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I am trying to do some surface mount soldering for my project and have hit a few snags.

My hot air gun works perfectly fine for most of the PCBs I am making but one type in particular is problematic. Due to the fine pitch of the chips I am using, and the number of very small components on the board I cannot seem to get a good board out of my current process as the components have a bad habit of shifting due to the air flow.

Due to the fact that the board is 35cm long it will not fit in any desktop ovens I have found and I just don't have the money nor the long term need to invest in any industrial equipment.

I've tried ordering two different induction hobs off of amazon in the hope that I can run them at somewhat accurate temperatures but I have hit the same problem in both cases. My tray started smoking and when I checked the temperature with a laser thermometer both heated up in excess of 440 C when I had them set around 160.

I think this may be due to the size of the tray I have to use, rather than a fault with the hobs themselves?

Anyone able to point me in the direction of an oven that is large enough for what I need but is under £250? Or perhaps someone can recommend a hot plate that will do what I am expecting / an entirely different process?

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closed as off-topic by JRE, dim, Elliot Alderson, Maple, Dmitry Grigoryev Oct 2 '18 at 8:20

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  • \$\begingroup\$ Sounds your options are (1) glue components [machine needed] before soldering; (2) perform hand soldering and (3) use professional assembly company. Pictures of assembly fault would help. Information about equipment you use will also help. \$\endgroup\$ – Anonymous Sep 27 '18 at 11:26
  • \$\begingroup\$ If none of the parts have pads underneath, then just whip out your soldering iron and do it by hand. I don't own a hot air anything or an oven anything, but have no trouble soldering fine pitch parts or passives down to 0402 size. \$\endgroup\$ – JRE Sep 27 '18 at 11:26
  • \$\begingroup\$ I'm currently looking at the T-962A+ I have some issues with fine motor coordination so doing the fine pitch parts by hand really doesn't work for me :/ Some of the parts do also have underside pads. \$\endgroup\$ – GigaJoules Sep 27 '18 at 11:37
  • \$\begingroup\$ I must be stupid. It seems it would take the same fine motor control to place the parts as it does to solder them. Either way, you have to place them correctly. And, doing hot air, manage not to knock the off again - or blow them off. Pads underneath is a problem, though. \$\endgroup\$ – JRE Sep 27 '18 at 11:43
  • \$\begingroup\$ Yeah placing the parts isn't too bad because I can make several attempts at it, whereas soldering tends to be something that is a lot harder to rectify when it's been messed up :( \$\endgroup\$ – GigaJoules Sep 27 '18 at 12:02
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The hot air guns are good for reworking a single component or two, but not a good choice for entire boards. I've soldered down QFN components using one, you have to be careful with airflow and direction, and/or use a pick to hold the component in place as it reflows, but I've hand soldered all the remaining 0603 package components. Any chip that isn't leadless can be hand soldered, even with fairly fine pitch, you have to develop a technique of dragging a solder ball across the leads, and the surface tension of the solder/solder resist between the pins usually results in no bridges. To reflow an entire board, there are a few projects that can be found on the web about converting toaster ovens, I bought one, with a convection fan for $40, it's fairly large, it does 12" pizzas with a fair gap at the sides, so a 35cm board should fit. the hacks only involve replacing the thermostat with a controller, but for a one-off it'd be possible to manually switch the heaters on and off to approximate the profile needed.

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Consider the following conclusion:

You must find someone locally who will do the soldering job for you. The individual must have proper equipment and skills to perform the job well.

You can of course continue yourself, but I am afraid by searching in the dark spots of your experience you risk to invest more than you expect, to say gently.

Let me highlight some thigs for your further thought

My hot air gun works perfectly

If you'd tell us model name of the gun, we may be able to make some useful suggestions; at this point I can only say that you, most probably, can adjust temperature of your gun and speed of airflow. Why not adjusting them so that small caps and other parts would not blow off?

I checked the temperature with a laser thermometer both heated up in excess of 440 C when I had them set around 160.

They are either faulty, or read the manual. No information on their models - unfortunately no further insight.

I'm currently looking at the T-962A+

Did you read manual for it? The quality of manual and the phrase "The warranty for mainbody is one year, for parts is three months, provide lifetime technical guide" should give you an idea that it is use-it-once device. High probability that UI will be in Chinese.

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  • \$\begingroup\$ So my hot air gun is a yihua 826D+ The PCBs it works well with use larger resistors that have a bit more weight to them but on the problem board they are very small and get moved at even the lowest airflows. Less direct flow simply doesn't melt the solder. As for the induction cookers, I did read the manual and it said no more than 12'' diameter pans which means they are out of the question. And yeah, the T-962 is supposed to be pretty crappy out of the box, but I've found a number of posts from people who have made some minor modifications and gotten significant improvements \$\endgroup\$ – GigaJoules Sep 28 '18 at 10:54
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At school we had a hot air tool that descends by adjusting a screw. Sort of like a drill press in motion.

As a budget, DIY solution you could affix your heat gun to some sort of vertical mount, and raise and lower the heat gun down to the surface of your PCB. You can take your time lining up the components under the heat gun, then turn the heat on while it's far away and slowly lower it to the PCB.

If you apply symmetric airflow in all directions, you should have some luck in getting the components to stay put. I'll try to see if I can determine the name of the tool I'm thinking of and post an image for reference...

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  • \$\begingroup\$ A "heat gun" is probably still going to have too much airflow in proportion to heat. SMT rework tools are more precision, with temperature actually controlled, and airflow controlled independently of that (of course the temperature controller must compensate for airflow). \$\endgroup\$ – Chris Stratton Sep 27 '18 at 17:23
  • \$\begingroup\$ @ChrisStratton It depends. I've done a handful of reworks with a "heat gun". \$\endgroup\$ – Kevin Kruse Sep 27 '18 at 17:30
  • \$\begingroup\$ Sure, you can use a heatgun with care in some circumstances. The point is that a heatgun is going to make the issues the asker is having difficulty with, a lot worse than they'd be with a proper hot air rework tool. \$\endgroup\$ – Chris Stratton Sep 27 '18 at 18:20

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