Miniature parts don’t necessarily call for Type 5 pastes.
Seems everyone is talking about the next big (or small!) thing in components: the 03015. It’s no wonder. The dimensions of this little gem are 300 by 150µm, which makes printing and placement immensely challenging, to say the least. The good news is, done right, 03015s don’t require a total upheaval of the entire process and, particularly for printing, use of exotic material sets.
Before we discuss how to print these nearly microscopic devices, let’s review area ratio rules and the current thinking on effective printing. The IPC specification for print area ratio is 0.66, which accounts for most standard SMT processes and what the industry association refers to as the revenue center of gravity (RCG) manufacturers. These are basically the products that make up about 95% of the market: laptops, televisions, power supplies and white goods, among others. Most assemblies within this category can still play by the 0.66 area ratio rule without issue. The state-of-the-art (SOA) accounts for the other 5% of the market. Within this category are the advanced smartphone and tablet manufacturers that push the boundaries of traditional area ratio rules, printing finer dimension apertures through thinner stencil foils. In the 5% SOA, we consistently see area ratios of 0.55 printed effectively and, in some cases, pushed as far as 0.45 with the use of novel squeegee technologies and stencil coatings.
Now back to the 03015 and some industry misperceptions. While at Productronica last November, I dropped in on a few presentations on 03015s, as it was certainly the hot topic of the show. It was interesting to note that many of the presentations – primarily by placement companies – applied the use of the traditional 0.66 area ratio in suggesting the printing material set. This isn’t unexpected: Placement specialists aren’t required to know the intimate details of printing, so applying standard SMT rules to an SOA device would seem logical. However, using the 0.66 area ratio rule for 03015 components results in some very challenging materials sets that really aren’t practical for high-volume, cost-conscious manufacturing operations.
Using the standard 0.66 area ratio rule to facilitate 03015 printing had some placement companies suggesting that the material set should include 60µm thick foils and Type 5 solder paste. It’s easy to see how the foil thickness was determined; 60µm foils at 150µm apertures produce area ratios around the 0.6 range. However, this is following the standard rule book which, in my opinion, is out of date and more suited toward the previously mentioned 95% RCG manufacturers. Practically speaking, there are a few issues with a 60µm thick foil, not the least of which is cost. The industry has standardized on foil thicknesses in one thou integers: Foils come in thicknesses of 0.006" (150µm), 0.005" (125µm), 0.004" (100µm), 0.003" (75µm), 0.002" (50µm) – you get the idea. Because these are the industry norms, these are the typical foil thicknesses (read: available in high volume at reasonable cost). That’s not to say 60µm foils can’t be sourced; they can, but at a much bigger price tag. The aim is to deliver a robust 03015 process using standard material sets, using a standard thickness foil and industry-proven solder paste materials.
That leads to the next challenge: Type 5 solder pastes. As with the odd stencil thickness, it’s not that Type 5 pastes aren’t available; it’s just that using these materials in high volume is challenging and expensive. Again, when the standard rule book is applied, it is clear as to why a Type 5 paste was suggested. The average particle size in Type 4 paste is about 30µm. Using the common recommendation of ensuring four to five particles on the width of the smallest aperture, the ideal particle size for a 150µm aperture (the size for 03015s) would be between 25 and 30µm. The Type 5 paste advice is based on simple mathematics using the RCG rule book. Not only does it cost significantly more to make Type 5 paste on a volume scale, but the issues of storage, mixing, printing and reflow are tremendous. With smaller particles, print resolution is excellent. However, the issues of shorter stencil life, shipping, storage and reflow (more surface area leads to more solder balling) outweigh the print definition advantage. Type 5 simply isn’t practical from a cost or high-volume manufacturing point-of-view. There are some excellent Type 4.5 solder pastes (reference my column from October 2011 on “Type Half” solder materials) that are highly-capable and would perform very well for 03015s.
Given all this, how are you supposed to print an 03015? The industry standard rules generally applied for the RCG masses can in fact be broken. Even SOA technologies can be effectively implemented using standard material sets, which is far more efficient from a manufacturing and cost perspective. Using activated squeegee technologies, nanostencil coatings, precision-cut stainless steel stencils and aligned platforms, printing 03015s using prevailing foil thicknesses and less exotic Type 4.5 solder pastes is possible. In fact, we recommend staying with 100µm foils so that robust heterogeneous printing – larger components next to the likes of the 03015 – can be carried out effectively and to ensure sufficient solder on the board for good joints and long-term reliability. Though 03015s aren’t available yet, and the dummy versions aren’t the most representative, I have full confidence these dimensions can be printed with standard material sets. Our team has already done it with metric 0201s, which actually have smaller dimensions than the 03015. The objective is to manage miniaturization without breaking the bank. The good news is it’s possible!
firstname.lastname@example.org. His column appears bimonthly.is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division, DEK (asmpt.com);