Lesson 9: Good joint, bad joint, and checking your work

This is the lesson that turns practice into skill.

You can make a hundred joints and get no better at soldering, if you never find out which of them were any good. Feedback is the whole mechanism of learning a hand skill, and unlike most hand skills, soldering gives you almost none. A bad joint holds the part perfectly well. The board might even work.

So you have to go and get the feedback yourself, by looking. The good news is that a solder joint wears its history on the outside: the shape it froze into is a fairly reliable record of what happened while it was liquid, which means you can read it.

By the end of this lesson you will be able to grade your own joints by eye, name what went wrong from the shape alone, and check a finished board properly before you put power anywhere near it.

9.1 The shape of a good joint

The aim of this section is to fix one silhouette in your mind, because everything after this is a deviation from it.

A joint that wetted properly is:

• Low. It hugs the board rather than standing up off it.
• Wide. It reaches out to the edge of the pad and stops there.
• Concave. The sides curve inward, hollow, like the base of a wine glass. They do not bulge.
• Continuous. No gaps, no visible seam where the solder meets either the leg or the pad.
• Even all the way round. Not piled on one side.

The everyday comparison is a small volcano hugging the leg: a wide base that tucks inward as it rises. That concave flank is the signature of wetting, and it is the single most reliable thing you can look at.

The one thing that is not a test

⚠️ Shine.

A leaded joint sets bright and mirror-like. A correct lead-free joint sets dull, and often faintly grainy or textured. Both are correct. See Section 3.2.

This matters more than any other sentence in this lesson, because the failure it prevents is a self-inflicted one. A beginner using lead-free sees a dull joint, decides it failed, reheats it, sees it dull again, reheats it again, and eventually the heat lifts the copper pad clean off the board. That damage is real and it is difficult to repair, and it was caused entirely by applying a leaded-solder standard to a lead-free joint.

Judge the silhouette. A dull joint with a good shape is a good joint. A shiny ball is not.

Figure 9.1: The shape to memorize. Low, wide, concave flanks, reaching to the edge of the pad. The finish can be bright or dull; only the shape is the test.

9.2 The fault gallery

The aim of this section is a diagnostic reference. Each fault is described by what you see, what caused it, and what to do. Come back to this page with a board in front of you.

Cold joint

What you see: a ball or a dome sitting on the pad, with a steep or undercut edge rather than a hollow flank. Often there is a visible line where the solder meets the pad, as though it has been dropped on rather than joined. Frequently dull and slightly rough.

What it means: it never wetted. The solder is touching the metal but is not bonded to it. See Section 1.2.

Cause: the joint was not hot enough. Usually the tip was not tinned, or solder was carried over on the tip, or the reader moved on too fast.

Why it matters: this is the fault that produces intermittent failures. It may conduct today and fail in a month, or work cold and fail warm. It is the most important fault in this gallery.

Fix: reheat it properly. Clean tinned tip, into the joint, let it come up to temperature, add a touch of fresh solder for its flux, watch it wet out, remove solder then iron.

Insufficient solder

What you see: a thin, starved joint. The leg is soldered but the pad is not fully covered, and you can see bare metal or the fillet does not reach the pad edge.

What it means: mechanically weak and easily cracked, though usually electrically fine at first.

Cause: not enough solder fed in, or the wire was pulled away before beat three finished.

Fix: reheat and add a little more. Easy.

Excessive solder

What you see: a fat, rounded dome or a ball that swallows the leg. Convex, bulging, no concave flank anywhere.

What it means: it may be perfectly sound underneath. The real problem is that you cannot tell, because the shape that would show you it wetted is buried.

Cause: too much wire fed in.

Why it matters more than it looks: it turns a visible fault into an invisible one, and on a dense board it is one nudge away from a bridge.

Fix: remove some with braid, Lesson 10, then reflow.

Solder bridge

What you see: solder connecting two adjacent pads or legs that should not be connected.

What it means: a short circuit. This one can actually damage things when you power up.

Cause: too much solder, or the tip dragged from one pad to the next.

