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The European Angle
By Pete Starkey
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Meeting Soldering Challenges of Miniaturization
“Over 50% of the world’s tin production is used in solder," observed Dominic Lodge, solders technology manager at the International Tin Research Institute (ITRI), setting the scene for a Soldertec webinar discussing the soldering challenges presented by the ongoing miniaturisation of electronic assemblies. “But it’s remarkable just how little solder there actually is in modern electronics. For example, there’s less than one gram in an iPad.”
Although minimum solder joint sizes have stabilised in the 0.3 to 0.4 mm range and soldering technology is currently under control, it is projected that joint sizes could reduce to the 0.1 to 0.2 mm range by 2020. What would be the consequences on manufacturability and reliability?
One of the biggest challenges is placing the right amount of solder paste in the areas--solder paste printing has become an increasingly demanding operation and is the source of the majority of assembly defects. Stencil geometries and mechanical parameters are critical in achieving a good print deposit, maximising paste transfer, and minimising deviation, but the solder paste itself is perhaps the most significant factor. Finer powders are required for fine-pitch printing: “Type 5” powder has a particle size range of 10 to 25 microns, “Type 6” powder is 5 to 15 microns and makes it possible to manufacture even finer grades although techniques employed are closely-guarded secrets. The finer the particle size, the higher the surface-to-volume ratio and the more work the flux has to do to protect against oxidation, as well as assist in wetting and coalescence. Head-in-pillow defect is acknowledged as a major problem currently facing the electronics assembly industry and, although component warpage is a contributor, flux plays a crucial role especially in lead-free assembly. Moreover, the smaller the paste deposit, the higher the ratio of surface area to flux volume.
As the dimensions of solder joints decreased, additional metallurgical issues arose: Intermetallic volume ratio increased and the microstructure of the as-soldered alloy was altered. Electromigration becomes significant, and creep properties are affected. In high-reliability applications, it is typical to specify underfill to add mechanical reinforcement to the relatively fragile solder joints.
Rework is a substantial area of concern in the assembly of area array packages. Visual inspection is of little benefit; techniques such as X-ray and acoustic microscopy are necessary. Furthermore, it is not practical to repair individual joints--the entire component package has to be removed and replaced.
Miniaturisation in electronic assembly is the focus of many research programs. Soldertec is closely monitoring the outcomes of projects such as PS-FP7 Factories of the Future, microBGA, and MCUBE.
This Soldertec webinar was well-attended and Lodge’s presentation gave a clear introduction to the challenges that miniaturisation presents to soldering technology. A copy of the webinar will be made available on the ITRI/Soldertec website at www.soldertec.com.
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