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Solder Alloy Powder Misconceptions

For a change of pace, again, I have asked another Technical Support Engineer, Chris Nash, to comment about powder sizes.  Chris is the Regional Technical Support Engineer for the Midwest region, and works from Indium Corporation HQ in Clinton, NY.

Small components such as 0201's and Micro-BGA's are being implemented into circuit board design and manufacturing more often. Many people still haven't had the chance to use these in their process but have heard that this will soon be upon them. In planning for this many immediately think they will need to use a solder paste with a smaller powder size and consequently start considering type 5 and type 6 pastes. This is usually not necessary.            
Type 5 and type 6 powders/pastes are currently being used in applications such as wafer bumping, substrate bumping, package on package, and dispensing with very small needle diameters. Wafer Bumping and substrate bumping (for flip chip assembly where there is not enough solder present on the chip) applications are using type 5 or 6 pastes with a printing process that may be a bit different than the typical SMT process. Many of these applications are using a mask instead of a stencil. Once the solder has been reflowed the mask is stripped and solder bumps remain. The apertures that are used in this type of process are typically around 50 microns, much smaller than the typical SMT aperture size. Package on package applications are using type 5 and 6 pastes in a totally different way. The packages are being dipped (not printed) into the paste before placement. Dispensing solder paste out of a syringe will sometimes require a small power size like type 5 or 6 due to the needle size that some applications require (26-30 gauge needles). As you can see there currently is a need for type 5 or 6 solder paste but this need has not spread to typical SMT assembly quite yet.
The selection of the appropriate powder size for a specific solder paste application is a fundamental step that will ultimately affect the print-ability of the solder paste with respect to the stencil design. Stencil design, focusing specifically on area ratio, plays an even more crucial role in solder paste print-ability. Area ratio is essential to the printing process and powder choice. Calculating the area ratio and choosing the correct powder size can help ensure proper stencil release. The area ratio is the ratio between the area of the aperture opening and the area of the aperture walls [area of the opening/area of the walls ≥ 0.66]. Once the proper aperture size has been determined, the appropriate powder size can then be chosen. For all apertures, it is important to maintain a minimum of 4 or 5 solder particles (the large particle size of the range) across the aperture.
Current manufactures (typically hand held device manufacturers) that are using 0201's and small Micro-BGA's have developed their process around the area ratio rule of thumb.  Many of the manufacturers have decided that the best way to achieve close to 0.66 is to decrease the stencil thickness to 0.004".  Some manufactures have seen improved transfer efficiency results with a type 4 solder paste with less than 0.66 area ratios and have used this smaller powder size successfully.  Typically speaking, an area ratio greater than or equal to 0.66 will allow for the use of a type 3 solder paste. 
Thank you to Chris for helping out on this blog.  More inforamation may be found at the Indium Knowledge Base (IKB).