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electrical resistor/motherboard/traces question

Z

ZippyDan

Platinum Member
er, forgot all my emag

question 1: i accidentally removed a tiny resistor from a motherboard. i tried to solder it back on, but its far too small to do accurately. thus, the solder is just basically connecting the entire reisistor to both ends of the connection

er, ill try to draw a diagram

--- is wire/connection point
=== is resistor
+++ is solder

supposed to be:

-------=======--------

now is:

------- +======= +--------
+++++++++++++++++++

where all the +'s are touching 😛

will this still work?

question 2:

are the green traces on a motherboard able to be 'connected'? er put another way, if i put solder on the motherboard, will i be joining the traces? or is there only conductivity and the endpoint (the tiny metal circles where the traces end)?

~Zippy!
 
2. The green stuff is called solder mask. Solder wont stick to it properly and it is non-conductive.

1. You need to remove the short circuit and to solder it properly. You might need to take it to someone who is set up for surface mount soldering.
 
it will not work since the solder is parallel with the resistor
recall parallel resistance calculation (R1*R2/R1+R2)
you have just decreased the resistance below the resistance of the resistor
 
it will not work since the solder is parallel with the resistor
recall parallel resistance calculation (R1*R2/R1+R2)
you have just decreased the resistance below the resistance of the resistor
 
No need for a calculation here. It's a short, plain and simple.

As far as soldering the resistor on....
Use a fine point soldering tip. Try putting a small blob of solder on each connection point. Now hold the resistor in place (with needlenose) and using no extra solder melt each blob. This will prevent all that extra solder you are getting now.

 
Its easy to do, given a $500 solder station and a $2000 stereo microscope.


Oh, you don't have those.~^

From where you are now you need solder wick or as solder sucker, remove the resistor and all execess solder. Get down to the pads if you can. Of course you must be carefull not to over heat the region or you'll lift the pads.

Then as stated above wet the pads with solder (as little as you can) here is where a fine tipped iron with temp controll is nice.

Set the resistor on the wetted pads and carefully heat each end.

What are the odds I give that you will be able to carry this out without the above equipment.

About 1in 10

good luck
 
It MIGHT work, but there's a big chance that it wouldn't. I'm guessing that resistor is either a current limiting resistor, or a series terminating resistor, the latter being most likely. Can you read the value off it? is it 47R0, or somewhere close to it?

Edit: Use some solder flux. Makes your life easier.
 
Yeah, there's no way in hell I could do it without the stereo microscope, tho I've got technicians who can solder freakin' 0402s with the naked eye. (Unbelievable.)

The right equipment's the fine-tipped soldering iron, the stereo microscope, solder wick, and a bottle of flux. Oh, and solder. And a dental pick and tweezers are pretty helpful... RossGr's got it about right.
 
You need to remove the solder before it will work - the best option is desoldering braid: basically, woven coper fibres, which act like a sponge for molten solder. Solder suckers are liable to suck up your resistor, and then you'll be stuck.

A soldering iron with a proper SMT bit is essential - normal hobby irons have tips so big that they can be bigger than the components themselves. I had very little luck with SMT work until I got a decent bit - although I've found that my cheap $15 iron is more than good enough.

I remember when I built my first SMT board - I'd originally designed it with 0603s and SSOPs, but I chickened out before getting them made and went for 0805s and SOICs. A wize move - but it didn't stop me from getting an entire row of LEDs on backwards - as for turning them round, now that was a problem 🙂
 
the resistor in question looks exactly like the brown one in this picture

ive been thinking that that is way too small to work with

so questions:

where can i get soldering iron that accepts fine tips and fine tips cheaply and quickly (locally)? currently i have a radio shack soldering iron that comes in their cheap soldering kit. tip is way too big and im not sure if its changeable

can i use a standard sized resistor in place of that tiny one? im sure i can as long as its the same resistance rating... so the question is how do i figure out the resistance rating? ohhhhmmm

~Zippy!
 
Originally posted by: ZippyDan
the resistor in question looks exactly like the brown one in this picture

ive been thinking that that is way too small to work with

so questions:

where can i get soldering iron that accepts fine tips and fine tips cheaply and quickly (locally)? currently i have a radio shack soldering iron that comes in their cheap soldering kit. tip is way too big and im not sure if its changeable

can i use a standard sized resistor in place of that tiny one? im sure i can as long as its the same resistance rating... so the question is how do i figure out the resistance rating? ohhhhmmm

~Zippy!
Click to expand...

If it looks like the brown one in that picture, then that's more than likely a capacitor, and not a resistor... notice the C7R1 to the right of the brown one in the picture, and the R7R1 to the left of the black one in the picture.

I suppose it could be possible to replace it with a standard sized component, assuming you can find one of the right value. But I don't see how it would be much easier to solder a full-sized component in place of an SMT component.

JW
 
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