|Ask the Experts|
February 22, 2021
Class 3 Cleaning Requirements
We recently received our first contract to assemble Class 3 circuit board assemblies. Are there any changes we need to make in our cleaning operations to switch from Class 2 to Class 3?
|Expert Panel Responses|
It is unclear as to what specification you are working under. There is no general specification for "Class 3" cleanliness. Apparently, there is a specification accompanying your contract that will dictate exactly what conforms to "Class 3" cleanliness in your particular application. Refer to that specification for the exact requirements.
Undoubtedly, it will require a verification step, so expect either a particle count per MIL STD 1246 or similar spec, or some type of solvent rinse of the surface with a GC / FTIR scan of the rinsing agent associated with the part. Unless it requires 100% verification (as on many NASA and black DoD programs), once you have proven the process you should be able to maintain QC by sampling.
IPCTM650 describes cleanliness testing methods and required results.Unfortunately, the cleanliness results outlined in both military and IPC standards were written so long ago, they are, in practical terms, obsolete.
IPC allows up to 10 ug NaCl equivalent /in2. This number is far too high. Due to the nature of ROSE testers (the industry's approved cleanliness testing method), contamination is detected then averaged over the entire assembly's surface area.
Because actual contamination is not equally distributed, actual contamination levels are concentrated in some areas of the assembly and less concentrated in others. Because of this, one must also consider other factors in determining an allowable contamination level. One must consider the density and diversity of component population, standoff height, flux application method, the device's intended environment, and flux type.
Most importantly, one must consider the cost of failure. There may be differing costs for failure between a class 2 device and a class 3 device. The more critical the reliability, the lower the allowable contamination level.While industry standards allow 10 ug NaCl equivalent /in2, I would initiate an internal requirement of 2 ug NaCl equivalent /in2, particularly for class 3 devices.
The answer is "it depends." Whether or not you need to make any changes to cleaning operations depends entirely on the requirements that your customer sets forth. If your customer is asking you to work to J-STD-001, for example, they would specify any cleaning requirements according to Section 8 of that document.
You'd want to review the requirements and the test methods and compare to your existing process. This would tell you whether your existing process is capable of meeting the desired level of cleanliness. In the event that your customer has failed to communicate cleanliness requirements, a discussion with the customer is suggested to avoid any potential for misunderstanding.
Also recognize that there may be a need for documentation of cleaning results for Class 3 products.You may need to be able to demonstrate that cleanliness was tested on the product, and have records of the results. Again, open communication with the customer is essential to ensure you are meeting their needs with regard to test methods, contamination limits, testing frequency and reporting.
Yes, there are certain spec changes, for example amount of rosin based residues left behind on the board surface differs from Class 2 to Class 3 assemblies. We would highly recommend reviewing the:
Application Technology Manager
It is really comes down to your company having data showing you can meet and control your process to the limits called out in IPC class 3 requirements. If you have not recently certified your cleaning process meets the JSTD-001 class 3 requirements of surface insulation resistance(SIR) and Ion Chromatography (IC) you should consider this course.
These are complex and time consuming test and only need to be performed initially and when significant process changes are made. You should select an appropriate test assemble with similar component types and use your flux, soldering process, and cleaning material sets when performing these tests.
Resistance of solvent extract (ROSE) testing is recommended to control the cleaning process on a ongoing basis once the IC and SIR testing passes. The important thing with ROSE testing is not the absolute value (assuming you pass) as much as the cleanliness data trend day to day.
There are not many inspection differences related to cleaning between class 2 and 3 in the IPC JSTD-001 assembly requirements and the 610 inspection requirements. You may want to send a lead inspector for re-certification to IPC 610 if they are not certified.
Austin American Technology
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