Issue link: https://iconnect007.uberflip.com/i/1002143
40 PCB007 MAGAZINE I JULY 2018 The mechanism of the test is to print solder paste in a pattern whereby the gap between solder paste deposits increases. When the sam- ples are reflowed, the solder paste will spread to represent solderability. The test vehicle is a tool that can provide a numerical value. The value is calculated by summing up the maximum spread values, (red line) and subtracting the number of breaks, (black rings) divided by 2. Therefore, the re- sult for Figure 13 is (6+7+7+7+6+6) – 9/2 = 35. These values were used to evaluate the im- pact of simple techniques to enhance produc- tion yields. The results show that the ionic contamina- tion is statistically related to the solder mask type and UV energy. It must be stressed that the UV energy is not a setting recommenda- tion, as the measuring techniques and equip- ment can give significantly varied results, but just to indicate more energy or less energy. In this test, more UV energy reduces ionic contamination and the impact of the UV ener- gy is also statistically significant regarding the solder gap test. In other words, a UV treatment prior to the immersion tin plating process sig- nificantly improves ionic contamination and soldering behavior. Conclusions The primary finding of this paper was that sulphur (>3%) can be found in solder masks that were identified as 'critical' and could there- fore be used as a marker to highlight them. The samples tested were supplied from the field and do not represent any supplier specifi- cally. As previously mentioned, it is the inten- tion of this paper to help educate users to ask the correct questions about the solder mask they are using. Whilst UV treatment prior to plating is effective, it can be seen that selecting a suitable solder mask can also impact yield. In the customer-based test, solder mask C had the lowest ionic contamination results and the best solderability results. Figure 11: Solder spread test vehicle. Figure 13: The results and significance for ionic contamination and solderability. Figure 12: Solder spread test vehicle after reflow.