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SMT-Dec2017

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64 SMT Magazine • December 2017 What Target? Common sense dictates that narrower is better. But how narrow is enough? A typical industry target for solder joint quality is IPC- 610, with subclasses 1 to 3. Performing a sim- ple DOE, one would find that most IPC criteria can be met across a fairly large distribution of solder volumes. However, mere compliance to IPC may not meet expectations when confront- ed with the subtle causes of defect modes like tombstones, voiding in thermal planes, or open corner balls of BGA. Besides, IPC criteria defin- ing ranges of acceptable solder volume from "wetting is evident" to "solder shall not touch a package body" hardly sounds like a valid predictor of product reliability under harsh conditions. In other words, you are left with the task of identifying the weak spots: your highest pare- to causes in yields, your first fail modes to oc- cur in highly accelerated life testing (HALT) or other test-to-fail methodology. Then, iden- tify best nominal targets and narrow distribu- tion for each, most of the time iteratively un- til you consider having reached stability. This is lots of work, far from the ease of using standard rec- ipes. With experience and methodology, patterns and targets get more predict- able, and the investments start paying off. What Process Inputs? In search of better per- formance criteria (e.g., yield, reliability…) once you sus- pect that distribution of sol- der volume is a significant contributor , what process inputs may impact your dis- tribution? Consider the 6M classification: measurement, machine, material, mother nature, method, manpower . Measurement "Know thy SPI" might say the Greek philosopher Thales, if assigned to the task. Measurement starts with proper measure - ment system analysis (MSA), which includes cali- bration and Gage repeatability and reproducibili- ty (R&R). However, wet solder paste samples may not remain sufficiently stable through the course of the analysis. Options include, for calibration: a 3D reference of a known volume, traceable to in - ternational standards; for Gage R&R: dried (long stabilized) paste, with care to present samples across the entire range of measurements. Additionally, SPI measurements may in- clude various artifacts originating from: incor- rect reference plane (referencing to solder re- sist instead of copper), PCB warping, shadow- ing from adjacent solder depositions, etc. The most recent generations of SPI include specific features to address these. Chances are that these are minor and only impact the few measure- ments that are so critical in your application. The point is to understand the limitations of your measurements, so their variations are not confused with other process inputs, or lead you to wrong conclusions. Make measurement part of a formal control plan (i.e., who, what, when, how is the process SOLDER PRINTING PROCESS INPUTS IMPACTING DISTRIBUTION OF PASTE VOLUME Figure 1: The volume of solder deposition has variations that may be char- acterized by a statistical distribution curve, whether normal or non-normal.

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