As consumers the expectation of increased functionality within new products is a given. However there comes a time where this tireless demand for product efficiency starts to stretch the design for manufacture (DFM) rules. Fabricating products with decreasing feature size and increasing complexity is not the issue nor is producing products that have larger components; the dilemma is when products require both.
This predicament is now upon the Surface Mount Assembly (SMA) community, the imminent role out of 0.3mm CSP looks to be pushing the feature size below 200 micron but still RF shields and connectors are required - or put another way heterogeneous assembly is looming upon us.
The main issue surrounding the stencil printing process when dealing with heterogeneous assembly is area ratio (the ratio between stencil aperture open area and aperture wall area). When complying to traditional design rules and maintaining area ratios greater than 0.66 then it becomes near impossible to design a print process for a wide mix of fine and large pitch components.
Whilst developments in solder pastes and stencil manufacturing techniques in recent years have allowed skilled operators to push the area ratio rule of thumb to 0.5-0.6 to accommodate 0.4CSP assembly, the next generation of component technology (0.3CSP's) is one step beyond this capability.
To address this, new techniques have been investigated with the aim of increasing solder paste transfer efficiency in the screen printing process. Of several techniques investigated one has stood out and has been the subject of intense laboratory trials. The results of this investigation have shown that existing area ratio rules can be seriously challenged and broken to permit 0.3mm pitch CSP assembly within a traditional SMT process. Details of these new developments together with substantive paste transfer efficiency data will be presented.
At the beginning of this paper we discussed the ongoing battle between miniaturization and the constraints that are present within today's manufacturing processes. We know that there are ways around these constraints but they are a half way houses at best, in fact they can lead to increased costs, decreased throughput and lower yields ; but because the miniaturization program is not waiting nor slowing these deficient solutions are been seriously considered.
The activation of solder paste through an energized squeegee assembly has been tested with 3 materials within this paper and although the test and material choice is not totally inclusive, the work outlined in this paper has shown that the squeegee system has been able to locally modify the print medium such that it is now possible to print extremely challenging Area Ratios using standard stencils and solder paste.
This technology has the capability to deliver to the SMA community a remedy to the miniaturization headache.
Yes we've proven these techniques with our solder transfer stencil technology and been using it in our products for years. They are right it works so well you can see visually if there is a problem with the solder connection.
