SMT Magazine

SMT-Nov2017

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36 SMT Magazine • November 2017 by Jonas Sjoberg, Chris Nash, David Sbiroli, and Wisdom Qu INDIUM CORPORATION There are many ways to increase packaging density. Examples of technologies include fine- pitch connectors, package-on-package (PoP), fine pitch CSPs, 01005, 0201, and reduced com- ponent-to-component spacing. Use of new technologies poses a number of challenges for solder paste selection, PCB de- sign, assembly process, and reliability. The type of end product will have different challenges, concerns, and requirements in all aspects. The assembly line for many of these end products will look very similar, but specification limits would be different. Typical reliability tests are drop test, vibration, thermal cycling test (TCT), and SIR; even if the names of the tests are the same, the pass/fail criteria vary between differ- ent end products. When it comes to material selection—and solder paste in particular—the type of end prod- uct plays a big role. An automotive, computing server, or router product would, in many cas- es, require an in-circuit test (ICT); thereby, the solder paste residues must be easy to penetrate with a test probe. A consumer product, on the other hand, typically doesn't focus on ICT. The concern is more on throughput, thereby requir- ing a solder paste that is capable of fast screen printing. There are many more examples, but with the increased need for high-density assem- bly, most, if not all, solder pastes must be able to print well on stencil apertures with area ra- tios (AR) way below the typical industry stan- dard of 0.66. The AR for some products could be as low as 0.5-.55. With the above challenges, finer solder pow- der is increasingly being used, and the type of solder powder is moving from type 3 towards type 4-4.5, and even type 5 for a number of ap- plications. This poses some new challenges. In addition, low-silver and low-temperature alloys are gaining traction, making the selection of solder pastes and alloys more complex. Once the correct solder paste material is se- lected, a feasible and robust assembly process must be developed and sustained. The assem- bly process ranges from screen-printing, place- ment, and reflow soldering in air or nitrogen to electrical and functional testing. Many factors FEATURE

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