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mosfet

I have this small mosfet with pins that are about 1mm in length that I need to solder to wires that are maybe 3 times larger than the pins.

Should I just drop blobs of solder on it or is there a better way to approach this problem?

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    \$\begingroup\$ Use smaller wire :) \$\endgroup\$ Jun 4 '12 at 17:30
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    \$\begingroup\$ What's the part number / SMD package? \$\endgroup\$ Jun 4 '12 at 17:30
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    \$\begingroup\$ I measured it and it's 12cm on my screen. That's not a small MOSFET! \$\endgroup\$
    – stevenvh
    Jun 5 '12 at 8:33
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I'd check out Proto-Advantage. They have a lot of nice SMT to DIP/SIP boards that are pretty cheap. Like Madmanguruman mentioned, it's far easier to properly solder the components to a board and utilize a plated through-hole for your wire.

Here is a board I found on their site that might work for you assuming the MOSFET you're using is a SOT-23 package. They have plenty of boards for other packages as well.

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  • \$\begingroup\$ Yes. Your part is a SOT-23 component, and so is this breakout. \$\endgroup\$
    – qdot
    Jun 4 '12 at 18:12
  • \$\begingroup\$ I'll also add that I've used Proto-Advantage personally and have had speedy shipments and a fulfilled order both times... so it's not just a random suggestion pulled from a Google search or anything. :) \$\endgroup\$ Jun 4 '12 at 18:19
  • \$\begingroup\$ Just be aware that the plated through-holes on these particular breakout boards will probably only fit 20AWG or smaller wire. \$\endgroup\$ Jun 4 '12 at 18:32
  • \$\begingroup\$ beldynsys.com and capitaladvanced.com are also good sources of these boards, which are sometimes referred to as surfboards although that's actually a trademark of Capital Advanced Technologies. \$\endgroup\$ Jun 4 '12 at 21:01
  • \$\begingroup\$ Soldering SMT components can be approached a multiude of ways. If the pads are big enough (i.e. you can press your soldering iron tip on them without pushing the component off) my personal approach would be to put flux on all the pads to help tack the IC in place and then get some solder on your iron tip and let the pad wick it up, which should give you enough tack to go and solder the other two legs without the IC shifting. \$\endgroup\$ Jun 5 '12 at 18:43
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You need to support the larger wire some other way than soldering to the pins. That wire can transfer enough strain to rip the pins off the part.

I suggest you glue the wire down, or wire-tie it etc, then make a small jumper wire to connect to the SOT-23 part itself. For soldering to the SOT-23 pin, I like small-gauge heat strippable magnet wire. Heat strippable means no nicks in the wire from stripping. Radio Shack sells a set of three magnet wire sizes, the smallest of which works well for this application.

Here is an example:

enter image description here

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  • \$\begingroup\$ +1 for the mention to the risk of pin loss. Just had this issue, with an SO-8. \$\endgroup\$ Jun 5 '12 at 6:46
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You could also pick up one of these for $0.95 from SparkFun. It's a little SOT23 breakout board.

enter image description here

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Make a little breakout board. RadioShack carries PCB kits. Cut the PCB into 1x1" segments and draw a shape like the attached image on it. Etch it, then drill holes from the underside to solder wires in.

smt pads

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  • \$\begingroup\$ Why drill holes? Just solder the wire on the copper side. \$\endgroup\$
    – stevenvh
    Jun 5 '12 at 5:03
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Ideally you'd have a breakout board to prototype this, but if you don't, this is what I've done for quick prototyping of small SMT ICs.

Use small wire: 30AWG wire wrap. Hold the part securely in a "helping hands" device or a small vise. I use a small Palmgren drill press vise as my general purpose soldering vise since it's not too big (about 3"x5") and heavy enough (about 5lbs) so it doesn't slide around on the table.

Wrap the wire once or twice around each lead and solder it in place. I use .031" (?) diameter solder for this. Now you can connect this very thin wire to a larger wire, such as 24g that's easier to work with. Alternately, you can solder the wire wrap wire to a protoboard to anchor the part in place for further use.

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Some people seem to be very handy with enamel coated wire:

enter image description here

Alternatively you can use wire wrap wire. 0.25mm and 0.4mm are standard diameters, and the Kynar insulation is easier to strip than enamel. (Real Engineers do that with their teeth.)

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At my workplace, we've designed little PCBs that have standard footprints much like the SOT23 MOSFET you're showing, with short traces that go to reasonable-sized plated through-holes. These are very useful for bench experimentation and doing rework on early prototypes, because the surface-mount device is securely soldered, the holes provide some mechanical support for the wires and the whole thing can be glued or RTV'd down without adding appreciable height.

It's almost impossible to reliably solder a wire to a small SM device, especially when the wires are much bigger than the part leads.

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...or you could do what @insta suggests by just taking a knife and scratching a pattern on copper clad board, then solder wired to that board. SOT-23 is huge compared to some other packages like SC70 or MSOP.

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