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Somebody told me not to keep a soldering iron on the pin of a through hole package for more than 2 seconds. I am not aware of where he got this number from.

When soldering we come across discrete passive and discrete active components, then we have integrated circuits that connect the die to the package via very thin bonding wires. We have very passive tiny surface mount components like SMD resistors and also really huge power transistors.

How do we know for how long we can keep the soldering iron tip on the component without damaging it? What temperature does one usually use when soldering, is it the same for all applications? If this time duration is not enough and we need to put the tip back to complete soldering, how long do we need to wait so we can apply the soldering iron for full duration without damaging the component? Finally what determines what shape of tip to use when soldering?

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  • \$\begingroup\$ Time is secondary here. You need to bring your parts to certain temperature (solder dependent), the time necessary for this will depend on the mass and the material of the parts (including the tip), power available to heat up the tip as well as ambient temperature (imagine, for example, using a preheater in addition to the iron or soldering outdoors in windy weather). Not sure what you meant by "we as engineers". \$\endgroup\$ – Oleg Mazurov May 3 '15 at 19:46
  • \$\begingroup\$ reference to any guide will be helpful. Of course we won't be using a thermometer to see if the part has come upto the required temperature. I wonder how it is done. \$\endgroup\$ – quantum231 May 3 '15 at 20:10
  • \$\begingroup\$ A datasheet for a particular part would contain this information. If not, find Japanese part in similar package and look there. \$\endgroup\$ – Oleg Mazurov May 3 '15 at 20:22
  • \$\begingroup\$ Make sure you are using a temperature controlled soldering iron like this one or better. \$\endgroup\$ – tcrosley May 3 '15 at 23:19
  • \$\begingroup\$ @ Oleg Mazurov, why Japanese? \$\endgroup\$ – quantum231 May 4 '15 at 0:33
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Like everything in engineering... IT DEPENDS

Time, by itself, is meaningless. There are a few competing concerns:

The temperature must reach the melting point of the solder material. This depends on the type of solder, the parts (mass and chemical composition), the soldering iron tip material and shape, power applied to the tip and its thermal conversion efficiency, and the ambient temperature (ex. preheaters).

It's not just about temperature, but how quickly you get there. If temperature rise is too slow, the total energy transferred past the pin to the internal part may be sufficient to damage or derate it. That is why all components come with either a JEDEC/IPC reference profile and/or a soldering temperature profile. This profile is an energy chart showing temperature vs. time.

If you ramp the temperature up too quickly you can get thermal fracturing of the part as the different elements inside expand at different rates breaking free of the ultrasonic bonding or lead frame.

If you leave the iron in contact for way too long you will overheat the part resulting in the destruction of the packaging (plastics don't fair so well, ceramics do much better) and the functional loss of the silicon die inside. There are many failure scenarios for microchips from overheating ranging from the obvious burn-out (physical fault) to the loss of function from ion-migration in the doped-semiconductors.

...but 2 seconds is a good average

The reason you hear 2-3 seconds as a "rule" is that for most parts and most irons and most situations and most PCB's and most... this time value works out safely.

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  • \$\begingroup\$ I will add that the type of component you're soldering is also a factor in how long you can take to solder it. Most simple, passive components like resistors and capacitors are fairly resistant to being heated for extended periods of time, while more complex components like transistors and ICs can be pretty sensitive to extended heating. Taking too long while soldering has killed many an IC... Resistors dying because of overheating, on the other hand, is practically unheard of. \$\endgroup\$ – Sander May 4 '15 at 4:41
  • \$\begingroup\$ What about those tiny SMD components that can be soldered by hand? Besides this, what if one was to heat up a component pin to soldering temperatures when the IC is actually powered up? \$\endgroup\$ – quantum231 May 4 '15 at 20:09
  • \$\begingroup\$ @quantum321 - Most passive components are pretty heat tolerant, but it is always best to follow the temperature profile as closely as possible. You should never solder on a live part for all kinds of reasons, but most importantly, your safety! The soldering iron tip/solder can short out connections creating a fire or explosion and the part itself will be much hotter operating than powered off resulting in a smaller thermal window to get the soldering done without damage. \$\endgroup\$ – DrFriedParts May 5 '15 at 0:43
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I soldered electronics for more than 10 years, and the time is definitely secondary. The size of the solder (.1-.5) makes a difference, as well as the size and type of component. A turret, for example, will be soldered around 700 degrees, usually with a .4-.5 solder, time factor about 4 seconds. a IC will be soldered with .1, usually silver solder, at about 375-400, and take about 1 second. The best way to choose the right tip is to FIND WHAT YOU LIKE BEST! Some applications, such as the turret again, need a very large tip, whereas IC's and your smaller components need a much finer tip. They key to soldering well is to do it as much as possible. Also, when soldering, you want to make sure the surface of the solder is shiny and smooth, with proper flow. When adding solder, you add it to the heated area, not the tip of the iron. Practice makes perfect-do yourself a favor, and get some spare PCB's and trash components, and just like Carnegie Hall, PRACTICE PRACTICE PRACTICE! The faster you can get a good, clean, mil spec. solder joint, the less likely you are to damage anything. Hope that helps.

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  • \$\begingroup\$ Dave from EEVBlog says that we should use chisel shape soldering iron tip rather than pointed tip, he does not give any reason. Do you think that flat is better than pointed tip? Today I saw him solder 0.65mm surface mount package, all by hand. It was mind blowing. He use chisel shaped iron tip rather than pointed. I do not understand what is up with tips. \$\endgroup\$ – quantum231 May 4 '15 at 0:32
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    \$\begingroup\$ If he were 100% right, pointed tips wouldn't exist! So, the chisel tip covers more area and it also brings more heat out to the tip. But sometimes physical constraints keep you from using the chisel. Sometimes SMT soldering is counter-intuitive, especially multi lead packages, since you effectively create solder bridges and then let the flux "break" the bridges. \$\endgroup\$ – gbarry May 4 '15 at 5:53
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    \$\begingroup\$ There is a large group that swears by their chisel tips. I hate them and use a conical tip almost exclusively. \$\endgroup\$ – Matt Young May 4 '15 at 14:50
  • \$\begingroup\$ Almost every expert (quick survey of blogs and asking around our office) prefers the chisel tip. It is more convenient because you get both better heat transfer from the flat side when you need it (which is most of the time) and then you may roll it 90degrees in your hand and use the side of the chisel just like it's a pointed tip (conical) when/if you need that. \$\endgroup\$ – DrFriedParts May 5 '15 at 18:50

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