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Process Characterization for Acceptable Levels of Flux



Process Characterization for Acceptable Levels of Flux
Analysis of residues and their effects has shifted from a global examination of ionic residues to a more site-specific examination of spot or local contamination.
Analysis Lab

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Authored By:


Doug Pauls, Collins Aerospace
Elizabeth Barr, Collins Aerospace / Iowa State Univ.
Adrianna Roseman, Collins Aerospace / Univ. of Florida
Mark McMeen, STI Electronics / Magnalytix, LLC
Mike Bixenman, KYZEN Corporation / Magnalytix, LLC

Summary


Surface Insulation Resistance (SIR) testing is a standard method used to characterize soldering and cleaning processes that result in acceptable levels of flux and other residues. Several different materials are used to assemble printed circuit cards. Residues can be present on the assembly from solder flux, solder paste, solder wire, underfill materials, adhesives, staking compounds, temporary masking materials, cleaning solvents, conformal coatings and more. Miniaturization of components increases risk due to tighter pitch, low standoff gaps, and residues trapped under the component termination.

In recent years, analysis of residues and their effects has shifted from a global examination of ionic residues (i.e. the entire assembly) to a more site-specific examination of spot or local contamination. The majority of an assembly surface may have acceptable levels of residues, with problem areas confined to a few components. Therefore, it was the desire to advance the state of the art in SIR testing and design cost-efficient test components and test vehicles that would allow an assembler to examine these problem point-sources of contamination. The goal of this research study was to design and evaluate an economical test board and laminate based components which mimic challenging components, and compare them to an accepted industry standard assembly, the IPC-B-52 standard test assembly.

Conclusions


The four goals for this research were:

1. Do test cards, all other factors being equal, processed with the laminate dummies (LD) provide the same SIR performance as test cards processed with the mechanical dummies?

It may be concluded that the laminate-based dummies provided an acceptable alternative to mechanical dummies for the BGA test patterns, and for the QFN test patterns.

There were significant differences between the laminate based dummies and the mechanical dummies for the QFP80 test component and test pattern. In Round 1 of the testing, the laminate based dummies had a 4 mil standoff, which led to lower SIR values and more variability. In Round 2 of the testing, the increase of the LD standoff from 4 to 8 mils improved the overall SIR levels, but not enough to rise to the SIR levels observed for the MD components.

An additional consideration, to be addressed in subsequent studies, relates to the lead and soldering pad length for the two component configurations. The gull wing configuration of the MD QFP80 parts occupied more solderable surface than the LD QFP80 parts, but the LD QFP80 parts had a larger overall footprint. This may mean that there was a higher amount of flux residues for the MD parts than for the LD parts. How the flux residues behaved during manufacturing may have been a significant contributing factor to the variations observed. Rather than attempting to craft a LD part with the same physical characteristics as an MD part, it may be wiser to craft the part such that it has the same flux amounts and outgassing characteristics as those displayed by an MD part.

A correlation can be drawn between the MD and LD components for the QFP80 comb pattern, although the level and variability improved significantly when the LD standoff was increased from 4 mils to 8 mils.

2. Can the SIR test system distinguish between LD and MD? Yes, as noted above.

3. Can the laminate dummies serve as a cost-efficient replacement for the mechanical dummies on the IPC-B-52?

Overall, laminate-based dummies can serve as cost efficient replacements for mechanical dummies for the BGA test component and the QFN48 test component.

Additional work must be performed on the laminate-based dummies for the QFP80 component to make them equivalent to the mechanical QFP80 components.

In many ways, the LD QFP80 components behaved very much like QFNs. The different footprints between the mechanical QFP80 (gull wing leads) and the laminate QFP80 (balled standoffs and larger footprint) led to different cleaning dynamics between the two components.

4. Can the Magnalytix B-52 Legacy 1 test assembly serve as an acceptable alternative to the IPC-B-52 test assembly?

Yes, although it cannot yet be considered a “drop in” replacement for the IPC-B-52 standard test assembly for all test patterns. Additional work must be done on the QFP80 component before such a recommendation could be made to the IPC committees.

Overall, this testing has greatly increased our knowledge of the SIR physics underneath low standoff parts, and how two variables, standoff height and flux volume, impact overall SIR levels and data consistency.

Initially Published in the SMTA Proceedings

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