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I'm looking to replace a resistor on a key sender unit from my Mum's car. The part is about 2 mm and too small for me to solder. Can I replace it with one that I can work with easier as it is bigger? I'd like to replace it with one with terminals so I can solder to the board.

Similar component shown here:

http://www.martingardner.com/MM/cap01.jpg

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    \$\begingroup\$ Is that a picture of the actual component? THat is a (100K) resistor as mentioned. (non polarised) SMD capacitors do not (usually) have any digits on them, frequently are a beige or grey colour. There are things like SMD ferrite beads,inductors,etc which can look remarkably similar, so a photo of the actual part (if it's not) is essential (and even then there is no guarantee of knowing for sure what it is :-) ) \$\endgroup\$ – Oli Glaser Aug 27 '11 at 11:10
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STOP !!!

Even though it may SEEM harder, you will probably find it easier to solder in a replacement part that is the same. You will never regret being able to do this once you learn how. If you have the original part still you can probably refit it this way.

ENABLING TECHNOLOGY :-) Get some needle point tweezers. They can be bought for very little cost. Like this or even "sharper". I have more thumbs than many people but can hold and locatesmall SMD components with tweezers like this.

enter image description here

Get some good flux. MUST NOT BE ACID FLUX. At a pinch you can do without the flux but it helps vastly. Use a piece of wire or matchstick or similar to put a small amount of flux on each pad.

Note that the following is probably not the method you will find recommended in most places and probably not what you'd use long term. BUT it works well with little or no experience. Using needle point tweezers rather than mastering solder surface tension is the key to not having thje resistor stick to iron, flip vertically, wander off across board etc.

  • Tin pads with std solder, nice shiny bloblet but not excessive or very lumpy.

  • Use tweezers to hold component and place in position. You probably don't need to see in detail as as long as you can locate it well enough the soldering step will work OK.Hold at mid body so pads at either end are not in tweezer contact. This is why you have needle points. This is much easier than you expected !!! :-).

  • Use smallest soldering iron tip you have. If you only have "fence posts" wind some thick copper wire around iron tip, cut a SHORT extension at end of coil to use as tip. Tin with solder. Make sure you are getting good heat transfer into coil if you use that method. A small blob more flux now will help.

  • Use tweezers to hold resistor in position against pads. Will probably be above usual location due to new solder.

  • Heat one end until solder "gives" plus a little longer. Keep some pressure on resistor so solder melted end presses into molten solder. Note that you can do this OK with a MUCH larger iron than necessary.

  • Allow solder to set (second or few). Keep holding component with tweezers and heat other end. You MAY want to add a little solder. May.

End result will not be beautiful and resistor may not sit down square but this should work for you.

Others are welcome to critique this as a recommended beginners method.
Remember what can go wrong for beginners with other methods.

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  • \$\begingroup\$ I think the easiest way to do small SMT parts (or just about any SMT part) is (1) holding solder wire in one hand and an iron in the other, add a blob of solder to a single pad (2) put down the solder wire and use tweezers to pick up the component. Slide the component into the solder blob while heating the blob with the iron (3) put down the tweezers and get the solder wire. Add solder to the other pad(s) (4) re-melt the first blob and use fresh solder wire (or really the flux in it) to get a nice shiny finish. If needed, use solder wick to remove the blob if it gets out of hand and redo it. \$\endgroup\$ – darron Aug 29 '11 at 1:18
  • \$\begingroup\$ @darron - Our methods are close. I recommended pretinning - I agree that using solder and no tweezers for pad 2 is useful if you are at least somewhat competent. // My instructions were for a person doing this for the very 1st tome. Main difference is retaining tweezer hold for second pad soldering. For a beginner this was so that overheating (which happens) which allowed remelting pad1 could otherwise result in movemnt of and possible total loss of the part in several possible ways. (Flicks away, sticks to iron tip, moves away, tombstones, ...). // Once confident, your way is good. \$\endgroup\$ – Russell McMahon Aug 29 '11 at 1:56
  • \$\begingroup\$ hmm... I don't remember having a problem melting the first pads while doing the second. It's possibly because I learned that trick after I'd already been soldering through hole a little bit. I'll take your word for it. I guess as long as your not trying to hold three things it's a good method. :) \$\endgroup\$ – darron Aug 29 '11 at 23:23
  • \$\begingroup\$ I'm too tin an only pad, place resistor, heat this pad and block resistor. Then go to solder other pad, but keeping holding component with tweezers, because in litle SMD resistor the heat could melt very easily the pad yet soldered. I solder this way also SOIC, tinning first pad and last, for block chip in correct position. \$\endgroup\$ – Antonio Jul 4 '16 at 7:46
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SMD resistors and capacitors may look similar, but a big difference is that capacitors almost never have their capacitance printed on them, while resistors always have their resistance value on them. So that should tell you whether it's a resistor or capacitor. (Also, most resistors are black, and capacitors beige.)
If it's a resistor the 3 digits printed on it represent its resistance value; the first two digits are the significant digits, while the third indicates the power of ten for the multiplier, i.e. the number of zeros following. In the given picture is shows 104, meaning 10 \$\times\$ 10\$^4\Omega\$ = 100k\$\Omega\$.
You can always safely replace a given size resistor with a larger one (same resistance, of course), since the larger version will be able to dissipate at least as much power as the smaller one. (In this case \$-\$ low voltage + very high resistance \$-\$ it would be even safe to use a smaller resistor, but you made clear that that's not an option.)

edit
I just wanted to add about soldering SMDs when I read Russell's answer. Russell always has good and detailed answers, but this time I don't quite agree with his choice of tweezers. I use Erem 102ACA tweezers, which are not pointed, but have the following tip:

tweezers tip

(the visible part is in reality a bit less than 20mm long.)

