SMT007 Magazine

SMT-Sept2018

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32 SMT007 MAGAZINE I SEPTEMBER 2018 on-demand use. In the Class II and Class III world, it's a megadifferent story. For example, demands for safety and hardware reliability in the military and aerospace segments are every- thing. Assembly parameters are traditionally deter- mined using manufacturers recommendations, trial-and-error methods, and in some cas- es, tribal knowledge. No matter how you go about that task, the important part is to look at supporting analytical data. No-clean flux is a prime example of a material that needs to be exposed to enough thermal energy for enough time to render the residues near benign. When you increase the belt speed to increase num- bers, you will also alter the chemical composi- tion after reflow soldering. A few years ago, I put together a study to determine the effect of speeding up the belt on a reflow oven on cleanliness. With the first group of test boards, I used the manufacturer's recommendation—maximum ramp of <2°C per second with a dwell time of 30–90 seconds at peak temperature. This was achieved with a belt speed of 1.5 FPM and a peak tempera- ture of 250°C was reached with a dwell time of ~60 seconds. Ion chromatography was per- formed at multiple locations and component types. Table 1 shows the ionic content for what we consider to be acceptable for normal field service operation. With the second group of test boards, I used a belt speed of 2.0 FPM and a peak tempera- ture 10°C lower than the manufacturers recom- mendation. The integrated circuit (IC) analysis in Table 2 shows elevated ionic content that is meant to be outgassed under temperature or fully bound in the outer resin/rosin shell of the flux. Ionic content at these levels increases the risk for electrical leakage with normal avail- able atmospheric moisture. The increase of belt speed will pump out more parts, but at what cost? Being able to pro- duce more parts in the same amount of time is always going to look great to the custom- er but needs to be judged against the amount of returns with "no-trouble-found" (NTF) des- ignations or hard failures. NTF is a megabad condition because when a part fails in the field, it can't be reproduced, which makes finding the root cause nearly impossible. When a part returns to a repair depot, the low hanging fruit becomes items such as missing components, ICs that needs to be flashed, or beer stains on the board—you know, standard stuff you see from field returns. Active flux residues aren't something you can see under a scope at 10x. You can see Table 1: Ion chromatography for the first group of test boards.

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