SMT Soldering for the Hobbyist -Visual Guide

UNDER CONSTRUCTION

We will solder an 8-pin SOIC IC to a chip carrier PCB.

The tools

All tools I use are common electronics tools. Nothing fancy. The most specialized tools I use for working with SMT are home-made or improvised

Soldering Iron

My soldering iron is a basic 25W no-brand iron. I do make sure it has a very clean tip, for doing SMT work. I do not use this iron for anything else. The tip style must be the sharpest one available for your iron type. The sharper the better.

Since heat control is more important in SMT work than in thru-hole, a way to vary the iron's power can be very useful. Some small SMT parts may be damaged by too much heat, and the heat source comes very close to the component's body. I don't really want to use full power for all soldering jobs, and for an IC, I like to turn it down a bit.

I use a home-made power controller, which is nothing more than a triac incandescend light dimmer ( 600W, $3.95 at a local hardware store) placed inside a plastic box. People buy these things and replace their wall light switches with them, but not too many folks think of putting one in a small box to control other things.

You don't have to have one, but it's nice to have.

Holddown Setup

This consists of a straightened paper clip and a rubber band. The photo below shows how it's used. A small piece of foam or other object is also placed under the tail end of the paper clip, to increase tension. The business end of the clip pushes the IC down on the PCB just enough to keep it from moving. Tension on the rubber band is moderate.  On a PCB containing many SMD components, you could move the tip of the paper clip around to hold down various other components, without re-setting the position of the rubber band or the black foam pad. Things move along rather quickly, once you get the hang of it.

The holddown jig is the key to working with SMT.

It needs to meet 3 conditions:  holds down parts, allows easy access to solder them without obstruction, and be easily moved from part to part without wasting time.   Trying to solder a board full of SMT parts without using some kind of system that does the above, often results in frustration and disappointment, and can cause many hobbyists to give up altogether on SMD work.

The above setup must also be used for resistors and capacitors.  Without positive holddown, 2-terminal passive components soldered one-side -at-a-time will exhibit "tombstoning". This is a phenomenon by which the part's unsoldered end flips up during soldering and the part stands vertically on the soldered end.  Surface tension is responsible for this, and surface tension can only be tamed with flux.  In SMD soldering it will become more important to add flux to solder a joint, rather than add solder.

The technique:

1. Keep the soldering iron tip away from the part being soldered.

2. Use solder flux

Never touch the soldering iron tip directly to the lead of a component being soldered.. Only touch the soldering tip to a PCB pad or trace 1-2 mm from the pin of the part being soldered., never closer. The soldering iron must heat the copper trace, flow solder onto the trace, (a very small amount) and the solder, together with flux (previously added to the PCB trace) will travel down the trace and touch the component lead, seep under the lead, and make a good connection.

This is the way it must proceed. And it will likely not happen without solder flux

Flux

Flux is a paste which reduces the solder's surface tension, and greatly enhances it's ability to seep into noooks and crannies, without effort. This is important, since touching an SMT component directly with a soldering iron is a no-no in SMT work.

Surface tension is the force, present in all liquids, which tends to want to reduce the surface-to volume ratio to a minimum. High surface tension means forming spherical droplets. Not wanting to assume complex shapes or seep into small crevices. Not wanting to "wet" surfaces. In real life we see this behaviour more with water, and less with oil. That's why oil is a lubricant. It seeps and wets. It assumes a shape with very large surface and doesn't care.

Solder has much higher surface tension than water. It REALLY wants to form spherical droplets. It REALLY does not want to wet the component lead. Without flux, you will have a hard time getting it to do otherwise.

If you are able to buy the solder flux used in industrial SMT work, by all means do so. But for those of us less fortunate, the flux sold at the hardware store (for plumbers) works very well. CAUTION: this must be a NON-ACID flux.

Any brand is fine. The brand I currently have is called Oatey No. 5 Lead Free Solder Paste, and comes in a small round tin. like a hockey puck. It has the look and consistency of bearing grease, and it is indeed greasy to the touch. Be careful that they use the word "paste" to mean flux, not flowable solder.

The industrial meaning of the word "solder paste" is a type of solder that is soft enough (like a cream) to be deposited in small droplets onto the PCB, and then "reflowed" or heated so it can solidify into a connection.

My experiments with the type of solder paste used industrially specifically for SMT work have not been as good as plain old  thin-gauge solder wire. The solder paste is a mixture of flux and tin/lead powder and has an unusual viscosity (it is granular, gritty, and does not flow or spread well) as a result, you end up spending more time trying to get little droplets of this material in all the right places (it never seems to go exactly where you put it) than you would using the technique above. SMT solder paste was meant to be applied by silkscreening  (printed actually), and it's properties are well  suited for that.

CAUTION 1:

If you buy industrial solder products, make sure you differentiate between solder paste and solder flux. Solder Paste contains both flux and solder. Flux is just flux. Do not interchangeably use the word "paste" although they are both pasty.  If the name is ambiguous, ask the composition. Again my experience indicates you do not need any special materials to work with SMT, and you are better off using what you have. My jar of solder paste ($25) sits lonely in the back of the fridge; used it once, and never again.

