SMT007 Magazine

SMT-Sept2018

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SEPTEMBER 2018 I SMT007 MAGAZINE 33 Using a thermal profile system on the actu- al product is the only way to effectively mon- itor the temperatures at locations that are known to be temperature sensitive. Using a bare board or thermocouples in air will tell you what the temperature is in the oven, but not what the actual temperature is on your product. This will certainly be a trial-and- error effort until you find the sweet spot of throughput, acceptable solder joints, and benign flux residues. The megatakeaway from this month's col- umn is the final reliability level of an assem- bly relies heavily on the thermal profile used for soldering. When the peak temperature is Using a bare board or thermocouples in air will tell you what the temperature is in the oven, but not what the actual temperature is on your product. Table 2: Ion chromatography for the second group of test boards. flux residues, but visual inspection alone isn't enough to determine if they are conductive or corrosive. When a board gets labeled with the NTF designation, many times, it's chalked up as a loss with no further follow up. That is most likely because the active flux res- idues will absorb loads of moisture and cre- ate an electrical leakage plate that will disap- pear as soon as the moisture dries out during shipping. The repair depot looks at the return and everything seems to be in working order. This condition will only increase with the act of increasing throughput without doing any of the hard work to determine if it's detrimental to the product. However, there are probably parameters that will allow you to increase the through- put while not producing PCBAs that aren't megaquestionable. Increasing the temperature in the preheat zone is one way to do it, but it's important to look at the recommended rise rate from the manufacturer of the flux and the component manufacturers. Sudden increases in temperature can crack some types of com - ponents, including LEDs, aluminum capaci- tors, fuses, inductors, and transformers with wire coils. It will do you no good to increase the throughput if you are destroying the com - ponents.

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