. While I’ve never come across any paper that explains the physics of this rule, in my experience it has proven to be absolutely accurate.

To calculate what type of paste to use, take the maximum particle size allowable in the 80% particle range per certain industry standards (e.g., J-STD-005A) and multiply it by five. In the case of Type 4 paste, for example, the particle range is 20 to 38µm, so any deposit size between 190µm and 200µm works with the rule (38 x 5 = 190), though it’s not advisable to go below 200µm deposits with a Type 4 paste. The Type 4.5 discussion centered on paste deposits that were around 180 to 200µm – sizes well within the five particle rule limit, as the upper limit particle range seems to be around 30µm (varies between suppliers).

So, winding that back to the 03015s, metric 0201s and 0.3mm CSPs referenced earlier and the 150µm deposits required, it becomes clear Type 4 paste is likely to be problematic, and even Type 4.5 is pushing the edge of the five particle rule. As I’ve written previously, Type 4 and Type 4.5 pastes can certainly be used effectively for highly miniaturized devices, but we are starting to reach the boundaries if there is to be any type of latitude in the process. Type 4 and Type 4.5 materials have been proven to be robust for 01005 and 0.3mm CSP printing when also using tools such as activated squeegees and stencil coatings, along with having a very tight and well-controlled process. However, as these ultra-small components begin to make their way into high-volume manufacturing across different regions and in varying manufacturing conditions, process specialists are finding they are more comfortable with a bit of breathing room in the printing operation.

Type 5 paste delivers this flexibility. With 80% of the particle sizes within 10 to 25µm, the five particle rule says a 125µm deposit is printable with Type 5 paste, yielding 25µm of wiggle room for those 150µm deposits. The five particle rule in combination with area ratio rules and proper printing technology will deliver the elements required to print well, even under very challenging circumstances.

As more high-volume manufacturers on the leading edge of electronics assembly (i.e., OEMs of handheld and wearable devices) begin to incorporate these miniaturized devices and do so onto boards that are thinner, more complex and, in many cases, flexible, moving to a solder paste that lets them open the process window a little bit might just be the difference-maker for yields. Type 5 pastes and five particles per aperture – perhaps five is the magic number for high-volume manufacturing of emerging 150µm deposits.

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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