| Stencil Horror Stories |
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| Written by Clive Ashmore | |||
| Monday, 30 June 2008 19:00 | |||
Why a 100 mm board shouldn’t be processed with a 300 mm squeegee.
Saving even a few dollars upfront, it seems, often takes precedence over making responsible decisions based on the overall impact on process cost. As the saying goes, many manufacturers “can’t see the forest for the trees.” On our visits to facilities around the world, we constantly are amazed by the choices made with regard to screen-printing process inputs. Frequently, our team is called in to conduct a process audit to analyze the cause of low yield levels; nine times out of 10, the root cause is a simple input error. And, I’m not talking about anything complicated here – just basics such as stencil design and manufacture, squeegee length and solder paste choice. I’ve spoken about proper stencil design and manufacture many times in this column, but the problem continues to rear its ugly head, so I will keep addressing it. In terms of stencil design, one common mistake is assuming that if the stencil follows the IPC spec for stencil design, everything should work. In theory, that’s all well and good, provided the board follows the spec as well. Unfortunately, that’s usually not the case. If apertures don’t match the pad geometries, the results will be disastrous. The only piece of the IPC spec that really can be applied, and is a common link, is the area ratio rule; the aperture design has to be at a ratio of 0.66 or above for a standard laser cut stencil. If you don’t break that rule, and assuming both the artwork of the board and stencil align, then at least your transfer efficiency will be OK. In addition to proper stencil design, stencil manufacture must be tightly controlled. This means ensuring high quality raw materials and manufacturing methods are employed. But, what our company is finding is that many firms opt to go the cheap route – saving maybe $100 per stencil on lesser-quality products – and what they are getting is awful. On the surface, the stencil may look OK, but when aperture walls are analyzed, it is apparent the cutting methods are rudimentary. The walls often have big lips, globules of metal or extremely rough edges, any of which alter the aperture geometry and, thus, the surface tension of the walls, which in turn results in poor paste release and non-uniform deposits. Squeegee selection – in particular, squeegee length – is another area where we find attempts to save a buck upfront, resulting in dire consequences down the line. Proper print rules dictate squeegee length should be equal to the board width, or a maximum 25 mm wider than the board on either side. Anything more causes problems. Believe it or not, in an effort to save, say, $250 for a set of varying length squeegees, certain companies are buying 300 mm squeegees as standard and using those for every product. So, a 100 mm board will be processed with a 300 mm squeegee. If the squeegee length is longer than the board width, the print pressure has to be increased to compensate. This then leads to a sharpened attack angle because the blade has to be pushed so hard that the 60° angle has suddenly become 30°, and solder paste escapes around the back of the squeegee, creating a wake effect (see “Playing the Angles,” May 2008). In the case of a QFP, for instance, this increased pressure can cause different volumetric depositions between the north/south apertures and the east/west apertures. The north/south apertures tend to overfill and the east/west generally fill normally. Too much material can cause bridging, and the deposit inconsistencies can affect device coplanarity. Additionally, the extra pressure required to accommodate for the extra squeegee length may also cause damage to the stencil. If a 100 mm board is being produced and a 300 mm squeegee is used, the stencil edges get pushed down because the support is only under the board. The pressure is wound so tight that after a couple shifts, the stencil bows from the force on the unsupported area. As a rule, pressure should be set at 1 kg per every 50 mm, which usually amounts to between 5 and 8 kg of pressure for a standard card size. What we’re seeing is manufacturers running 13 kg, which is outrageous and causes irrevocable damage. At the end of the day, then, in trying to save on squeegee costs by sourcing one size, you end up spending even more money on stencils. Not to mention the (likely) lower yields and rework costs. Last, make certain the correct solder paste grade is being used. Frequently, a manufacturer will change a technology, move to a smaller assembly, and fail to change the solder paste type. Although type 3 paste is most common, it is not the ideal particle size for highly miniaturized products. As a general rule of thumb, five particles of paste should fit end-to-end inside the smallest aperture of the stencil. If you can’t do that with your current paste type, it’s probably a good idea to change, lest you have paste sticking in the apertures. Combine this with a poorly manufactured stencil and improper squeegee length ...and, well, you get the picture. You reap what you sow. If you scrimp on quality inputs – mind you, we’re not talking about a lot of extra expense for quality stencils and proper squeegee sizes – you will get substandard outputs. Ultimately, you may pay in lost business. Invest wisely! Clive Ashmore is global applied process engineering manager at DEK (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears bimonthly.
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