How Clean is This Spot?
The question of “how clean is clean enough for reliable operating performance?” is an ongoing issue for higher-reliability assemblies. Due to recent advances in electronic assemblies, it is now necessary to understand whether a residue in a critical area is corrosive or insulative. Ideally, this testing should be done on the production floor so problems can be addressed immediately.
That is why Foresite developed the C3 (Critical Cleanliness Control). The C3 puts process cleanliness control of invisible and complex residues from today’s manufacturing processes in the hands of the manufacturing and quality engineers.
The C3 can be used to automate the extraction protocol for ion chromatography analysis. It can also be placed on the production floor and used to assess incoming material cleanliness and to monitor assembly and repair processes. The production floor samples from the C3 can be used in conjunction with ion chromatography, FTIR or SEM/EDX to identify the type and level of residue that created a corrosive event on the test electrode.
An Example: C3 Use by a Medical Device Manufacturer
One medical device manufacturer uses the C3 to check bare board, component, and assembly cleanliness levels. They are also using the C3 after repairs to show rework operators the effects of using a small amount of extra no-clean flux and not heating it adequately near where the flux spreads from the solder joint. The C3 has repeatedly indicated that the remaining flux was conductive in high-humidity environments, which corresponded to a problem the manufacturer had been experiencing.
As a result of this production floor information, they have implemented a second, controlled heating process to ensure that the no-clean flux sees the proper amount of heat to complex completely and leaves the intended benign residue on the board surface.
C3 Principles of Operation
The C3 operates on three main principles:
Extraction
Any production floor or analytical test for cleanliness is only as good as the technique to remove the residue from the surface of the assembly. The C3 extraction process has been designed to achieve effective ionic residue removal using a heated delivery system consisting of 3 stages:
Solution heating/delivery to the extraction site
Soak and ionization time
Aspiration of solution to collection cell
This cycle is repeated 9 times to effectively remove the surface residues from a 0.1 in2 area, generating approximately 2.0 ml of extraction solution to be used during the testing and afterwards for any desired additional testing.
Electrical Testing
Using a sacrificial Y-pattern electrode immersed in the extraction solution, a 10 volt bias is applied to the electrode and an internal timer is started to measure the time it takes to achieve a leakage event. The system measures the leakage across the electrode generated by the extraction solution plus the residues extracted from the board surface. The electrical measurement is determined by assessing the time it takes for the extraction solution and the 10 volt biased electrode to reach a 500 µΑ event. The system works under the theory that the more corrosive or conductive the residue the faster it will achieve this event. The less corrosive or conductive the residue, the longer it will take.
Data Interpretation
The output from the C3 is recorded in seconds (+/-0.1) and typical run times are up to 180 seconds. Since corrosive residues will create short run times and benign, insulative residues take longer, the challenge was to determine the limit between clean and dirty. After repeated testing and correlation between SIR and ion chromatography, a division line has been set at 60 seconds. Electrical tests that run are less than 60 seconds are identified as dirty samples. Sample runs greater than 60 seconds are identified as clean.
Some examples of dirty and clean samples are as follows:
Ion Chromatography Testing of Extraction Solution
Using ion chromatography to assess the extracted residue will determine which type and level of contamination is present to cause the measured electrical effect. The ion chromatography analysis is conducted using IPC TM 650 2.3.28 test method.
The following are test results using both the standard one (1) hour 80° C extraction and the C3:
Conclusions
Using a localized extraction we are able to quickly and efficiently assess various individual areas on any circuit board assembly, bare board, component or material to identify if corrosive residues are present.