Miniaturization’s impact on printing material selection and management.

Toto, I have a feeling we’re not in Kansas anymore.

This legendary line, spoken by main character Dorothy in the iconic film The Wizard of Oz, sums up where the electronics assembly market is today. With metric 0201s and 030105s increasingly making their way onto production floors and 0.2mm CSPs coming around the corner, manufacturers are in unfamiliar territory. This extraordinarily expedient miniaturization transformation has implications all the way down the line, where SPI has to be precise, intelligent placement capability spot on and reflow ovens optimized. And, of course, the entire process begins at the printer, which has famously been identified as the culprit for more than 60% of defects. (Certainly that's not the case for those who read my column regularly and apply the learnings … but I digress.)

The integration of exceptionally small devices into high-volume production is happening; our company is facilitating it daily at customers around the world. From a printing perspective and outside of the sheer machine capability and stencil design, one needs to be acutely aware of the importance of printing materials selection and management when it comes to these small dimensions. Now more than ever, tight control of solder paste and understencil cleaning materials is critical.

Type 5 solder paste. The general move from Type 3 to Type 4 solder paste happened over a somewhat manageable time period (about five years). The shift from Type 4 to Type 5 materials has not been so gradual or forgiving.

Just a little over two years ago at Productronica, it seemed the entire show floor was talking about 030105s; now we’re assembling them. The nearly microscopic dimensions of some components dictate the use of Type 5 solder pastes for a robust process. In fact, when applying the guidance I’ve so often discussed in this space – the five ball rule and area ratio models (reference my past columns on these topics) – Type 5 materials are all that make sense for certain assemblies.
However, managing Type 5 materials – at least today – takes a bit more fortitude than Type 4 or even Type 4.5 pastes. Based on personal experience in a lab environment, my observation is currently available Type 5 pastes must be managed with a far more disciplined approach than their predecessors required. Of course, materials vary by supplier, but in our work with four well-known Type 5 solder paste brands, we did notice the materials, compared with Type 4, were more sensitive.

Whereas Type 4 materials can, in general, be left out for a couple of days in a relatively low relative humidity (RH) environment (our lab is at about 40% RH) and still print very well, this was not our experience with Type 5 materials. After being removed from refrigeration and left out for two days, the Type 5 materials became quite viscous, didn’t print as well, and crystallization of the flux was observed. When fresh out of the jar, the materials print just fine. My advice is that, if you plan to use Type 5 materials, make sure a rigorous refrigeration, tracking, stock rotation and conditioning process is implemented so unused material is stored properly and consumed within manufacturer-specified time limits. Printer tools that permit paste roll monitoring and on-demand, automated delivery of paste to the stencil would also avoid the material-compromising temptation to put the whole jar of paste on the stencil in an effort to save time and limit unplanned production interruptions. These small components are challenging enough; the paste and printing performance must be optimal and process variation reduced.

Understencil fabrics and cleaning chemistries. Understencil cleaning fabric and chemistry selection also need to be considered in the age of miniaturization. Higher grade fabrics that are lint-free and very absorbent have been discussed and available for some time. While it certainly hasn’t ever been desirable to have lint, a stray fiber in a print deposit of today that measures 145 x 120µm can be the cause for a sub-par print process and a contributor to that 60% defect rate. Higher grade fabrics are produced to be lint-free and absorbent, enabling thorough and fast cleaning of very small stencil apertures within an HVM environment.

The same importance should be given to selecting the understencil cleaning solvent. Especially for miniaturized, high-throughput assembly requirements, the solvent should be quick acting and drying. For small features, a fairly aggressive cleaning agent is recommended to enable breakdown of the flux. High-volume manufacturing dictates a fast-drying formula as well, so as not to be a drag on production rates and to remove the possibility of any residual solvent remaining in the aperture and affecting the stability of subsequent prints.

Indeed, proper material selection and management are simply a must for advanced print processes. Just as Dorothy found out on her journey, navigating the new normal for electronics assembly specialists will take courage, brains and heart – not to mention some serious process expertise. No amount of heel-clicking and wishing will take us back to large, forgiving apertures and Type 2 solder paste. But, as this industry has done time and time again, I have no doubt we will be successful. Welcome to Oz!

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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