Circuits Assembly - November 2007 - (Page 38)

Reflow Soldering Creating Ideal Solder Joints Zulki Khan Effects of stencils and thermal profile are reviewed. tencil quality and the major elements comprising it play a pivotal role in ensuring solder paste is properly deposited on a PCB’s surface mount pads. Experience in defining and cutting perfect stencils is critical because there are virtually no textbook solutions on how best to make stencils. When properly combined, stencil quality and other associated factors comprise a critical step before a thermal profile is applied to reflow soldering. Creating the ideal thermal profile is equally important. In many instances, the common practice is to develop three basic thermal profile recipes: one for small PCBs, another for medium-sized ones, and a third for large boards. On the other hand, the perfect thermal profile meets a particular PCB’s requirements, and needs to be created separately, one part number at a time. Suffice it to say, a poorly devised thermal profile can have devastating (read: expensive) repercussions. Reflow soldering is a superset of a 100% accurate thermal profile. It is the most common means to solder a surface-mounted component to a PCB. The steps involve applying solder paste, placing components on the PCB, and reflowing solder in a con- S veyor oven. Reflow melts powder particles in the paste, joining the surfaces through the solidified solder to create a strong metallurgical bond. Stencil quality. A stencil dispenses solder paste on surface mount pads. Stencil thickness, aperture sizes and frames (or nonframes) are factors that contribute to paste dispensing and ultimately the solder quality on the PCB. Thickness can vary from 0.003" to 0.010" and determines the amount of paste dispensed on the pads. Larger SMT component packages (1206, 0805, 0603) require an 0.008" thick stencil, which deposits a larger quantity of paste. Smaller package sizes (0403, 0201) require a thinner stencil, about 0.005", and hence a smaller amount of paste on the PCB to accommodate the smaller pads. Even thinner 0.003" to 0.004" stencils are used for 0201 and 01005 packages. In these instances, very small amounts of paste are used, usually a small drop. Using the wrong stencil thickness creates havoc on a PCB’s performance. For example, an 0.008" thick stencil used with 0102 components will result in an inordinate amount of deposited paste. During reflow, capacitors, resistors and inductors are prone to shorts because of the excessive paste. Conversely, using a thinner (0.004") stencil on 1206 or 0805 packages could result in cold solder or voids during reflow because of an insufficient amount of paste applied to the pads. 38 Circuits Assembly NOVEMBER 2007 circuitsassembly.com http://circuitsassembly.com

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Circuits Assembly - November 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Pb-Free Manufacturing from a Tier III EMS Perspective ‘Living Documents, Subject to Change’ Creating Ideal Solder Joints An Alternative Drying Process for MSDs ‘Customer Satisfaction is More than a Score’ Optoelectronic Substrates: Will They Happen? Tech Tips Wave Soldering Process Doctor Test and Inspection Getting Lean Materials World Equipment Advances Product Spotlight Assembly Insider Technical Abstracts Ad Index

Circuits Assembly - November 2007

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