As consumers, the expectation of increased functionality with reduced form factor is a given. However, the tireless demand for product efficiency is starting to stretch design for manufacture (DFM) rules. Fabricating products with decreasing feature size is not a major issue, nor is producing products that have larger components; the dilemma is when products require both. This conundrum is now upon the Surface Mount Technology (SMT) community. With the impending roll out of 0.3mm pitch CSP's pushing feature sizes below 200 microns - on assemblies still requiring large RF shields and connectors, the challenge of 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 with traditional design rules and maintaining area ratios greater than 0.661 then it becomes near impossible to design a print process for a wide mix of fine and large pitch components. To address these concerns a new squeegee printing technique has been developed. Initial laboratory trials 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 without the need for any material change. Details of this new development together with substantive paste transfer efficiency data will be presented.
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