Authored By:
Richard Brooks
Spartronics LLC
FL, USA
Mike Bixenman and Mark McMeen
Magnalytix, LLC
TN, USA
Summary
Cleanliness of materials, components, and manufacturing processes are integrated together as the overall PCBA contamination level is determined by each step in the process. Complex components, such as integrated circuit packaging, leadless, and bottom terminated components trap contamination within the component’s interior. Secondary processes that include wave/selective soldering, manual soldering, and topical cleaning can spread contamination to neighboring components. Controlling the process for acceptable levels of flux and process contamination improves reliability.
Miniaturization of electronics means smaller PCB layouts, smaller components, lower standoffs, and multiple PCB layers. Contamination risks are not uniform across the assembly. Localized contamination is more likely to cause a failure than the risk based on the average overall contamination level found on the PCB. Each of these factors contributes to the overall cleanliness due to the difficulty associated with the required cleaning steps.
The purpose of this research is to design electrical twin test boards in the waste area of a panel of production boards, which are highly representative of production hardware. The selected components used on the electrical twin exhibit a high risk for trapping contamination. The electrical twin test boards will be used as a process control monitor using the temperature-humidity-bias test method. The data will be plotted into an ẌR chart. The CpK will be tabulated and monitored.
Conclusions
The cleanliness of materials, components, and manufacturing process steps represent the contamination that can be present in a finished circuit assembly. The soldering process causes significant contributions due to the presence of flux residues. When building to a no-clean standard, the activity of the flux residue during the soldering process depends on the kinetics of the thermal decomposition process, which is never complete due to variations in the temperature profile and short exposure time to higher temperatures.[2] For assemblies that are cleaned, the removal of flux residues under low-profile components is challenging. Partially cleaned flux residue changes the morphology of the flux and can be a potential reliability factor.
The industry has long used chemical extraction methods to test for conductive residues. Printed circuit assemblies are far too complex to accurately detect problematic residues using these methods. Adopting the “Electrical Twin” that sees all the assembly processes is a low-cost and accurate method for controlling for active residues.
This research showed the effectiveness of electrical testing to qualify and control the assembly process. SIR testing has long been known as the “Gold Standard” for detecting the negative effects of process residue. This method can accurately detect process drift across the different assembly steps. The method provides timely information to monitor and control the assembly process.
Initially Published in the SMTA Proceedings
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