Issue link: https://iconnect007.uberflip.com/i/1464168
APRIL 2022 I DESIGN007 MAGAZINE 79 will most certainly affect the assembly stake- holders in a negative way. Let's examine the Goldilocks zone for solder mask clearance. e two-mil clearance example in Figure 4 challenges fabrication capability for some vol- ume production suppliers. If the image misreg- isters even a little, the board fails acceptability due to solder mask material on lands. If the solder mask clearance is too "robust," as shown in Figure 5, the clearance will void masking between lands, possibly allowing sol- der at the assembly level to dri or "bridge" and short to neighboring lands. Notice that significant portions of the traces entering the land are not covered with solder mask. is condition allows solder to spread out away from the contact points of the land pattern and creates the notoriously problematic condition of inconsistent paste geometries. Target condition for all classes: No solder mask misregistration. e solder mask is cen- tered around the lands within the clearance space provided. Figure 6 shows solder mask coverage which tightly surrounds the lands and provides for optimal solder dams. But for many volume suppliers this tight manufactur- ing constraint capability cannot be met due to process or equipment limitations or advanced design topologies. In this case, should the PCB supplier's CAM engineer be allowed to grow the clearance between the contact land and the solder mask opening to the point that it will not adversely affect solderability but allow for potential misregistration? Read on. Avoiding DFM Confusion By setting the solder mask clearance in your design to a certain value you are in effect dic- tating the design parameters for solder mask clearances unless you make some notational provisions. We've covered the case in which designers set their solder mask clearances based upon checking in with their own favorite fab shops. is may work for the design in the proto stages, but if the PCB designer cannot check in with the production supplier who will be building the PCB in production, what is the point? e point is that far too many design- ers do not understand that just because "their own" custom prototype shop can accurately mask with nearly zero increase or "swell" of the solder mask opening doesn't mean that many volume-capable, offshore suppliers possess the capability. is fact is compounded by another fact that by the time the design goes to volume, the PCB designer's favorite proto-shop is long removed from the EMS assembly stakeholder which will be ordering the boards from their own suppliers which most certainly will have entirely different manufacturing constraints. When there is a lack of vision the people per- ish, says the proverb. So, who is "the hub" of the manufacturing process helping by interpreting DFM as "dic- tate for manufacturing" when they have abso- lutely no vision regarding the capabilities of the EMS provider's supply chain? Figure 5: Too much clearance. Figure 6: Clearance is just right and matches supplier capability.