Electronic – How do surface mount components withstand heat of reflow while through hole components can not


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?

Best Answer

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.