Fix: Lesson 10. Usually the quickest cure is to put the tip in the gap between the two joints and draw it up one leg, so the solder follows the leg instead of spanning the gap. Braid handles the rest.

⚠️ Where to look hardest: rows of closely spaced pins. Headers, chip sockets, connectors. Look along the row from a low angle with a light behind it, which makes a bridge obvious in silhouette when it is invisible from above.

⚠️ The one exception. A pair of pads marked JP is a jumper, and on some boards you are meant to bridge it deliberately to select an option. See Section 7.2. Check the documentation before you clear a bridge you did not make, because occasionally it is doing a job.

Disturbed joint

What you see: a joint with a crystalline, frosted or cracked appearance. Sometimes a visible fracture line. It looks like it has a texture that a normal joint does not.

What it means: it was moved while freezing, so it solidified in a fractured structure.

Cause: the board or the component shifted during beat four. Much easier to do with lead-free, because of the pasty range in Section 3.1.

⚠️ Do not confuse this with normal lead-free dullness. These two are the most confusable pair in the whole course, and getting it wrong is what lifts pads, because you reheat a perfectly good joint.

The difference is texture and evenness, not brightness. Both are dull.

Run a fingernail-and-eye test: a sound lead-free joint looks like matte paint. A disturbed one looks like something that set while being stirred, because it is.

Fix: reheat, let it flow, and this time do not touch anything for two seconds.

Overheated joint

What you see: the joint may look fine, but around it the board is discolored, the solder mask is browned or blistered, or the silkscreen has yellowed. In bad cases the pad has visibly moved.

What it means: you held heat on it far too long. Section 9.4 covers the serious version.

Cause: sitting on a joint hoping. Usually because the pad was on a ground plane, Section 7.5, or the tip was not tinned.

Fix: the joint itself may be fine. Check that the pad is still bonded to the board and that no track has lifted.

Lifted pad

What you see: the copper pad has detached from the substrate and is standing proud, or has come away entirely, sometimes with a piece of track attached.

What it means: the most serious damage in this course. The pad is glued to the substrate and prolonged heat destroys that bond.

Cause: too much heat for too long, most often from repeatedly reheating a joint. Frequently caused by reheating a perfectly good lead-free joint because it looked dull, which is why Section 9.1 matters.

Fix: genuinely difficult. It usually means running a bypass wire from the component leg to the next point in the circuit. Doable, ugly, and much better avoided.

⛔ The prevention is a rule: never reheat the same joint more than twice in quick succession. If it has not worked twice, stop, let it cool completely, work out why, and come back.

Cold joint's cousin: no wetting on one side

What you see: the solder has wetted the leg beautifully but not the pad, or the pad but not the leg. Fine on one side, a clean line of separation on the other.

What it means: the tip only touched one of them. Section 8.3 beat one.

Cause: the tip was balanced on top of the leg rather than laid into the corner between leg and pad.

Fix: reheat with the tip properly in the corner.

Figure 9.2: The fault gallery. Each silhouette next to the good one, so the deviation is the thing you see.

9.3 Checking a whole board

The aim of this section is a routine to run before you apply power, in the order that finds the most faults for the least effort.

Do this before you plug anything in. Every time.

1. Look along, not down

Hold the board so you are looking across the solder side at a shallow angle, with a light on the far side. This is the single most effective inspection technique and almost nobody does it.

From directly above, every joint looks like a gray dot. From a low angle with backlight, you see silhouettes: the tall ones, the balls, the bridges between pins. Faults that are invisible from above are obvious from the side.

Rotate the board and do it from all four directions.

2. Count the joints

Compare against the board. Every leg through the board should have a joint on it. Missing joints are surprisingly common, especially on multi-pin connectors where one pin at the end gets forgotten.

3. Check the polarized parts, again

LEDs, electrolytic capacitors, diodes, chips. See Section 7.3. It is much better to find this now, before power, than after a bang.

4. Look for bridges along every row

Headers, sockets, connectors. Low angle, light behind.

