Issue link: https://iconnect007.uberflip.com/i/1464168
78 DESIGN007 MAGAZINE I APRIL 2022 the main purpose of solder mask material is— are you ready for this?—to mask solder. During the reflow phase, the solder melts and will flow away from the contacts if not masked. How Much Clearance? In all cases, unless allowed by design, sol- der mask material is not allowed to end up on solderable land surfaces because it blocks or masks the solder from coating the intended surface to be welded. For instance, if a solder- able land surface on a PCB happens to receive a lost blob of solder mask which mistakenly finds itself on a land during processing, or is missed in inspection and continues to the assembly soldering process, the anomaly will corrupt the solder joint by reducing the surface area of the land. is is considered a nonconforming defect in the pages of the IPC-A-600 Accept- ability of Printed Boards (Figure 2). But there are other manufacturing condi- tions which can cause solder mask material to end up on solderable surfaces. Misalignment of the artwork, material movement, and expan- sion or shrinkage of the PCB over the entire span of its length are conditions which must be considered. ese are conditions for which additional solder mask clearance is required. erefore your design tool supplier has added a control feature in your design tool. But how do you set it? What values do you use? For years, I have asked these questions of the students in the IPC CID classes I teach and I've heard some very interesting opinions. One will say, "I have had very good luck by setting my solder mask clearance to 6 mils (0.006")." Another will brag that his supplier is so good that clearance can be set at only 2 mils (0.002"). Still another argues that for consideration of "DFM," everyone in their design group opens up the clearance to a whopping 10 mil (0.010") clearance. The Goldilocks Zone Again, per the widely accepted IPC accep- tance criteria, no solder mask is allowed to end up on solderable lands as a manufacturing defect related to imaging, alignment, or sloppi- ness. So, with regard to DFM, a PCB designer might conclude that an overly robust clearance would be the best choice to help with PCB manufacturing as it makes the target zone hard to miss. But wait a minute here. As "the hub" of the PCB manufacturing process, a PCB designer must consider all the stakeholders' requirements. While the PCB supplier might applaud this DFM consideration as something they'll not have to worry about, the decision Figure 2: Find information about manufacturing target conditions in the IPC-A-600 Acceptability of Printed Boards. Figure 3: Setting the solder mask clearance values of your design tool. Figure 4: Too little clearance.