Chapter 1 Excerpt: The Printed Circuit Assembler’s Guide to Process Validation

Estimated reading time: 2 minutes

Quick and Convenient Contamination Testing

The original principle underlying the IPC’s ionic cleanliness requirement was that the mobility of surface ions, detectable from SIR measurements, would correlate to a maximum value of detectable ionic elements permissible on the circuit surface.

The industry required a test that was both fast and accurate for process control. Thus, a simplified ion chromatograph that was able to detect ionic elements without differentiation was created. Due to patent restrictions, there were—and still are—only two different instrument configurations: closed-loop and open-loop, which became referred to as static and dynamic instruments. Each instrument configuration must be capable of measuring the presence of a known amount of ionic contamination.

Based on an extrapolation of insulation resistance measurements, it was determined that there was an upper limit beyond which ionic elements would become mobile on the assembly surface, thereby potentially compromising electrochemical reliability. The number, although occasionally altered during the infancy of this test, was 1.56 µg/cm² NaCl equivalence. Despite the many technology changes employed in the period after the mid-1970s, many—if not most—felt it convenient to simply continue applying this rule as adequate evidence of the electrochemical reliability of their end-product.

The IPC has now withdrawn the requirement for 1.56 µg/cm² NaCl equivalence for a number of reasons as explained in IPC-WP-019A, which states, “For many years, military (e.g., MIL-STD-2000) and commercial (e.g., IPC-J-STD-001) manufacturing standards have required manufactured circuit card assemblies (CCAs) to meet an ionic cleanliness requirement of 1.56 microgram (µg) of sodium chloride (NaCl) equivalence per square centimetre (cm²) of extracted surface, using resistivity of solvent extract (ROSE) testing per IPC-TM-650, method 2.3.25.”

Therefore, it is the position of the IPC committees that the value of 1.56 µg NaCl equivalence per square centimetre should be considered as obsolete for the following seven reasons:

• This test methodology was originally developed in the 1970s. It was never intended to be used as a cleanliness test, nor as a test for product acceptability; it was only intended to be used as a process control method.
• The use of the ionic contamination value as a measure of product acceptance was the result of a U.S. Department of Defense desire to implement pass/fail criteria.
• This ionic contamination value, and those derived from them, were originally developed for high solids (35% solids) rosin fluxes and ozone-depleting chemical (ODC) cleaning. The flux chemistries and cleaning solutions used today are completely different from those used when the ROSE limits were established.
• Modern assemblies are simply too complex in terms of residues to have a single “one-size-fits-all” cleanliness criterion.
• There is mounting evidence that as CCA component density increases, so does the sensitivity of the circuit to ionic contamination. Modern circuit assemblies have far greater component densities than found in the 1970s. This also means that residues that had minimal impact on component technologies from the 1970s can now have a significant impact on component reliability.
• For many assemblies, ROSE testing is no longer a sufficient test regimen to adequately predict acceptable levels of ionic residues. IPC has compiled a list of technical presentations showing the inadequacy of ROSE to predict ionic residues for high-performance electronics.
• It is recognized that ionic residue testing is critical for reliable circuit function, so the ROSE test has continued in use until a more suitable alternative can be identified and implemented.

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