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Schemes you're a part of

By openschemes, on October 20th, 2009%

4,386 views

If you’ve read the first part of this series, you already know how to remove an IC from a printed circuit board. Specifically, the IC we removed in our example is an EEPROM used as a BIOS, so we will need to replace it in order to get the device to function again.

We don’t want to risk having to rework the board again for a second EEPROM removal, so it is desirable to place a socket into the old EEPROM’s location so future changes are a little bit easier. If you’ve considered this yourself, you may have thought about building an adapter from the SMT pads on the PCB to an IC socket – and you would have been right! But there’s an easier way, especially in the case we have here where there are SMT sockets available that fit the standard footprint of the IC we are replacing.

A few PLCC Sockets of the SMT style, suitable for use as a replacement.

Fig 1 – A couple of PLCC Sockets, SMT style

Hey, look at that! These sockets will drop right onto the PLCC footprint. Another hot air rework and you’re done. However, trying to hold a socket in place while you apply hot air from a heat gun is tricky business with a high risk of cooked fingers. There’s also the nearby BGA chip that we wouldn’t want to have to cook again for very long, so we present an alternative route: Break out the flimsy bottom of the socket and simply hand-solder the socket the the PLCC pads. It’s super easy and there’s virtually no risk of affecting the other parts on the board.

PLCC Socket with Floor Broken Out

Fig 2 – Modified PLCC Socket

The bottom of the socket is exceptionally easy to remove – simply flip the socket over and press down with a screwdriver on the center of the socket. It should crack out nicely, although there will probably be a few plastic sprues holding bits of the bottom in place. These can be cleaned up manually. Try to press in the center when breaking out the bottom to avoid bending or damaging any of the socket’s leads.

Continued on Next Page…

By openschemes, on October 20th, 2009%

Heat Gun Homebrew SMT Rework Tutorial

By openschemes, on October 16th, 2009%

6,328 views

Hey, if you came from a direct link, be sure to check out part 2: Installing a Socket on this PCB

Since it’s a Friday and we’re all taking it easy for a change, we’ll take this time to talk a little bit about using an off-the-shelf heatgun to work with SMT devices. I know this is a topic that’s been covered before on the web, but usually the instructions given are something like: “Just use a heat gun, dude”. A little light on the details, to say the least.

We’re here to tell you that it works. Very well actually, and we’re proud to say that when we’re evaluating a new IC this technique is essential. Perhaps you’re abusive and tend to kill a lot of chips, or perhaps you just want to see the statistical variation in 10 or 20 IC’s – you’re going to be doing a lot of chip removal and resoldering.

Trying to do it with a soldering iron may work for something like an SO-8, but if you start talking about >16 pins or any type of quad package – forget it! The extended heat you put on the board will burn the PCB and even worse – cause the traces to peel, ruining that board! So if you don’t want to build 20 boards for 20 IC’s, you need a simple and fast method that does not damage the PCB.

Heating the PCB with a heat gun to remove IC's

Fig 1 – Skipping ahead to show the action shot

Now it doesn’t matter much WHICH heat gun you use, although it should be able to produce a good bit of air at >300C. In our experience, long heating times due to using a weaker gun are the biggest cause of PCB burn, bulge, or trace peel. Your mileage may vary, but we prefer to use both the highest air and highest heat setting in order to melt the solder on our target device in the shortest time – lowering the chance that the solder on the neighboring device has also melted.

A drawback of high air setting is the risk of blowing neighboring devices (usually tiny SMT resistors or capacitors) off their pads and maybe even off the board! You will need to weigh the risk at rework time and choose your airflow accordingly – if you have a device that’s simply SURROUNDED by tiny SMT caps, you may want to use lower air or make a foil cone for your heatgun to contain the hot air to the desired area. Here’s the heat gun we used here, peak temp is about 550C.

Fig 2 – The Heat Gun Used to Remove SMT Components

There’s really nothing magic about this heat gun – we’ve used any old gun the lab had on hand: Some piece of junk Sears gun, a paint removal gun, chinese no-names. It’s more the strategy of getting in, liquifying the solder on the component you want while minimizing the effect on nearby components, and getting the target component out. You’ll need a pair of tweezers in addition to the heat gun, unless your fingers are really strong. Kidding, kidding – get the tweezers.

Continued on Next Page …

By openschemes, on October 16th, 2009%

The Cyrustek ES51925 DMM IC

By openschemes, on September 15th, 2009%

1,785 views

The Cyrustek ES51925 DMM IC is used in the MS8910 Smart Tweezers featured in our hack.

This chip is manufactured by Cyrustek, and includes most of the active circuitry needed to implement a simple DMM. The external circuitry required is mostly filtering and scaling stuff using passive components.

For the full description of the smart tweezer hack using this chipset, please look at our old project page at . . . → Read More: The Cyrustek ES51925 DMM IC

By openschemes, on September 15th, 2009%

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Installing a PLCC EEPROM Socket onto a MOBO

Heat Gun Homebrew SMT Rework Tutorial

The Cyrustek ES51925 DMM IC