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Solder temperature?
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Old 2nd December 2019, 03:12 PM   #1
northumber82 is offline northumber82  Europe
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Default Solder temperature?

Hi, I need to know the maximum solder temperature for these transistors:

KSC1845/KSA992
KSC3503/KSA1381
BD139/BD140

I'm using 300C solder tip, correct? How much time?
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Old 2nd December 2019, 06:11 PM   #2
Enzo is offline Enzo  United States
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SOlder temperature is more about heating the internals, which is more a matter of how long the heat is applied. In other words you need enough heat to quickly heat up the joint so it flows solder. If you use too little heat, then it takes longer to heat the joints and component leads, and that can lead to the copper traces lifting off the board.

Learn to solder with confidence. I see novices solder a joint, then decide maybe it needs a little more solder, then maybe it isn't shiny enough so they reheat their joint. and sometimes even decide to remove the solder and do the whole thing over again. meanwhile they have just baked not only their part, but the circuit board as well.

My basic Weller WTCP solder station comes with 600, 700, and 800 degree tips. Degrees F. I have always used 700 degrees, which is maybe 380C? I have been soldering for over 60 years, and I have never ruined any components by my soldering. I make plenty of other mistakes.
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Old 2nd December 2019, 06:17 PM   #3
rayma is offline rayma  United States
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With leads and pcb pads that are free of oxidation, soldering should be just a few seconds.
If there is oxidation, remove it by scraping with a knife, and then tin the exposed copper before soldering.
Agree that 700F is the best tip temp, keep it clean with a wet sponge just before each joint.

Last edited by rayma; 2nd December 2019 at 06:21 PM.
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Old 2nd December 2019, 09:33 PM   #4
tschrama is offline tschrama  Netherlands
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300..350 degrees C max
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Old 2nd December 2019, 10:48 PM   #5
Mark Tillotson is offline Mark Tillotson
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If the pcb starts to discolour, the iron is too hot (!) Through-hole transistors are unlikely to suffer before the pcb material as the heat has to travel along the lead, into the leadframe, and then along a bond wire or the epoxy case. The FR4 of the pcb is charring already...(*)

Surface mount components are a different matter, the heat has a much more direct route to the semiconductor die.

I'd recommend an adjustable temp-controlled iron, for 2 reasons:
1) Able to set the temperature according to need (different bits and different situations).

2) Low voltage in the lead - no risk of electric shock from accidentally melting the lead - most temperature-controlled irons use 24V elements

A good iron will have silicone insulation on the iron's lead to reduce heat damage if you do accidentally touch it with the iron.


(*) In the (bad?) old days of germanium transistors with max junction temp 100C, it was a real issue, silicon is much tougher at 175C.
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Old 3rd December 2019, 12:12 AM   #6
avtech23 is offline avtech23  Australia
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Solder temperature?
I tend to run the iron hot for through hole parts - usually 380-400c.

The way I do it is first clean the tip, then apply a dab of solder to wet it. Then apply heat to both pad and lead, touch solder on (it should flow pretty much right away) then withdraw the iron in an upward motion along the lead away from the component. You can see the solder wick down into the hole and that indicates to me the point to withdraw the iron.

The only time I've baked components is when trying to use lower temperatures and having to loiter on the leads because the solder wouldn't flow, or when I haven't preheated a 2 Oz copper ground plane connection.

One other tip: if I have multiple transistors to solder, I solder one leg of each, then middle leg of each and so on. It gives each transistor a period to cool between solder operations.
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Old 3rd December 2019, 12:25 AM   #7
PRR is offline PRR  United States
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> BD139/BD140

I had a PC motherboard which used a similar type as a voltage regulator. I put in a faster (hungrier) CPU. All worked fine. Then I noticed the transistor was "loose" in its solder pads. It had MELTED its solder but was still "working". Silicon can stand a LOT of heat.

Note that this regulator had a serious driver which would absorb the base leakage of a too-hot transistor; I would not expect most "audio" circuits to be happy (even alive) at such heat. And I only ran it 15 minutes.... long-term temps that high are sure to perish the transistor.

I put a much bigger heat-sink on the transistor and used the mobo for years.

As everybody is saying: fairly hot iron, CLEAN surfaces, get in and get out quick, no problem. Use a not-hot iron and slow-bake when solder is not taking is the slow path to harm.
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Old 3rd December 2019, 12:54 AM   #8
jackinnj is offline jackinnj  United States
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Solder temperature?
Old enough to remember when we had to use flea clips or alligator clips to dissipate the heat on germanium transistors.

I use 750F on my soldering station and use 63/37 all the time.
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Old 3rd December 2019, 03:16 PM   #9
lcsaszar is offline lcsaszar  Hungary
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Also important is to use good resin core solder. Resin is half of the success. In old days I had a nail polish bottle filled with a small amount of resin (the type of rosin used by violinists) diluted in ethyl alcohol. It came handy at difficult soldering tasks.
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Old 6th December 2019, 09:12 PM   #10
Mark Tillotson is offline Mark Tillotson
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Resin is the generic name for various sticky gummy chemicals. Rosin is (usually pine) tree resin and may be combined with a few other chemicals to make it solid for uses like violin bows and solder flux. It turns to amber after a very long period of time. Its also known as colophony..

As a flux it works due to being molten at the right temperatures and being reactive enough to reduce metal oxides - its acidic only when molten and not water soluable - other acidic fluxes destroy the thin traces of pcbs over time unless scrupulous cleaned off.

Last edited by Mark Tillotson; 6th December 2019 at 09:20 PM.
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