SMT007 Magazine

SMT-June2016

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98 SMT Magazine • June 2016 When performing rework on printed circuit boards, the issue of the moisture having to be baked out of the PCB is often debated. Whether it is localized mini-wave rework, hand-soldering or convection rework, the board, as well as neighbor- ing components, needs to be taken into account in terms of moisture protection during the reflow cycle. The board components' protection of mois- ture ingress and subsequent part "popcorning" or heat damage is governed by the IPC-J-STD-033 Handling, Packing, Shipping and Use of Moisture/ Reflow Sensitive Surface Mount Devices, whereas the board moisture ingress and its potential mea- sling or delamination is governed by the IPC-1601 PCB Handling and Storage Guidelines. Along with the components mounted to the PCB, in some cases, the boards need to be considered as MSD devices. In the case of PCB rework, the component to be removed is typically going to be discarded, which means there is no concern with the removed part having moisture damage. If the device is to be re - moved and sent back for further failure analysis, then baking of the board/parts is recommended. If you look at the rework guidelines from TI, Kyo- cera, Intel and other major component vendors, you will find that they recommend to bake the boards and parts prior to another reflow. In addi- tion, care must be taken to make sure that parts that are temperature-sensitive, such as batteries, capacitors, plastic connectors and under filled or glued components, may need to be removed or protected correctly. As part of the process the board rework areas should be properly masked and shielded so that the components are protected from reflow tem - peratures. There are a variety of materials that can serve to protect neighboring devices (Figure 1) in order to make sure they remain unharmed during the rework process both from a parts removal and replacement standpoint. Shielding of the compo- nents so that they remain undamaged by localized reflow temperatures may be necessary depending upon their proximity to the reflow location. Other reflow sources or complete part removal are other strategies that will protect the components from thermal damage or MSD exposure. After baking is complete, make sure to prop - erly tag and mark the board or parts such that the exposure history can be documented. Place these components or boards into a dry environment such as a dry box in order to house the parts or boards prior to being exposed to the environment again. An alternative is to place them in a mois- ture barrier bag, and putting desiccant and a mois- ture indicator card on the inside prior to sealing in order to have solid MSD controls in place. Review of the IPC/JEDEC J-STD-033C The J-STD-033 describes the handling, pack- ing, shipping and use and reflow of moisture-sen- sitive electronic components on PCB assemblies. Keep in mind the handling of the components themselves and how sensitive to heat damage they may be by referring to the IPC-J-STD-075, Classi- fication of Non-IC Electronic Components for As- sembly Processes. by Bob Wettermann BEST INC. To Bake or Not to Bake (in Rework)— That is the Question KNOCKING DOWN THE BONE PILE Figure 1: Shielding gel protects heat-sensitive components in areas close to rework.

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