Cracking of the conformal coating shown in figure 1 was found after the circuit board assembly was returned from the field for investigation. The printed board assembly was in a medical application produced in a tin/lead assembly process in medium volume. Some form of contamination under the mask had led to cracking on the coating. There was no evidence of corrosion or electrical failure on the sample board assembly. Large amounts of no clean flux have been seen to cause lifting, not corrosion on boards. This is probably due to expansion and contraction of the material during thermal cycling. This movement of the material can lead to cracking of some brittle masks. It would be necessary to review the board cleanliness and the coating compatibility with the intended application. Temperature cycling is probably the cause for the cracking but something under the mask had to cause the mask to expand and contract. If a mask is cracked it cannot provide suitable protection to the surface of the board. If a no clean flux was the root cause the volume of material left on the surface of the board during assembly needs to be reviewed. The NPL database is a unique resource for readers to search for solutions or submit problems online for possible solutions at
http://defectsdatabase.npl.co.uk/
EPP Europe 409
