| | | | Where I can find some informations about repairing of SMT boards? | | | | | | | | Thanks | | | | | | | | Marcos | | | | | | | A first place to start is IPC's 7711 and 7721 encompasing repair, modification, and rework procedures. | | | | | | Earl Moon | | | | | While we are on the subject... would any one care to answer why we use under side heating of multi layer pcb's ? | | | | Joe: OK, consider this, your board is a multi-metalic strip. (I just made-up that term so don�t reuse it without permission.) This multi-metalic strip is very similar to the more renowned bi-metalic strip used in household and other heating and cooling systems.
You remember how a bi-metalic strip works. Right?? Good, because I barely do!! Bi-metalic arms are made from strips of two carefully selected, dissimilar metals, laminated together. When heated, the two metals expand at different rates, which makes the arm bend in the direction of the metal which expands the least.
When cool, the bi-metalic arm makes contact in an electrical control system. Current flows and all that. When the bi-metalic arm bends, due to heat, it breaks that contact, current stops flowing, and the control system turns something on.
Now, think about the number of layers of materials in a multi-layer board!!!! Each circuitry layer has two layers of copper, each layer of copper has an adhesive layer attaching the copper to the core, and then there�s the core. And often cores are different thicknesses. Next, there�s the c-stage resin attaching the layers together. There�s the solderability protectant on the pads. There�s solder mask and components bonded to the board. And on and on. All with different coefficients of thermal expansion and thermal conductivity.
If you heat one side of this lamination, but not the other, the lam will tend to bend. This bending of each of those bi-metalic arms generates a small electrical voltage in each arm. Small in each arm, yes, but when all these small voltages are added together, it becomes huge. Sometimes on the order of thousands of volts. I don�t have to tell you about the severe personal hazard this presents to rework techs without the proper safety gear.
One tech without thermodetic gloves and head gear that takes a "hit" while working on a board without bottom-side pre-heating and you�ll have OSHA (US work regulation compliance people) crawling all over your plant like ants. I can tell you other stories about companies that were shut down because of this.
Sure, you have a choice: (1) bottom-side preheat, (2) proper safety gear, or (3) OSHA. Not a pretty choice. And not to say that it�ll happen every time, but sooner or later it�ll catch-up with ya. Fore warned is something or other ... ;-)
Darn, I almost pulled that off (so to speak) without smirking.
If you can rework boards quickly, without delamianting the boards, don't preheat the bottom side.
Ta
Dave F
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