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48 SMT Magazine • November 2014 specifically for civilian airborne radio commu- nication and long-haul flights is the tuning control unit: a circuit board fitted with several hundred tiny plastic-coated SMDs (surface- mounted devices). The function of this SMD as- sembly is to reliably tune the antennae, thereby ensuring overall radio communications. Unexpected adhesive Problem Conformal coating came to an abrupt end one day when it was discovered that the trans- parent protective coating had lifted on around 50 transistors. Since nothing whatsoever had changed in the assembly, pre-cleaning or coat- ing process, it became apparent that the root of the problem could only lie with the com- ponent material itself. And sure enough, when questioned, the supplier confirmed that it had changed the composition of its plastic blend. This is a problem frequently encountered by processing companies that are reliant on plas- tic components produced externally since even the slightest alteration to the composition can be enough to totally change the surface char- acteristics of the material. This news from the supplier was alarming, because there was no alternative to the new plastic blend. There was simply no other certified manufacturer avail- able who could produce these particular elec- tronic components. "We had to find a solution as quickly as pos- sible to ensure the adhesion of the conformal coating," explains Michael A. Schneider, me- chatronics engineer in charge of production technology at Rohde & Schwarz. "Without a reliable adhesive bond it would no longer be feasible to continue manufacturing the tuning control units." surface activation As is well known, materials are made more receptive to adhesion by means of activation; in other words, by pretreating them so as to in- crease their surface energy. This is the most im- portant measure for determining the probable adhesion of an adhesive layer or coating. Reli- able, long-time stable adhesion is conditional on the material surface being ultra-clean and the surface energy of the solid material being higher than the surface tension of the liquid—in this case the coating. Various pretreatments are avail- able to achieve these two conditions, with wet chemical substances still the most widely used. Difficult choice But finding the right pretreatment for these highly sensitive electronic components seemed almost impossible at first. Schneider: "Activa- tion using a solvent-based primer was not an option for us. Partly because these substances are extremely harmful to the environment and partly because they would incur enormous costs in terms of health and safety (e.g., explosion protection) and disposal." The electronic engi- neers also ruled out laser pretreatment on the grounds that the uneven surface of the mate- rial would have made the coupling efficiency unpredictable. CO 2 snow blasting, which cleans but has no activation capability, was also reject- ed. The final method under consideration was a low-pressure plasma treatment; a highly effec- tive activation process, but not suitable for this purpose because the vacuum would have drawn the fluid out of the wet electrolytic capacitors contained in the SMD assembly. "We seemed to be very far from finding a way around this avionics: atmosPHeric PressUre PLasma case stUDy continues arTiCle Figure 2: avionics specialist Michael Schneider (left) and plasma expert Peter langhof (right) standing in front of the openair-Plasma system. (Photo: Plasmatreat)