SMT007 Magazine

SMT007-Nov2018

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80 SMT007 MAGAZINE I NOVEMBER 2018 ing environment. These materials can include, but are not be limited to, the following items: • Underfill materials underneath the components • Staking materials on the PCB • Insulative coating materials • Conformal coatings • Thermal grease These materials can dry up, become non- functional, soften, or migrate to other areas of the assembly, which all have a detrimental effect. For example, underfill materials have a softening temperature below that of the reflow temperature of the solder. When an under- filled device reaches this temperature, it push- es out solder that reaches reflow temperature and causes shorts underneath the neighboring device. Thus, it is important to know the mate- rials on the board and be aware of technical information on the datasheet from the materi- al supplier or have experience about the mate- rials' properties. There are numerous steps the rework tech- nician can take to avoid these neighboring reflow "gotchas." One of the first steps to pre- vent having to scrap a board or perform rework a second time (if allowed by the customer) is to investigate the materials and components on the board before starting the rework oper- ation. It is better to take a moment to ques- tion the material composition of nearby com- ponents and pull a few component or material datasheets then to start again. After the risk areas are identified, protect at-risk parts by either removing or shielding them. The use of ceramic non-woven tape, a metal shield, or shield gel has been shown to be the most effective thermal shields [1]. Final- ly, placement of thermocouples around the component being reworked when performing rework profiling will help identify issues. If a hot-air or an infra-red (IR) reflow source is used, this will help to pinpoint potential prob- lem areas instead of guessing. The reflow of surrounding devices in the rework area is a yield detractor in PCB rework. If you are not careful with the reflow source being used, nearby materials may be destroyed or cause the PWB not to meet the initial spec - ifications or design of the assembly. Howev- er, careful planning, shielding, and sometimes removing neighboring device or material will ensure the highest possible rework yield. SMT007 References 1. Shielding Effectiveness of Polyimide Tape During Rework by Gaynor Adam and Robert Wettermann, Circuits Assembly, October 2014. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. To read past columns or contact Wettermann, click here. Figure 2: Use shielding materials, like gel, to mitigate the effects of heat in the rework area.

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