I pulled out main relay from my car because of the problem I described
in this thread:
Although there is no way to be 100% certain that my diagnoses are
correct without looking at the unit myself, not at a photo of it, I
still made my best effort to study the image and based on what the image
portrays, I have labeled the contacts that appear to have cracked solder
with a red 'X' and ones that are questionable with a '?'. I resoldered
my main relay myself with a 25-watt soldering iron and it works like a
charm now. If you don't have a 25-watt soldering iron, you can buy one
pretty cheap, or just borrow one from somebody who does. DO NOT use a
high-powered soldering gun (75+ watts) for they can scorch the circuit
board and ruin the relay. Just melt the solder back down and eliminate
all the hair-line cracks. You may or may not have to add more solder to
the joints, depending on how badly the cracking is. (Radio Shack should
have the correct solder)
The old solder on the dry joint is now a bad conductor and has no
flux. Re-melting the solder most likely will result with a solder joint failure.
The car may stall just like a bad igniton switch.
The correct method is to remove the old solder. Use rosin core solder
or buy a rosin paste flux and a standard solder.
it's good to be prudent, but a 75W iron shouldn't scorch anything - it's
supposed to be the same temperature as the 25W iron. what it /is/
however is clumsy, and clumsiness can lead to bridging - /not/ a good idea!
personally, i like to fully de-solder and re-solder from scratch. the
cracks are full of oxides and simple remelting leaves those oxides in
place. it'll work, but it'll crack again soon after.
Count me in on that. My favorite technique will probably cause the soldering
purists to faint, but... I hold the work upside down, if possible, and let
the old solder run onto the iron tip. When the tip starts to get blooby
(technical term) I shake it off onto the work bench or kitchen table covered
with newspapers or whatever. The connection is already tinned, and a touch
of fresh rosin-core solder makes it nice again.
I agree the wattage isn't as important as the handling. Don't press the tip
on the printed traces in any case, and you'll do fine.
how? a soldering iron at 250C [or whatever it is] is a soldering iron
at 250C!!! all the wattage does is ensure it can heat larger objects
quicker by dumping more heat into it. otherwise the built-in thermostat
ensures temperature constancy.
A 25W is
And that is where the problem is;too much heat transfer too quickly.
The soldering GUNS take too long to heat up,and thus the user holds it to
the work for too long a time,the total heat transferred is way too much for
PCBs and small parts.
It's not only temp,but time+temp that matters.
that's a much better explanation. but i think what you really mean is
that it comes down to the skill of the operator. when i repair my
relays, i use a 63W hakko iron. it's magnificent! great heat control.
huge thermal capacity. makes the whole job real quick and simple. a
25w weller doesn't have the heat capacity to make a good quick job of
every joint because some of them are the relay chassis, and they are
huge heat sinks. attempting to solder those with too low wattage takes
ages and as you say prolonged heating is much more likely to lead to damage.
Wattage does count, as that's what generates the heat. I
wouldn't recommend an extremely high-Watt constant power
soldering iron. This wouldn't be appropriate for a beginner.
Higher heat is acceptable (or preferred) when skilled techs are
experienced with the higher temps. Typically, an effective iron
tip is supposedly (400C) 750F /25 Watt.
It's the heat capacity that matters most, and that's a combination
of a large enough tip to transfer the heat quickly, and enough
wattage to generate the heat.
Not all soldering irons are temperature-controlled. There is a
*huge* difference between constant power and constant
My older Weller irons have ferromagnetic tips that regulate temperature,and
the selection runs from 600 to 900 degF.Then there's the selection of tip
widths,allowing more or less heat transfer.
With no temp control,an iron's tip temp can climb much higher than 700degF.
A solder sucker is a good idea too. Radio Shack or Fry's has them and
they are cheap.
I've been using a derivative of this technique as well. I actually
hold whatever needs desoldering in my hand and when blooby, bang my
hand on the table - it throws the solder onto the table.
Just be careful following either technique: Getting solder spashes in
your eye is not much fun. (ask me how I know :)
Those soldering guns just take too long to get up to temp,that's why they
damage things;the person holds the tip to the workpiece while it is still
And big soldering irons have too much thermal mass for some items.they
store so much heat in the tip that it overheats the foil and causes it to
But I used a 60W weller with a 1/4" tip to resolder my MR,no problem.
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