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SIR Test Vehicles - Comparison from a Cleaning Perspective
SIR Test Vehicles - Comparison from a Cleaning Perspective
This study compares different SIR test vehicles to determine which test vehicle is tougher to clean and therefore a challenge the cleaning process.
Analysis Lab

Authored By:
Naveen Ravindran, Umut Tosun
ZESTRON Americas
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Summary
PCB design has evolved greatly in recent years becoming ever more complex. Board density is increasing, component standoff heights are decreasing and long term reliability requirements are greater than ever, particularly for Class III products. Given the quality and reliability demands for complex PCBs, manufacturing processes are qualified; that is, the PCB design, including component and solder paste/flux selection, material compatibility and process steps, must meet the long term reliability requirements demanded and quality standards desired. As a result, cleaning is becoming a mandatory step within the manufacturing process.

Analytical tests are key elements to any qualification process. Through the IPC, numerous tests have been developed and have become industry standards. In particular, IPC-TM-650, method 2.6.3.7 or SIR (Surface Insulation Resistance) is frequently used regardless of the solder paste/flux type. Per the specification, this test can quantify the deleterious effects of fabrication, process or handling residues on SIR in the presence of moisture. Measuring changes in surface resistance is a standard way of testing cleanliness and long-term reliability of a test board or complete process assembly based on industry standards.

There are numerous test vehicle options available to the industry for conducting SIR analysis. This study was designed to compare different SIR test vehicles, from a cleaning perspective, in order to determine which test vehicle is tougher to clean and therefore challenge the cleaning process.

The three (3) test vehicles selected were the IPC-B-52, IPC-B-36 and the SMTA Saber. Each test vehicle was populated with specific components. The authors chose to reflow the test vehicles with water soluble solder paste only, since the high activity flux in the water soluble paste would increase the chance of SIR failure if left partially cleaned.
Multiple test vehicles were prepared. Cleanliness verification and validation was completed by visual inspection underneath all components as well as by performing SIR tests. All test vehicles were cleaned prior to reflow and ion chromatography was conducted on selected test vehicles initially to ensure they were free of any ionics. An inline cleaning process was used for all cleaning trials.
Conclusions
Conducting this study using water soluble solder paste enabled the authors to differentiate achievable cleanliness levels of the three (3) different SIR test vehicles. As expected, if residues remain on the board surface, SIR tests yielded failed results. As mentioned in the introduction, the purpose of SIR testing should influence SIR coupon selection. It is interesting to note that utilizing the low concentration aqueous cleaning agent yielded passing SIR results for all coupons cleaned under the "best case" scenario whereas the DI-water cleaned coupons yielded failed SIR results under "worst case" scenarios for IPC-B-52 and IPC-B-36. Additionally the authors noted:

Differences in visual cleaning results could be noted on boards cleaned with DI-water only.

The IPC-B-36 test vehicles were found to be the most difficult to clean with residues noticed underneath components even at 1 ft/min with DI-water. Using the aqueous cleaning agent, all coupons were clean at 4 ft/min.

Saber boards required a conveyor belt speed of 1 ft/min to be 100% clean, whereas the IPC-B-52 test vehicles were completely cleaned at 2 ft/min with DI-water.

SIR test results showed failures on the IPC-B-52 and IPC-B-36 test vehicles cleaned under "worst case" settings, which was to be expected since these settings resulted in residues undercomponents.

The Saber board passed the SIR test even under worst case settings. This is due to the fact that the SIR measurements are taken on the QFP component of the Saber board which is an easier component to clean as compared to other components on the board.

In general, the SIR values for test vehicles cleaned with "best case" settings (with chemistry) were higher than boards cleaned with "worst case" settings (DI-water only).
Initially Published in the IPC Proceedings
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