|Ask the Experts
February 26, 2024 - Updated
November 3, 2014 - Originally Posted
What are the latest techniques/methods for testing the cleanliness of an assembled PCB after going through our in-house cleaning process?
|Expert Panel Responses
I would recommend using an ion chromatography approach to quantify and qualify the amount of ionic contamination present on the assembly. Ionics are not present to the eye or even in an aided magnification state but are present on pcb assemblies.
Anions, Cations and Weak organic acids are present at some level and this is the cleanliness level that needs to be understood. Depending on the type of flux system determines the amount of ionic contamination present. There are a lot of very good qualified analytical labs that perform IC testing - ion chromatography testing.
The test output will be quantifiable and can be used as a baseline for any future tests to insure repeatability and consistency. There are no specific industry defined guidelines for IC test results but each analytical lab has their recommendation and most large OEM's have their own guidelines ion contamination levels so there are industry benchmarks to be used if IC testing is performed.
VP Engineering Services
STI Electronics Inc.
Aside from visual inspection, the industry accepted practice of ROSE/Ionograph testing remains the standard for testing in the production environment. This process has it's limitations as it cannot identify localized contamination and should not be relied upon when no clean residue is present. Solvent extract testing measures the ionic contamination level as an average over the entire surface area of the assembly.
If there were a small pocket of contamination under a low-standoff device, the test would not be sufficiently sensitive to 'see' that contamination and the risk it poses. There are localized test methods that can be employed to focus on areas of particular concern. These are useful for process development and verification, but would be difficult to adapt to as 'go-no go' in the production environment.
The issue with no clean fluxes is that the extraction process has limitations and results can be misconstrued. An ionic contaminate that is insoluble in and IPA and water blend may be undetected. Additionally, the solvent extraction method, can liberate ionic components of a flux residue that are not harmful under normal conditions. Both instances could be misleading as to the state of the cleaning process and condition of the assembly.
Additional tests, including Ion Chromatography and FTIR can be used to characterize a cleaning process, the materials used and identify risk areas. Best practice remains, qualifying the current process and materials, establishing repeatable baseline for the process and monitoring the process to ensure it is maintained within the established parameters.
Director of Product Management
One new technique is a localized cleanliness tester. There is one called C3 from Foresite. With this type of cleanliness tester, you can test specific areas to see exactly where the potential cleanliness issue is, unlike using an Ionograph where it tests the entire PCBA.
Esterline Interface Technologies
The newest cleanliness test methods are focusing on the time it takes to create and electrical event (defined short) in a test solution extracted from the circuit on an exposed biased test coupon of fine pitch conductors. This method is not gaining traction with users because the data is difficult to interpret because pure 18 megohm DI water will form dendrites on a perfectly clean coupon within seconds depending on the metals exposed, the voltage gradient present between the test conductors.
The best current analytical method is ion chromatography. This method identifies all ionic species present and at what concentration. Techniques are well developed and documented for analyzing the entire assembly or just a single component.The drawback of this method is the time and expense, taking an experienced technician several hours to prepare the extract and perform the test.
Cleanliness testing is all about predicting the statistical life of a circuit assembly in a common failure mode often referred to as electrochemical migration. The likelihood of this type product failure depends on depends on 3 major factors;1 the environment, 2 the circuit performance requirements, and 3rd the amount and nature of the surface contamination. A search for the best cleanliness test to predict the life of an electronic product must consider the specific product performance requirements in the worst case environmental conditions expected.
The current IPC JSTD 001 and the older Mil Specs both suggest a three step approach to qualifying and controlling the cleaning process. The first level is visual inspection. Is it clean visually? Is the flux removed on the surface and under the part? The second step is to identify the amount and if possible the location of contaminates remaining after cleaning using ROSE testing or ion chromatography. The third step of cleanliness process qualification involves accelerated product life testing in humidity temperature chamber under bias. This can be accomplished with actual assemblies of test coupons made with identical product materials.
Once qualified, a cleaning process should be monitored and controlled daily by visual inspection. Periodic "ROSE" testing is recommended with SPC control limits.
Austin American Technology
If you want to understand the cleanliness of a process with typical components doing the average cleanliness of the total board with ROSE or even Ion Chromatography you will get a general cleanliness average over the entire PCBA with a volume dilution but to understand the entrapment localized cleanliness levels then tools such as the C3 test system should be used.
This tool allows for screening the cleanliness of the production floor with an electrical measurement of the corrosivity of the residue. Using the C3 and Ion Chromatography allows the understanding of the amount of residue trapped under low stand-off SMT components or residues under connector locations.
President/Senior Technical Consultant
Ionic contamination testing is still the most cost-effective method available. There are several
commercial systems available.
The following are well-known cleaning and cleanliness identification from IPC standards:
Application Technology Manager
Ion chromatography (IC) testing for anionic, cationic and weak organic acids against IPC-TM-650 para. 2.3.28 procedures is currently the latest method available to quantify individual flux contamination residues on a circuit card assembly. IC testing extracts the residues from a unit under test (UUT) and analyzes the individual residue components in terms of chloride, bromide, sulfite, sulfate, etc., each of which provides a window into what is going on in your process. We maintain an in-house Critical Process Control (CPC) laboratory for cleanliness testing and you can read more about IC testing and validation of your cleaning process using this method on our website at www.techmfgcorp.com and then click on CLEANLINESS TESTING at the bottom of the page.
This is a higher level testing that identifies residue components and is not to be confused with ROSE testing, which extracts the residues from the UUT but expresses the residues in an aggregate manner in terms of a generalized sodium chloride (NaCl) content. For example, J-STD-001 allows for ROSE testing results in the 1.56 ugm/cm2 (or 10.00 ugm/in2) from an acceptability standpoint, but this testing method is by no means the latest and dates back several decades when cleanliness was not well understood and was established as more of an arbitrary baseline than anything else.
It is also worth noting that in a high reliability environment, it is also valuable to perform ion chromatography testing on the bare circuit board itself (especially for HASL platings) which will give you an understanding of the baseline residues originating from the your board supplier and their cleaning process. In my experience, processing boards that have residual flux contaminants and are not completely clean right off the bat only contributes to the overall cleanliness (or lack thereof) in the finished product so it's also something worth considering.
Sales & Marketing Manager
Technical Manufacturing Corp.
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