We receive many questions related to electronic assemblies and their cleanliness. If none of these FAQs address your specific question, please contact us.
Q. What is the source of my problem and how do I solve it?
A. This is, of course, a very complex question. There are more ways to screw up a process than you can shake a stick at, and often several solutions for fixing any problem. In general, when we backtrack through a manufacturing process, we look for several things. Has something changed in the process recently which transforms good product to bad? What is the failure mechanism: electrical leakage, corrosion, a visible residue, metal migration? In a vast majority of cases, you can isolate the problem by looking at the materials of construction and the materials and processes used in manufacturing.
Q. How do I determine if my cleaning process is adequate?
A. Ion chromatography is a good first step. By examining the baseline process, we can get a pretty good indication of the cleanliness of the process. We can then vary and examine the cleaning process to see if the harmful ions are decreased. A good explanation of the different residue tests can be found in section 8.0 of IPC J-HDBK-001.
Q. What kinds of cleaning processes are available and which are dominant?
A. Excluding the niche applications (such as CO2 super-critical fluid cleaning), electronics cleaning processes can be broken down into three groups: solvent, semi-aqueous, and aqueous. Solvent cleaning is used primarily where components are water intolerant or are not sealed. In the past, solvent cleaning was the norm for military and high-reliability applications, but its use is now greatly reduced with the elimination of freon and trike. Co-solvent blends, hydrofluoroethers (HFEs) and hydrofluorocarbons (HFCs) are taking the place of freon. Semi-aqueous cleaning is done primarily on older military contracts as the semi-aqueous cleaning was a good transition material to other technologies – Axarel and Bioact from Petroferm are the primary materials here. Aqueous cleaning is the most prevalent cleaning process today and has many advantages, especially when a saponifier is added
Q. How clean do components have to be?
A. That depends on the components and the applications. A high-impedance device, which assumes an infinite resistance between leads, is more sensitive to plating residues and other contaminants than a low-impedance component. High-voltage and high-frequency components are also very sensitive to contamination. As to what cleanliness levels you need, you would have to do correlation testing to determine the needed cleanliness levels for the application.
Q. What equipment should I be using in assembly?
A. That depends on your product and whether you are a high volume or low-volume/high-mix manufacturer. Different manufacturers have products with different strengths and weaknesses. We have extensive equipment experience. For more information, please contact us.
Q. What IPC specifications should I use to make sure the coating sticks to my assemblies?
A. The short answer is, there are no such specs. The IPC specifications, over the past few years, have gone to a system of materials qualification, using standard substrates and standard tests to level the playing field for coating manufacturers. The work of determining if any particular conformal coating will stick to any particular assembly or flux residue is left to you, the assembler.
Q. What resources exist to help me understand conformal coating issues better?
A. The IPC Conformal Coating subcommittee has a task group under its domain that is working on putting together a conformal coating handbook, compiling the collective wisdom of IPC members on this topic. This handbook can be obtained from the IPC.
Q. What IPC specifications will tell me if I have good hardware?
A. A complex question. Ideally, IPC-A-600 and IPC-A-6012 address bare board quality issues. IPC-A-610 and J-STD-001 look at assembly issues. All are good guidelines for assembly quality, but no spec. is perfect. Only you, the assembler, can determine how “bulletproof” your hardware is in your end-use environment. Unfortunately, IPC has no specifications at this time on how to perform accelerated testing to determine your hardware reliability in various end-use environments.
Q. Which specification should I be using, IPC-A-610 or J-STD-001?
A. This depends a great deal on what your contract administrators and your attorneys say. A-610 is a visual guide and is not a specification, per se – it does not have any testing or criteria. The recourses available to you if your subcontractor does not comply are limited. J-STD-001, on the other hand is a standard/specification, with all that implies for legal purposes. In our experience, J-STD-001 is used by the Class 3 “big boys” (e.g. defense contractors and other high-rel shops). A-610 is used by the Class 2 and Class 1 assemblers. J-STD-001 is a good specification, but does not guarantee that the hardware is “good”.
Q. I have several IPC manuals such as IPC-610, IPC-7711/7721, IPC-6011, IPC-6012, etc. Unfortunately, I have not been able to find any information regarding ionic contamination for completed printed circuit assemblies – any pass/fail criteria and/or quality standards. Can you refer me to any IPC publications that contain information regarding these issues?
