Issue link: https://iconnect007.uberflip.com/i/1284035
24 SMT007 MAGAZINE I SEPTEMBER 2020 tions are that facilitate the CAF. For that, you want to use ion chromatography. That will tell you exactly what ions are present and at what concentrations. Those results can be matched back to chemistries used in the plating process, and then the optimization is focused and can happen a lot faster in most cases. The IC data in Table 1 shows typical ionic content from an inner layer cleanliness issue, high levels ace- tate, sulfate, and sodium residues. These ions are normally found in plating chemistries and suggest that the final rinse is insufficient to completely remove all the residues. Ion chromatography should also be used on normal production PCBs to determine the level of cleanliness on the outside surface. If IC is to be used for process monitoring, you will want to perform global extractions for base - line data. Localized extractions over concen- trated distributions of plated through-holes, over-plated pads, and overly-bare solder mask areas should all be considered to get the clear- est idea of just how clean each of those parts of the process is. Solder mask cure is another critical param- eter that should be examined. When a mask is properly cured, it will exhibit a continu- ous smooth texture, like a marble countertop. If the solder mask is under cured, the surface will be rough with nooks and crannies, like an English muffin. The same way that muffin will hold delicious butter and jam, the solder mask will hold flux, wash chemistry, and other pro- cessing residues. The IPC test methods related to solder mask cure are 2.3.23B and 2.3.23.1A. These are chemical tests that use drops of methyl chlo- rine or methyl chloroform on the solder mask, followed by using a wooden spudger to see if you can scratch the mask. If it easily scratches, give it a "cure bump" with either UV or thermal exposure and then repeat the test. If the mask is then unaffected, you can go back to your supplier and have them adjust their cure pro - files. Uneven solder mask coverage can expose the base metals to less than optimal environ- ments, and that alone can be enough to cause issues like corrosion (shown in Figure 5). There are many different tests specific to bare boards, so it's a good idea to consider the end-use envi - ronment, warranty period, and any other prod- uct-specific details to determine which test is most applicable for your product. Many of the same processes used for the plating of bare boards are also used for com- ponent leads. Both processes use chemistries that can increase the risk of corrosion or issues related to electrochemical migration if not fully removed. This happens with components when those chemistries find a way up into the die area, causing corrosion and dendrite growth. This can easily happen when there is a Table 1: Ion chromatography results, CAF analysis. Figure 5: Corrosion in solder mask with pinholes .