CAUTION 2:

Avoid "Lead free" solder products if at all possible. It has higher melting point, and is more difficult to work with. The industry has had a hard time adjusting to it, and has not even now worked out all the quirks and problems inherent in lead-free soldering. These problems will be amplified for the hobbyist.
You have a learning curve ahead of you becoming proficient in SMT soldering, you do not want an additional variable added in the process, which could throw a wrench in your enjoyment of the hobby.

The amount of flux required for excellelnt results is so small, you will wonder how it can ever work.

Below is a photo of the amount of flux required to solder one side of the SOIC-8 IC (4 pins). A small resistor lead was used to dip into the tin of flux and pick up some flux on its tip. You can barely see it. That amount is more than needed for one side of the SOIC, and there will be excess left over on the PCB. 

This is the main drawback of flux, for the hobbyist as well as in industry. Flux residue stays behind on the PCB and needs to be removed, since it's electrical properties are not exactly inert --it has finite resistivity and dielectric constant. Excess flux needs to be washed away with a solvent like acetone. The less flux you add , the less you have to clean up.

Streaking the flux across the finger pads. Trying  NOT get flux between the traces is a futile effort anyway. Even if you succeeded, as soon as you apply heat and flux melts, it becomes so thin it quickly runs sideways and ends up between traces anyways. You'll still have to clean it up later. Either way, the solder follows the copper trace, not the flux, so it will not form bridges between traces, unless you've added grossly too much solder.

Placing the IC on the PCB.  Initial placement under the paper clip cannot be very accurate. So it must then be nudged into final position (while under tension) with the nose of a pair of tweezers.

Final position, ready for soldering.

Soldering tip well away from IC body.

Applying the solder. May not always be necessary if the copper traces are pre-tinned, and the soldering iron tip already has some small amount of solder on it.

Soldering opposite side pins with the holddown removed.

This is controversial. The chances that some degree of "tombstoning" will occur with the IC are fairly good. This means that as you solder one row of pins, surface tension of the solder "pulls" the IC down on the soldered side, causing it to lift slightly off the board on the opposite side. Good tension on the holddown arm may prevent this, but DON'T COUNT ON IT. Always inspect the opposite side pins carefully to make sure thay are properly seated before making connections.

Best practice is to solder one corner pin first, then solder a diagonally opposite corner pin, to make sure you achor the chip before proceeding to other pins.

Actually, go 1-2-1.
Solder first pin. Solder a second pin, diametrally opposite to first.
Then re-heat the first soldered pin to relieve any tension that may have been caused while anchoring the part at 2 opposite points.

TO RECAP:  Sequence of events:

Add a tiny amount of flux right at the point where the component leg will touch the PCB.

Place component.

Touch solder tip to copper trace 2mm or so away from the IC pin. Heat the trace. Wait 1 second

Touch a thin gauge solder wire to the copper trace, at the soldering tip, and melt a VERY small amount of solder, the size of a pin head.

Continue heating the trace. Solder will flow outward and towards the component.

It helps if you place a small piece of cardboard under one end of the PCB, so as to create a slight slope for solder to flow more easily away from the iron, and towards the IC.

When molten solder reaches the IC pin, it melts the flux, mixes with it, and is then able to wet and get into very small spaces. Molten solder heats the IC pin, then wets it and binds to it. Without the flux, the molten solder could very well be in contact with the IC pin and NOT make a connection to it. Or perhaps a poor connection

The entire process should take about 3-6 seconds, so you need to keep the soldering tip applied to the copper trace, motionless, for this entire time.

You will actually see a change in the solder reflection at the IC pin, as a solder melts and makes a connection to the IC pin. You may need to wear a head-mounted magnifier, to see this, unless you have very good eyesight.

A lighted, head-mounted magnifier will actually be very usefull for all SMT work, and it's a good investment to get one. Once you experience it's usefulness, you'll kick yourself for not having gotten one sooner. Besides, electronic components are going towards SMT with the speed of a bullet train, so in the future, you're ony likely to be doing more SMT work and less thru-hole.

You should always do a continuity check of all IC pins soldered this way. Don't trust your eyes.

Below, continuity check. One probe the the root of the IC pin, where it meets the IC body. The other probe tip on the copper trace, a few mm away from the IC pin. You really need an audible continuity tester for this, unless you have 2 pairs of eyes..

Note:
To clean up leftover flux, you will need Acetone.

This must not be nail polish remover, since that product contains acetone diluted with water (won't dissolve flux any more), and has other cosmetic additives to soften the skin (more contaminants on the PCB). Some of those products don't even contain acetone, but something else.

Other solvents that will work are: Dichloroethane, Dichloroethylene, Trichloroethylene, and plain old paint stripper (liquid, not gel).  Use with good ventilation and avoid inhaling vapors. 

Alcohol, paint thinner, Turpentine, Mineral Spirits, household cleaners WILL NOT WORK. Don't even try.

Some hobbyists use a toster oven to reflow SMT boards, but I do not recommed doing so , unless you have modified the oven to provide accurate temperature measurements. Do some background reading on reflow soldering. You will see that the components should not spend more than 10-30 seconds at the reflow temperature, and they must be properly pre-heated and cooled before and after the reflow. An unmodified toaster oven will not be able to provide a safe temperature profile. Many components will be damaged if the reflow temperature is a little too high, for a little too long.