A. When you talk of pass-fail levels for ionic contamination, the ONLY IPC specification is for the standard ROSE test (e.g. Omegameter, Zero Ion, etc.), which is the historical military limit of 10 micrograms NaCl equivalent per square inch, adjusted for the instrument used. Most assembly level committees, recognizing that this level is valid ONLY for high solids, rosin fluxes, allow for an alternative pass/fail as agreed upon between vendor and customer. As an example, if you don’t have an agreed-upon value with your customer, the default is 10. If your customer wants to see 7, then the pass/fail is 7. It is largely a contractual issue. The bare board specifications, such as 6011, have a pass/fail for bare boards of 5 or so, which was a carryover from MIL-P-55110 and MIL-P-28809. In our view, those pass/fail levels are antiquated and unreliable as a protective measure for assemblers.
We would suggest that if you are looking to become better educated in the area of ionic cleanliness and how it is determined, that you read Chapter 8 of the J-STD-001 Handbook. You should also get a copy of EMPF report RR0013 (An In-Depth Look at Ionic Cleanliness Testing). You should be able to contact them online at www.empf.org
The test methods of interest here are found in the IPC test methods manual IPC-TM-650. The methods are 2.3.25, 126.96.36.199 (ROSE method). A more advanced measure of ionic cleanliness is ion chromatography, which is test method 2.3.28, which we wrote. Comparing ROSE to IC is like comparing a broad blade ax to a surgical scalpel. All of our failure analysis and process troubleshooting is done with ion chromatography.
For assembly level documents, you can look in IPC-A-610 or J-STD-001 to find pass-fail requirements for ionic cleanliness. Basically, they take the old military limit of 10 and perpetuate it, unless you have other arrangements made as I outlined above.
At Foresite, we don’t believe there are any “golden” numbers or ionic cleanliness criteria that can be blindly applied to all technologies and all flux types. Based on our own failure analysis work, we have recommended limits based on factors such as metallization type (e.g. HASL vs. gold), flux type (water soluble vs. no-clean) or application (hybrid vs. PCB).
Foresite does not place much confidence in ROSE testing as a guarantor of quality. We have had dozens of clients who had adequate numbers from their test instruments, only to have corrosion and metal migration in the field. The RR0013 report can help you understand why this occurs. Our Process Rx column in Circuits Assembly magazine also covers this.
Q. Why does IPC-TM-650, method 188.8.131.52A specify FR-4 and bare copper as the substrate? Wouldn’t HASL metallization be better?
A. This test method is controlled by the J-STD-004 specification on fluxes. This specification uses a “level playing field” approach, and so specifies FR-4 as the base laminate and bare copper as the metallization. HASL, or some other coating, might be more representative of the bare boards you get, but then you have the complicating factors of HASL and post-HASL cleanliness complicating the materials characterization testing of J-STD-004.
Q. What’s the difference between Bellcore NWT-TR-000078, issue 3, and Bellcore GR-78, from the standpoint of materials testing?
A. Not a great deal. The SIR test remains essentially the same with the same sample sizes and test patterns. The SIR test humidity level went from 90 % minimum to 85 % minimum to allow greater control. The minimum levels required to be considered compliant remained the same.
The Resistance to Electromigration test is essentially the same, but the test environment went from 596 hours at 85oC / 85 % relative humidity to 596 hours at 65oC / 85 % minimum relative humidity. The requirement of no corrosion or electromigration remained the same.
Q. Are there any rework or repair operations that I should avoid?
A. First, determine which solder defects are cosmetic and which are true defects. More damage is done touching up solder defects that don’t compromise reliability than any other action. Second, never use the addition of liquid flux of any kind as a touch-up soldering aid. This causes far more problems than it helps, especially for low-solids fluxes. Third, localized cleaning with isopropanol is not recommended. In essence, this only spreads the contaminant out on the board, “infecting” a larger area. Fourth, do not mix cored wire solders in the work area. Keep workstations as either water soluble cores or no-clean cores. Mixing them will assure that at some point you will put the water soluble solder on the no-clean assemblies.
Q. How should I strip conformal coating for field repairs?
A. That depends on the coating. Acrylics are fairly easy to remove locally with a variety of materials. Most coating manufacturers have a coating stripper formulation for acrylic. The next easiest coating to strip is urethane, followed by silicone, paralene, and epoxy. If you only have to repair a solder joint or two, burn through the coating with the soldering iron and do a spot recoat afterwards. If you have to remove a single component, then strip the coating in that area. Placing an RTV Silicone dam around the area allows you to selectively strip. The IPC Conformal Coating Handbook and IPC-7011 and IPC-7021 should be consulted for more information.