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I have read some online tutorials about soldering through hole components which say that transistors and ICs are delicate components and can be easily damaged by heat. So they recommend to keep the soldering iron in contact with the leads not more than 2-3 seconds and also to use heatsink while soldering.

Here is a quote from one of the tutorials

Some components, such as transistors, can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the component body.The heat sink works by taking some of the heat being supplied by the soldering iron and this helps to prevent the component's temperature increasing too much.

But when it comes to soldering surface mount IC and components, some prefer to use a reflow oven which heats up the entire board as well as the delicate IC to a temperature above the melting point of solder.

SO why don't those components get fried?

What makes the tiny components survive such temperatures while big through hole components can't even if they have larger surface to dissipate heat?

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    \$\begingroup\$ I haven't seen heatsinks clipped to transistor wires for soldering since the germanium days. Come to think of it, I've never seen germanium SMD parts either... \$\endgroup\$ – Brian Drummond May 7 '18 at 18:06
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One of the key points to answer your question is thermal stress. When you apply heat to one pin of a device, there is a suden and huge temperature difference between that point and the rest of the device. That difference is stress, and the result can be a material breakout.

On an oven, on the other hand, all the board is put under a controlled, gradual thermal rise. ALL the points of the device are at almost the same temperature, so there are no thermal stresses (or they are much smaller than) they were when you applied the soldering tool to ONE pin and the rest of the device is at room temperature.

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  • \$\begingroup\$ In addition to the above. The assembly house will/may perform the soldering and/or reflow in stages. Maybe the SM parts are reflowed then the use of an ACE (selective/localized soldering) process as the last thermal process. Thus minimizing any thermal shock/stress to the most sensitive parts. Control of the dwell on different zones (for TH parts) will also help manage any thermal stress. \$\endgroup\$ – Steve May 7 '18 at 18:18
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    \$\begingroup\$ But isn't the reflow temperature higher than the maximum junction temperature? How do they survive a temperature higher than the highest temperature they are designed for? \$\endgroup\$ – Rupesh Routray May 7 '18 at 18:49
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    \$\begingroup\$ "Maximum Operating / Storage Conditions" \$\ne\$ "Maximum Conditions"; the component neither has to work at that temperature, nor has to survive for hours. Usually, soldering takes minutes at full temperature, and the manufacturers explicitly state a temperature/time curve ("solder profile"). So, they survive that temperature because they were designed so that they are solderable. \$\endgroup\$ – Marcus Müller May 7 '18 at 19:45
  • \$\begingroup\$ Thermal stress (subject of one of my patents) is not something that happens over the tiny distances of computer chips. Rather it is the maximum temperature, as Muller above points out, that does the damage. Also, the last time I observed a solder reflow machine for through hole components, it was not in an oven. During reflow, the time of thermal contact was very brief, just enough for the solder to wet the component leads, and much briefer than a radio ham with a soldering iron can manage. \$\endgroup\$ – richard1941 May 10 '18 at 18:26
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TO-92 and similar types of through-hole transistor packages are not that temperature sensitive. They're soldered by running the bottom of the PCB over a fast-flowing river of molten solder which transfers heat rather quickly. The boards are typically preheated a bit, but only to about 100°C.

Here is a video of wave soldering. The vapor you see coming off the board is mostly from the flux.

Some parts are just unsuitable for reflow soldering because of the type of plastics used, or other material concerns. In some cases they have been adapted by using more expensive plastics, in other cases there is no solution because the plastic is part of the component- for example there are no SMT polystyrene capacitors because of the low melting point of PS. There are SMT film caps using dielectrics such as PPS (Polyphenylene Sulfide) but they're not necessarily as good performing (especially with regard to dielectric absorption).

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