Continuous Flow Soldering

Published:2011/8/17 1:35:00 Author:Amy From:SeekIC

From reader correspondence we know that lots of you are still hesitant about soldering SMDs (surface-mount devices), particularly integrated circuits.

We won’t say it’s a piece of cake, but maintain that there’s no rea­son to start panicking about SMDs, as accurate soldering does the trick. Using the right tools and some basic skills, sol­dering SMD ICs with ’gullwing’ or ’J’ leads should be within your reach.

What’s needed

In any case, make sure sufficient solder flux (flow agent) is avail­able, so that the liquefied solder will easily flow towards the desired solder pad instead of to creep up adjacent pins or fill unwanted holes! Small pieces of cloth and flux cleaner are also recommended to mop up solder spills. The actual soldering is best done using an iron with a so-called ’wave’ tip. This tip has a hollow part capable of holding an amount of liquid solder. Finally, we would recommend the use of an illuminated desktop magnifying lamp.

What to do

1. Apply flux to the copper pads in the corners of the IC foot­print.
2. Put the IC on the footprint, carefully aligning the pins at diagonal corners with the sol­der pads.
3. Place the wave tip on the junction between Ihe IC pin and the pad in one of the cor­ners. If necessary add some solder flux.
4. Remove the tip. Wait for the solder to solidify. Next, solder the opposite solder pin.
5. Put some solder in the wave tip to create a ’bead’.
6. Hold the solder tip such that the bead is in contact with the upper surface of the IC pins. Slowly, in one smooth move­ment, move the solder tip across the contact row, such that a uniform amount of sol­der is applied to each pin.
7. Repeat step 5 and 6 for the other side(s) of the IC.
8. If necessary clean and inspect the result.

The above method may also be applied to ICs with ’J’ leads.

Background

For clarity’s sake: the advantage of the proposed method is that you do not have to add solder to the tip for each IC pin. The dispens­ing of solder from the ’bath’ in the solder tip is based on a principle from physics called ’capillary action’. Because every solder pad/pin surface ratio is identical, each contact will attract an identi­cal amount of solder, the excess amount being retracted by the sol­der tip. In addition, solder is attracted by a heat source, in this case, the solder tip.

The above process only works if sufficient flux is present on the sol­der pads. After all, solder does nothing except melt during heat­ing. For best results, however, it should flow and bond, too.

Copper, tin and lead are ’porous’ metals that protect themselves from corrosion by forming a thin oxide layer. Agents like solder flux have an etching effect that removes oxide layers.

It is recommended to use solder fluxes based on resin, as they are the easiest to remove using (inex­pensive) flux cleaners based on alcohol.

Solder iron tips for continuous flow soldering are produced by, among others, JBC, Xytronic, Met-cal, Weller and Pace.

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