Issue link: https://iconnect007.uberflip.com/i/713934
72 The PCB Design Magazine • August 2016 other sites had no issues to report regarding the coating, and they confirmed that they were ex- periencing neither the delamination problems we were investigating, nor indeed any prob- lems related to a lack of adhesion. It was time to check the records. Following a search through their database, the customer discovered they had coated four different styles of boards using the same batch of conformal coating material. Significantly, however, all of the adhesion issues were con- fined to a single board style. Moreover, these adhesion issues were first reported and recorded at about the same time. On checking the serial numbers of the coat- ed boards, it became apparent that the adhesion issues all began on a batch of bare boards that had been obtained from a new vendor as part of a cost-cutting measure. Samples of these in- coming bare boards were subsequently analysed and found to have a surface energy of 26 dyne/ cm (i.e., much, much less than the 40 dyne/cm level recommended for good adhesion). Our customer then decided that perhaps it was high time to audit the new board supplier. During this process, they found many corners had been cut to save time and money, but two things, in particular, stood out. Firstly, the board shop was performing multiple passes through a UV oven to "cure" the solder resist, instead of the single recommended UV pass and a second- ary thermal bake. Secondly, the auditor noted a "shiny film" floating on the surface of the rinse bath during the final rinse process. Indeed, each UV exposure was found to re- duce the bare board surface energy by two dyne/ cm, while the final rinse was found to contain significant amounts of oils and lubricants used during the manufacturing process. By eliminat- ing the additional UV processes, implementing the recommended thermal cure and ensuring a clean final rinse process through careful moni- toring, the supplier was able to produce boards with a requisite surface energy greater than 46 dyne/cm—enough to ensure excellent adhesion of the conformal coating. Quite apart from the important role that good record-keeping played in this customer's problem-solving exercise, there are a number of lessons to be drawn from our customer's experi- ence which, I hope, you'll take onboard when you embark on the search for new suppliers. Firstly, incoming bare boards will provide an important determinant as to whether or not your coating achieves good adhesion. Remem- ber, perfectly sound incoming boards can be made bad during the SMT assembly process, but bad incoming boards can never be made better! Secondly, in your purchase specification, state the level of surface energy you require for incom- ing boards (generally greater than 40 dynes/cm) and TEST all incoming batches. REJECT any batch if it does not meet your specifications. Thirdly, do not be afraid to audit new sup- pliers and, in particular, their manufacturing processes. This is the only way you will feel comfortable that they have a full understand- ing of what they are making, and how it might impact you should their manufacturing param- eters drift out of specification. Well, that's all from me for the time being and I hope you have found my columns on coatings helpful and of interest. Next month, my very able colleague, Alistair Little, technical director of Electrolube's global resins business, will be hosting this column. Alistair's technical capabilities cover a broad range of related chemistries, including benzox- azine and phenolic resins for high-temperature applications; epoxy, epoxy meth/acrylate, ure- thane meth/acrylate, silane terminated poly- mer, polyurethane and unsaturated polyester systems. Alistair is also an expert in vinyl ester resin systems, and he is a member of the Royal Society of Chemistry, so rest assured the tips and tales from him will be expertly documented. So, please do look out for his new column, which will focus on everything resin-related, including the types of products available, their special features and the issues that manufactur- ers might have to face when using them. Have a great summer and thank you for all your feed- back. PCBDESIGN Phil Kinner is global business/tech- nical director of the Coatings Division at Electrolube. CONFORMAL COATINGS: BEWARE THE BOARDS THAT 'BARE' ALL!