SMT007 Magazine

SMT-July2016

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July 2016 • SMT Magazine 51 The challenge of achieving stable and con- trolled condensing conditions over large areas is understood by a consideration of the dew point and the onset of condensation. This work shows that the SIR of the uncoated cou - pons, begins to drop noticeably when the tem- perature of the platen was dropped below the dewpoint. At 40°C and 85% RH the dewpoint is 37°C, and the onset of degradation was no- ticeable at 36.5°C. At 40°C/93% the dewpoint is 39°C and the SIR drop was noticeable at 38.5°C. These results are important in show- ing that there is good agreement between the platen and dew point temperature, and con- firms that the platen arrangement is perform- ing as expected. The experiments described here show clear- ly that while test boards may pass simple hu- midity testing, as the geometry of the surface becomes more challenging test boards will fail under condensing conditions if complete cov- erage is not achieved, as was observed with the acrylic-1 results. In Figure 8 the SIR results for acrylic-1 are very good, for the 1-oz copper tracks in Figure 9, there is some susceptibility to condensation cycles but the SIR values are still high and above 109Ω. However, in Figure 12 with the 3-oz copper track the SIR values are now at the 106Ω level, which is classed as an SIR failure. Hence, the difficulty in achiev- ing good coverage with these acrylic materials became apparent as the copper track thickness increased, and even with the application of two separate coating layers the performance was problematic. These results clearly demonstrate the need for condensation type testing for finding the weaknesses in coating coverage, and further- more the control that can be achieved with this experimental setup allows this to be read- ily achieved than has hitherto been possible. However, while specific condensing conditions can be achieved the desired test parameters still need to be developed, and this will be the scope of further work. Conclusions A review of condensation testing reveals that there are many approaches to achieving the re- quired condition. These approaches struggle to achieve a known and uniform steady state across the test vehicle, and typically attempt to achieve condensation while fighting the inher- ent control system of the humidity chamber. CONDENSATION TESTING—A NEW APPROACH Figure 13: The 3-oz SIR pattern coated with acrylic-1 with evidence of corrosion from the track edges.

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