I find them to give me better control than needle shaped tweezers and I can even solder 0402s with them. (disclaimer: I consider myself clumsy)
If you have a good pair of tweezers and a fine tip temperature controller soldering iron you may have more success soldering an SMD part than a much bigger leaded part, particularly an 0805 should give you little trouble.

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  • \$\begingroup\$ Thank you so much, I'm off to buy a resistor with legs, this one is about the size of a pinhead and under a magnifying glass the soldering iron tip looks like a lamp post coming in, I appreciate the help, thanks again. \$\endgroup\$ – Martin Gardner Aug 27 '11 at 11:34
  • \$\begingroup\$ @ Martin - You can measure the component (roughly, a ruler with mm markings should do) to figure out what package type it is. 0805 mean 8x5 mils (1/1000th inch). Common sizes are 1206, 0805, 0603, 0402, 0201 (last two not really feasible for normal hand soldering) 1206, 0805, 0603 translate to 3.2mm x 1.6mm, 2.0mm x 1.2mm, and 1.5mm x 0.7mm respectively. It is usually possible to solder one size up onto the smaller footprint (e.g. 0805 component onto 0603 footprint) Personally I would probably take this route than try to solder a leaded component to SMD pads. \$\endgroup\$ – Oli Glaser Aug 27 '11 at 12:05
  • \$\begingroup\$ Hi Oli,Just measured the resistor, it is 2mm in length. \$\endgroup\$ – Martin Gardner Aug 27 '11 at 12:17
  • \$\begingroup\$ @Martin - then it's an 0805. Actually the 08 and 05 are hundredths of an inch, thousandths. \$\endgroup\$ – stevenvh Aug 27 '11 at 12:25
  • \$\begingroup\$ @ Stephen - you're right, my mistake - they are 1/100ths of an inch (10 mils) so 80 x 50 mil for 0805. \$\endgroup\$ – Oli Glaser Aug 27 '11 at 12:33
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It is possible to replace an SMD resistor with a larger one, and it can all be done with a regular soldering iron.

The short description of how to do this is to remove the chip by adding solder while pushing on it with the iron. Once it is removed you use solder wick to clean the pads and then add fresh solder to the pads. Then you just rest the end of the wire from your larger part on the solder blob and push it in with the iron.

A full description of this process is here in my blog post: Fixing the contrast on a Grove LCD RGB Backlight

SMD resistor replaced with through hole resistor.

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Looks like a 100k resistor to me. You can replace it with one with terminals if you find that is easier to solder.

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  • \$\begingroup\$ Hello,Thanks for your help, that is the part next to the one that fell off and has the same number on it,looks like a 104, just for future reference (probably daft) but how does 104 translate into 100k resistor? Thanks so much for your help, M \$\endgroup\$ – Martin Gardner Aug 27 '11 at 11:27
  • \$\begingroup\$ The numbers work in the same way as the colours on a traditional resistor. 0 = black, 1 = brown, 2 = red, 3 = orange, 4 = yellow, etc. So a 104 would be brown black yellow, which is 100KΩ. First 2 bands are numbers, third band is the number of zeros to add to it. 1 0 0000 which is 100,000 which is 100K \$\endgroup\$ – Majenko Aug 27 '11 at 11:37
  • \$\begingroup\$ @ Martin - note that there can be 4 digits, e.g. 1212, which would mean a 12k1 (or 12.1k) ohm resistor. Works the same way, the last digit always being the multiplier. However, there may be cases where the above rules do not apply, see here for some useful info on SMD codes: marsport.org.uk/smd/mainframe.htm \$\endgroup\$ – Oli Glaser Aug 27 '11 at 11:53
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The image you have posted is of an "0805" (i think - hard to tell the scale) resistor. It is 100KΩ (One, Zero, Four Zeros - that's what the numbers mean) and rated at 0.165 watts.

Is that a picture of the actual component, or just one you found that looks a bit like it? If not, is it possible to post a picture of the actual component itself?

The proper way to replace these would be with a Hot Air Rework Station, but I regularly work with these size components using a regular soldering iron.

Yes, you could replace it with a regular axial leaded resistor, say a quarter- or eighth-watt resistor, but you may find it quite hard getting it to solder to the pads.

Working with 0805 components is not that hard as long as you have a steady hand and lots of flux. Tweezers and a nice fine needle are good to have on hand as well.

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  • \$\begingroup\$ Thanks to everyone who helped out on this, I've done the resolder and hopefully, I haven't damaged the pcb to the point where it won't work, I'll test it on the car and let you know if it works... M \$\endgroup\$ – Martin Gardner Aug 27 '11 at 14:05
  • \$\begingroup\$ @Martin Gardner - how did you do it? Original resistor? Tweezers or ...? \$\endgroup\$ – Russell McMahon Aug 27 '11 at 14:30

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