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Repair of SMT boards

Marcos

#11161

Repair of SMT boards | 4 June, 1999

Where I can find some informations about repairing of SMT boards?

Thanks

Marcos

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Earl Moon

#11162

Re: Repair of SMT boards | 5 June, 1999

| 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

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#11163

Re: Repair of SMT boards | 5 June, 1999

| Where I can find some informations about repairing of SMT boards? | | Thanks | | Marcos | Marcos: Ya know, repair is that "dirty little" part of the business that all try to ignore, don't want to fund properly, and creats more angst than propably anything else. A short list:

1 Circuit Technology Center has an excellent down-loadable guide at http://www.circuitnet.com/repair1.htm 2 IPC (http://ipc.org) produces two excellent standards: IPC 7711 Rework of Electronic Assemblies and IPC 7721 Repair and Modification of Printed Boards and Electronic Assemblies 3 An excellent text available from SMTA (http://www.smta.org), SMTnet, or SMT Plus (http://www.smtplus.com/). "How to do SMT/FPT Rework and Repair," Blankenhorn, 5th Edition SMT Plus, 1996; 180 pages. (snipped from an SMTA review) "Introducing SMT can waste time and money if you don�t understand the changes in testing requirements, fixtures, techniques, and workmanship standards. This book will explain these changes and show how easy rework and repair can be. This edition contains new information on the soldering process, soldering temperature, flux types and interactions, cleanliness, standards, tools, soldering irons, soldering tips, budget planning, techniques, temperature settings, sockets, connectors, and repairing lifted lands. A must for both novice and experienced operators." 4 Training courses provided by the sources listed in the archives. 5 Equipment suppliers often offer training after purchase of their equipment. 6 Get scrap boards, an iron, a solder sucker, some wick, a good light, tweezers, and get to liftin� pads. 7 Our friend Bob Willis (http://www.bobwillis.co.uk/) sells several products 8 Other consultants offer excellent services

Good luck

Dave F

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Joe

#11164

Re: Repair of SMT boards | 7 June, 1999

| | 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 |

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Joe

#11165

Re: Repair of SMT boards...a question!! | 7 June, 1999

| | | 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 ? |

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Brian Wycoff

#11166

Re: Repair of SMT boards...a question!! | 7 June, 1999

| | | | 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 ? | | | One thing to keep in mind is the difference between rework and repair. I have often seen them used interchangably, which makes some customers leary.

IPC-700-C has some good repair procedures in it.

One reason we use underside heating od multi-layer pcb's is to provide enough heat to do the job properly. I have worked on many multi-layer boards that have multiple ground plane layers in the same area. In those cases, a soldering iron alone may not provide enough heat to rework the assembly without damaging pads and barrels. I have even used solder mini-waves to rework those boards. I have even used resistance soldering in rare cases.

Brian

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#11167

Re: Repair of SMT boards...a question!! | 7 June, 1999

| | | | 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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Earl Moon

#11168

Re: Repair of SMT boards...a question!! | 7 June, 1999

| | | | 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 ? | | | | Minimize thermal shock/stress to MLB's and components.

Earl Moon

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