preheating SMD parts prior to hand soldering?

[coppercone2]

If you are doing something funky like hand soldering a QFN package or even doing a ceramic capacitor, does it make any sense to preheat the part prior to soldering to reduce thermal shock? Like on a hot plate. If you have aversion to hot air pencils (its easier to drag solder or use solder channels to route solder into a flat no leads IMO).

[thm_w]

I don't think thermal shock would be much of an issue unless its a sensitive part or you are doing something strange.
Preheating is always useful if you have thick copper or large ground plane to solder to, usually not an issue with QFN but might come up with a FET/motor driver part. But then they'd usually have a center pad, which is really better to do with hot air.

Pre-soldering the pads by hand then using hot air to reflow the package is also an option.

[coppercone2]

yea thats what I usually do, hot air for realignment, its hard to do things right without it since they often have a pitch to them. But I am just wondering if it would be better to heat em up to like 70-90C before doing the hand soldering part.

With ground plane parts its even worse because usually to do it by hand, I will tin both the part and the pad carefully, reflow it on, then hand solder the other connections, then reflow it again, the part must take a beating from this.

[ar__systems]

waste of time, unless you have some super fragile component.

For resitors/caps I put a bit of solder ball on one pad, a bit flux, then melt the ball and do the other side with solder wire. Doing another round of hot air reflow is a waste of time and unnecessary stress to the circuit board and the component.

[rx8pilot]

I pre-heat with most silicon parts and ceramic capacitors out of fear of thermal shock reducing the life of the part. While prototyping, I may not bother but re-working a PCB that I want to last, I will pre-heat. For most parts with a lot of legs, it is much easier to solder with everything at 150C already.

I have read that ceramic caps are particularly susceptible, so I just got in the habit. A lot of my stuff goes to very far off locations and has a high expectation of reliability. For random stuff in your lab, perhaps overkill if you are good at soldering and don't cook your parts.

[ar__systems]

https://www.murata.com/support/faqs/products/capacitor/mlcc/mnt/0001

Murata says to preheat. But yeah, for prototypes I don't bother.

[xaxaxa]

For soldering some QFNs where pads are heat sinked to the ground plane, I use hot air at 150 to 180*C when drag soldering the pins. Otherwise the solder on the pins just will not melt.

[coppercone2]

I pre-heat with most silicon parts and ceramic capacitors out of fear of thermal shock reducing the life of the part. While prototyping, I may not bother but re-working a PCB that I want to last, I will pre-heat. For most parts with a lot of legs, it is much easier to solder with everything at 150C already.

I have read that ceramic caps are particularly susceptible, so I just got in the habit. A lot of my stuff goes to very far off locations and has a high expectation of reliability. For random stuff in your lab, perhaps overkill if you are good at soldering and don't cook your parts.

Do you have a bifty small chic hot plate dedicated for this?

[coppercone2]

Its probobly pushing it but i wonder if you can make a one shot ramp down that interfaces with a hot air station to give it a ramp down profile after hand soldering!

I would not do this unless i found a really nice arm to hold the hot air station. Like a dial indicator arm.

[schmitt trigger]

With DPak and D2Pak devices, many times one uses a copper pour both as a heatsink and low impedance connection.

But those same characteristics make it difficult to solder with a regular soldering iron.
In those instances, I preheat the board with elements from a discarded coffee-mug warmer. It heats to about 95C, and makes soldering easier.

[coppercone2]

I think I am going to stick a strip heater on a small aluminum plate with some rubber feet, thermostat etc to make a little prewarmer. Cover with kapton and ground for good measure.

[wraper]

You should preheat pcb from the bottom, not only component itself.

[rx8pilot]

You should preheat pcb from the bottom, not only component itself.

Definitely that. If the majority of the PCB is heated to 125-150C nice and gentle...the soldering get so much easier and safer. Especially parts that are attached to a ground and power planes.

[Siwastaja]

Such a "super sensitive device" might be the bog standard 1210 MLCC. The manufacturer may explicitly forbid any hand soldering activity completely, and gives very strict guidelines not only about board flex, but also about the footprint, max number of reflow cycles (such as only one), and the reflow profile.

I have had a Murata 1210 MLCC die after a bit careless hand-soldering operation. It died after running for 1000 hours no problem, then sitting unbiased in humid summer air, during the next power up. It has became a low-value resistor (high enough not to blow properly sized fuse, and not to vaporize instantly) - glowing bright yellow, nearly starting a fire. After looking at the matter, indeed, the manufacturer forbid the hand soldering completely. (The actual mistake I made was to solder multiple capacitors too closely together, in a shared copper fill with solder mask openings, without thermals. But, there was no mechanical stress involved, which is the thing most people seem to think is the only issue with MLCC's).

I know everybody here says "nah, haven't happened to me, it's not a problem". But it may be a problem. Follow the standard advice for MLCC, especially with 1210 and up.

Yes, preheating MLCCs to reduce thermal shock might indeed make some sense, and may not be overkill. Although, preheating the board is much more important, since this will reduce the soldering time considerably, reducing the thermal shock as well. Might as well put the MLCC's sitting somewhere on the board to preheat as well, "for free" - that won't hurt.

Preheating the board to about 100degC for manual assembly makes a lot of sense, since this is a temperature the FR4 board and already placed components (such as electrolytic caps) can take basically forever, with a good margin, so you don't need to think about any negative side effects.

[rbm]

I do mostly hand soldering and assembly so this discussion is pertinent to me.  I've not been preheating all this time.  Am I correct is my understanding that you preheat the board to 100°C, then place each component and solder?  Or is it place all components, then preheat both board and components, and then solder?  Apologies if this sounds naive.

[Siwastaja]

I do mostly hand soldering and assembly so this discussion is pertinent to me.  I've not been preheating all this time.  Am I correct is my understanding that you preheat the board to 100°C, then place each component and solder?  Or is it place all components, then preheat both board and components, and then solder?  Apologies if this sounds naive.

Not important IMHO, but as I guesstimated previously, susceptible components such as large MLCCs might get some sort of reliability benefit from being on the board for some time before soldering so they heat up to the preheated board temperature, as well.

The key idea of preheating the board is that the amount of heat flow (actual thermal power) your soldering iron needs to supply to the joint is about halved (for example, assuming leaded solder, 170-100 degC = 70 degC difference instead of 170-20 = 150 degC difference). This means you get away with a smaller tip, or lower tip temperature, or shorter contact time. All are big positives: fewer tip changes, can reach difficult places with a smaller tip, longer tip life with reduced tip temperature, quicker time-to-working-prototype. The only minus I guess is related to the handling of the hot board without burning your fingers.