Why does drag soldering SMD work?

[dknguyen]

I was just curious about something, when soldering through hole components, you have to heat the pad and the lead so the solder will bind to both for the joint to properly form (I assume the same is true for reflow also since both the pad and solder are heated). But in drag soldering it just seems that only the solder is heated? You drag the ball of solder on the tip along the pins so the solder forms overtop of the pin and pad...there is insufficient heating of the pin and pad, so why does it work?

Are there different mechanical requirement for throughhole and SMD or something that allows that to work? Am I missing some detail?

 

[hjames]

The pad and pin have very low thermal mass, so they pretty much get heated immediately. Surface tension pulls the solder in. It's usually recommended that you flux the surface so that the solder deoxidizes itself and presents a nice shiny bead of solder to the pin and pad.

In comparison, a through hole pin is probably an order of magnitude better at conducting heat away from a solder joint, and also requires a lot more solder to join properly.

 

[Nigel Goodwin]

Yes, it's the presence of the liquid flux that helps to draw the solder to the correct places. In conventional soldering you heat the joint and apply the solder to the joint - this is so the flux inside the solder can act on the surfaces. If you simply apply the solder to the iron the flux is lost.

 

[AllVol]

Hello,

This whole SMD thing has me reluctant to continue experimenting with any new devices. Since I am an old _art, I have lost the keen eye and steady hand necessary to use those tiny components. That leaves me forced to use the larger DIP's, when they are available. As it is, I spend as much time on the floor looking for parts as I do soldering.

Audioguru sometimes chides us for using out-dated parts, but often that's all we can see.

Anyone else have this problem?

AllVol

 

[JimB]

AllVol wrote:

Dear Abby:

What can I do about all the sex, nudity, language and violence on my VCR?


I line with other posts on this board, you need to give is more information before we can help you.
Is the problem due to too much or too little of the "entertainments".

JimB

 

[dknguyen]

Ah, lower thermal mass. That makes sense.

Audioguru sometimes chides us for using out-dated parts, but often that's all we can see.

Anyone else have this problem?

AllVol

Can you use a toaster oven for reflow? THat takes half of the shakey handwork away. It is also said that the components don't have to be placed too precisely on the pad since surface tension will resposition them (reflow only).

 

[ThermalRunaway]

There was a project in Elektor Electronics which allowed you to turn a small electric oven into quite a good reflow oven. It was all microprocessor controlled with adjustable parameters to suit different ovens. Quite good.

Brian

 

[kchriste]

I find working with small lead pitch parts easier if you tack solder a couple of pins first to hold it in place. Then I lay the solder wire down one side of the chip and move the iron along the pins into the solder wire. Solderwick will fix any solder bridges. Larger chips can also be tacked down with a little glue to keep them from moving around. One nice thing about SMD parts is that there are few holes to drill in the PCB.

 

[AllVol]

Just found a super site concerning all this:

https://www.fpga4fun.com/SMD.html

Thanks for all the suggestions. Are you guys using individual pads and headers, or what?
Aside to JimB: I dunno.. maybe GIGO