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PCB Surface Finish Receives High Marks from HKPC
January 29, 2013 |Estimated reading time: 4 minutes
Editor's Note: This article originally appeared in the December 2012 issue of SMT Magazine.Many OEM and PCB fabricators currently wrestle with traditional PCB surface finish issues. Of primary concern is the fact that traditional finishes cannot withstand modern environmental elements, which results in corrosion failures. In addition, limitations with shelf and floor life of these traditional finishes also cause solderability issues for EMS providers and contract manufacturers (CMs) using surface-mount technology (SMT) assembly. Earlier this year, WKK Distribution Ltd. initiated the evaluation and benchmarking of modern PCB surface finish solutions to better serve the PCB industry and its customers in the greater Asia-Pacific region. For the testing, completed in May, the company contracted Hong Kong Productivity Center (HKPC) to perform the tests. HKPC is a government entity, tasked with providing an independent source of testing for companies in Hong Kong. During testing, the control copper surfaces, control immersion tin, and immersion silver-coated surfaces showed major degradation; Semblant Plasma Finish (SPF)-coated surfaces showed no degradation.
Rather than use a simple test for copper protection, HKPC chose to test using IPC’s test method. By using a standardized test method, the company felt that if the PCF coating survived this very harsh test, it would be a demonstration of an excellent protective finish. The test identification number chosen for review was TM 650 2.3.22. For this test, the high-pH chemical ferric chloride was used to determine the robustness of the coating. The concentration of the ferric chloride solution maintains a Baume of 40 in a hydrochloric acid base.
The initial review of the material was quite interesting, so the company decided to use an independent testing board to further stress the material. The control sample used was bare copper and test samples were:
- Control, bare copper;
- Bare copper, SPF coated;
- Bare copper, SPF coated, and passed through a simulated reflow cycle three times (Figure 1);
- Immersion tin;
- Immersion tin, SPF coated; and
- ENIG-coated panel.
Figure 1: Simulated reflow profile at 250°C. Figure 2 shows the test vehicle. The same substrate pattern was used for all tests to ensure there were no anomalies that could relate to different circuit configurations. The test requires that the PCB substrates are aged using a controlled environment for a set period of time and then subjected to a concentrated solution of ferric chloride for a controlled period.Figure 2: Test vehicle substrate pattern.
The test panel is mounted at approximately a 45-degree angle to the work bench. Using an eye dropper or pipette, the test solution is dropped onto the test area of the test panel. The length of time for contact is usually between 5 to 10 seconds (a 10 second interval was used for this test) and the substrate is then rinsed and dried. Inspection is performed using either natural vision or a low-power hand loupe.
Results follow with pictorial verification of the data. Please note: The images on top are before test; images below are after. The first test was a bare copper, baseline test. The result was a total failure of the substrate with major copper surface breakdown and major damage to the copper surface via surface oxidation.
Figure 3: Pictorial verification of Test 1: Bare copper, baseline test. The second test was of copper with SPF coating. The result here was no damage to the copper surface after chemical treatment.
Figure 4: Pictorial verification of Test 2: Copper with SPF coating.
The third test scenario was the immersion tin with no SPF treatment. The test solution dropped onto the test area of the untreated test panel. Again, the tin surface shows major corrosion after chemical treatment.
Figure 5: Pictorial verification of Test 3: Immersion tin with no SPF treatment.
The next scenario tested was an immersion tin with SPF treatment. The result was no damage or oxidation of the after chemical treatment.
Figure 6: Pictorial verification of Test 4: Immersion tin with SPF treatment.
The next test looked at immersion silver with SPF treatment (sample lost for bare immersion silver). With this situation, there was no damage or oxidation of the surface after chemical treatment.
Figure 7: Pictorial verification of Test 5: Immersion silver with SPF treatment.
The sixth test used electroless nickel, immersion gold (ENIG) surface. The expectation is no surface damage, which was indeed the case.
Figure 8: Pictorial verification of Test 6: ENIG surface.
The last test scenario was bare copper with SPF coating, 3X reflow simulation. The SPF coating provided a robust protective film for the exposed PCB surface areas, for long periods of time, after several simulated assembly processes, and even when treated with the harshest of chemicals available.
Figure 9: Pictorial verification of Test 7: Bare copper with SPF coating, 3X reflow simulation.
Results showed that the SPF material provides superior protection, both in the short term and for at least a shelf life of one year--comparable to an ENIG or an ENIPIG coating. Semblant has developed a unique product for the protection of exposed copper and other metal surfaces for bare PCBs both for long-term storage as well as prior to assembly. One such product is its SPF coating. This material is deposited out of plasma in a vacuum chamber on all surfaces of the finished PCB.
Lionel Fullwood serves as technical director of PCB Arena Company and technical director of WKK Distribution Limited, both subsidiaries of Wong’s Kong King International Holdings Ltd. Fullwood has spent 34 years in the electronics industry, specializing in PCB manufacturing. He has designed, selected equipment, and been responsible for the management of four new PCB manufacturing facilities in Asia and the U.S. Fullwood is active in IPC, chairing the 2221, 2222, and 2226 committees, and has authored or co-authored 33 technical papers.