Topic

Is your board clean?

There are a number of ways of assessing whether a board is adequately clean. Arguably the most objective, and the easiest to relate to the final application is to measure Surface Insulation Resistance. SIR tests measure the electrical isolation between two points on a circuit board under accelerated conditions of heat and humidity. Suitable tests are referenced in J-STD-004 (on fluxes) and IPC-9201, the Surface Insulation Resistance Handbook. Experience suggests that it is difficult to meet the specified SIR limits if you have any level of detrimental residues.

However, there are practical implementation issues. As one commentator put it, “if you want to measure SIR on an actual finished assembly, save your sanity and don’t try unless your assembly has built in test patterns. Burn-in testing would be a better alternative.”

Unfortunately, both SIR and burn-in tests also take time, and it is hardly commercially acceptable to hold entire batches until lengthy tests have been completed. Also, there is the cost implication, not just of conducting the tests themselves, but also of paying for the good parts which have to be disposed of after any humidity tests.

The IPC-TM-650 manual lists four test methods that are cheaper and easier to carry out than SIR, although they are less direct measures of the effect of non-cleanliness. Instead, the tests attempt to measure the actual residues:

The most commonly used are the three ROSE methods, variations on a test in which the assembly is immersed in a solution of 2-propanol and de-ionised water to dissolve the contaminants. The dissolved ionic substances cause a change in conductivity of the test solution, which is measured and converted into a contamination value in micrograms per square cm equivalent of sodium chloride.

ROSE tests are carried out both on bare boards and assemblies, usually using proprietary instruments such as the Cookson Omegameter and the Multicore Contaminometer. Typically these use a precision conductivity cell with precious metal electrodes, and provide a plot of contamination against elution time. Great care has to be taken to ensure that the system is not itself the source of contamination!

What is considered as acceptable cleanliness varies between specifications and from company to company. As you would expect, a densely-populated nickel-gold board would have a different residue sensitivity to a through-hole non-dense assembly. Practitioners in the field counsel their clients1 to ‘define a battery of electrical tests which can separate good from marginal from bad product. This might be ESS testing, or HALT testing, or established burn-in tests, and develop designed experiments correlating residue levels with electrical testing. Until you can say what residue levels correlate to unacceptable performance, why do the tests?’ ‘Your testing should be structured so that you can tell good from bad product, both from an infant mortality standpoint and from a wear-out standpoint, and then optimise your manufacturing process to consistently meet what you define as “good” for your product’.

However, in J-STD-001C, IPC have suggested allowable levels of ionic contamination which vary purely according to the application:

Class 1: General Electronic Products. Includes products suitable for applications where the major requirement is function of the completed assembly.

Class 2: Dedicated Service Electronic Products. Includes products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically the end-use environment would not cause failures.

Class 3: High Performance Electronic Products. Includes products where continued high performance or performance-on-demand is critical, equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required, such as life support or other critical systems.

What class is your product?! And is this level correct for it?

The ROSE test has been criticised on many grounds:

Ion chromatography is still an elution process, but one in which the ions present can be identified and quantified. Board manufacturers report an increasing use of ion chromatography (IPC-TM-650, Method 2.3.28, revised May 2004) to qualify bare board cleanliness and set more realistic bare board control limits. Delco-Delphi in Kokomo has their Q-1000 bare board spec which qualifies the bare board cleanliness using ion chromatography, ROSE, and modified-ROSE testing. The ion chromatography results indicate if the fabricator is acceptable and the corresponding ROSE and mod-ROSE values are used for process control.

1 Doug Pauls on TechNet (16 Jun 1999).
2 Doug Pauls commented “I have seen many cases where ROSE data showed squeaky clean boards, but experienced all kinds of corrosion and metal migration problems due to entrapped or absorbed residues which ROSE will often not detect.”

 

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