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Rework PCB's with coins

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CJ

#61364

Rework PCB's with coins | 15 March, 2010

Does anyone have experience with reworking PCB's with coins bonded to the them? I have been having trouble with coins loosing the bond after rework and falling off. The material bonding the coin to the board is suppose to be able to withstand temperature up to 300C. I have been monitoring the temperature of the coins and the max temperature I have reached is 220C, but 2 out of 6 boards the coin has come off. Any tips would be appreciated regarding this problem.

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#61412

Rework PCB's with coins | 19 March, 2010

Could it be a CTE mismatch issue?

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#61418

Rework PCB's with coins | 19 March, 2010

We can't let this thread pass-by without trying to learn more about these 'coins.'

What's the deal? * Are they like currency, money? * Are they coin-shaped metal slugs? * Why are they on a circuit board? What's their purpose? * Can you attach a picture?

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CJ

#61419

Rework PCB's with coins | 19 March, 2010

Coins are sometimes used on PCB's with RF Power Field Effect Transistors. These boards are usually mounted in an housing. They provide a ground path for the transistor to the PCB and thermal path to the housing. The coins are usually made of aluminum or copper. The coins I am working with are made of aluminum. The attached pictures are an example of want I am working with.

Attachments:

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#61420

Rework PCB's with coins | 19 March, 2010

The aluminum material is typically plated by a zincation process, followed by nickel plate and a gold flash layer.

What do the soldered side of the component and the soldered side of the aluminum coin look like? Do they both take solder?

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CJ

#61421

Rework PCB's with coins | 19 March, 2010

Both the coin and the device flange that is soldered to the coin have a gold finish. A solder preform is placed in the pocket of the coin before the device is placed prior to reflow. Solder adhere's to both surfaces

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#61422

Rework PCB's with coins | 19 March, 2010

That's how it's supposed to work, but is working correctly on the components that are loosing heat sinks for you? We have two choices: * Poor solderability of either the component or the heat sink ... OR * Wrong solder preform material

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