I have had two experiences with shorting under the capacitor, and the adhesive was a contributing factor(at a previous company). 1. A dual dot pattern was used to secure the component to the PCB. We switched from an adhesive developed for low volume dispensing to an adhesive designed for high speed dispensing. Due to the different chemistries and viscosities, when the new adhesive was used and the component placed on the adhesive, the adhesive kept its two distinct dot shapes, it did not always get "mashed" together. ( With the previous adhesive, when the component was placed, the adhesive "flowed" together to make one big mass.) This left a small channel between the two dots, under the component. During the wave solder process, flux would be sucked under the component, between the dots (due to capillary action). The solder would then also wick under the component, following the flux. Usually a small amount of solder was left under the component, and this shortend the distance required to create a short under the component. For the short to occur, a number of variables have to fall into place. This problem occured on a random basis, but was usually limited to the same components. For the short to occur, the circuit has to have a voltage applied. The solder under the component decreased the distance between the ends of the component or pads for the short to grow, the remaining flux under the component,, moisture from the board, or outside humidity, will provide the medium for a small dendritic growth to complete the short. If the component is shifted on the pads, it decreases the distance required for the growth also. Like I said, this was a random occurance, it took us a while to solve the issue. We solved the problem by changing to a single dot pattern (but see the next experience below). Another solution is to put a dummy trace between the two pads, under the component. The dummy trace and the dual dot pattern acts like a dam. Several SMT books discuss this solution. 2. About a year after solving the above problem, switching to a single dot, the problem occured once again. The root cause was voids in the cured adhesive dot and the size of the dot. If the dot was slightly oversized, and was touching one of the pads, during the wave solder process, solder would wick into the voids from the molton pad and travel under the component due to the capillary effect of the voids. As stated above, this shortened the distance required to create a short under a component. When the dot size was such that it touched both pads, we had actual shorts. Once again, this occured on a random basis. The voids in the adhesive occured due to improper storage; it froze, laid on its side. etc. We also discovered a certain amount of moisture inherent in the PCB, especially those from overseas (solved by requiring sealed desicatted shipping packages and proper inhouse storage). We also switched to a improved adhesive from the same company that was designed to be less hydroscopic (obviously others had seen the same problem and they had developed a new formulation). The cure profile was also a contributer. Instead of curing quickly at a high temperature(150?C, we cured at a lower temperture 130?C for a slightly longer time. At the high temperture, it was determined that the entrapped moisture in the PCB was flashing to steam, and sometimes the vapor path from under the solder mask was through the adhesive dot. This created voids in the glue as the adhesive cured. The lower temperature allowed the moisture to escape while the adhesive was still soft, so it would eventually solidify into a solid dot. However, the key improvement was reducing the size of the dot so it never touched the pad. You can have voids in the adhesive, but if it never touches the pad, the solder will not wick into the dot. General Observation: The capacitor short could be caused by a number of reasons. In this case, several variables worked together to create the problem on a widely random basis. The voids in the adhesive, the dot size under the component, the shifted component and the environment. The solder wicking under the component shortened the distance required for a dendrite to grow. In this case, the humid environment, the contaminants on the board or under the component, the shortened distance and a constant voltage provided the means for a dendritic growth under the component. For your information, you can take a capacitor, apply 12 volts across it and place DI water to the top, and eventually, you will create a dendritic growth (try it, it is fun to watch) Hope this helps, you can contact me for more information if required. John Maxwell (SMT Consultant) helped us solve this issue several years ago. He monitors this forum also. Ed Holton Hella Electronics [log in to unmask] ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################