5. The gentle tug

Give each component a very light pull or wiggle. Not a yank. Anything that moves has a joint that is not holding, which almost always means a cold joint.

⚠️ This is a mechanical test only. A joint that passes the tug test can still be electrically bad. Section 1.1: the two jobs can come apart. Never treat "it feels solid" as proof.

6. Continuity, if you have a meter

A cheap multimeter with a continuity beeper turns opinion into fact. Set it to continuity and check that each joint connects to where the circuit says it should, and does not connect to its neighbor.

Two checks worth doing on any board:

• Power to ground should NOT beep. If it does, you have a short somewhere, and you have just saved yourself an expensive power-up.
• Each joint to the track it belongs to should beep, confirming the joint is actually conducting rather than merely present.

7. Clean the flux off, and look again

Isopropyl and a stiff brush. With no-clean flux you can leave it, but sticky residue hides faults, and a cleaned board is much easier to inspect. Do the low-angle look again afterwards; you will find things you missed.

Figure 9.3: Look along the board, not down at it, with a light behind. Faults invisible from above are obvious in silhouette.

9.4 Knowing when to stop

The aim of this section is to prevent the one genuinely serious mistake, which is caused by trying too hard rather than too little.

Almost all real board damage in beginner work comes from repeatedly reheating the same joint.

The sequence is always the same. A joint looks wrong. You reheat it. It still looks wrong, because the reason was never heat. You reheat it again. And again. And then the pad lifts, and now you have a genuine problem where you previously had a cosmetic doubt.

⛔ The rule: two attempts, then stop.

If a joint has not come right in two goes, put the iron down and diagnose instead of persisting:

• Is my tip actually tinned? Lesson 6. This is the answer far more often than anything else.
• Is this pad on a ground plane? Section 7.5.
• Is the joint dull because it is lead-free and actually fine? Section 9.1. Very often, yes.
• Does it need fresh flux? An old joint has none left. See Section 3.3.

Then come back to it cold, with the right change made.

And a piece of perspective: an ugly joint that has wetted properly works perfectly. Electronics is full of functional, slightly untidy joints made by people who knew when to leave them alone. Wetted and ugly beats beautiful and lifted, every time.

Figure 9.4: Almost all real board damage comes from persistence rather than carelessness. Two attempts, then diagnose.

9.5 Test it for real

The aim of this section is to point you at the moment the whole thing pays off.

Inspection tells you a board looks right. Powering it up tells you it is right.

If you have built one of amomii's kits, you can go further than looking. amomii Link at amomii.com/pages/link talks to a built board in the browser over USB, no software to install, and runs it through its paces so the board itself tells you which parts are working. There are per-project pages for the Testudo shields.

We built it so that a beginner who has just finished their first board does not have to sit there wondering, which is the least enjoyable moment in the whole hobby. It is free and it needs no account.

If you built something else entirely, the principle still holds: have a plan for testing before you power up, so that "does it work?" has an answer better than "the light did not come on".

What this sets up

Lesson 10, desoldering and rework, is how you fix everything in the gallery above. Read it before you attempt any of the fixes here.

Lesson 11, wires and connectors, has its own set of faults, because a wire joint fails differently from a board joint.

Frequently asked questions

What does a good solder joint look like? Low, wide, with concave sides that curve inward, flowing out to the edge of the pad and stopping there. Like a small volcano hugging the leg. The finish can be bright or dull depending on your solder; only the shape is the test.

What is a cold solder joint? A joint where the solder never bonded to the metal, because the joint was not hot enough. It looks like a ball or dome with a steep edge rather than a hollow flank, often with a visible line where it meets the pad. It causes intermittent faults that appear weeks later.

Is a dull solder joint bad? Not necessarily. Lead-free solder sets dull and slightly textured when it is completely correct. Judge the shape rather than the shine. Reheating good lead-free joints because they look dull is a common way to lift a pad.

How do I fix a solder bridge? Desoldering braid is the usual answer, and sometimes simply reheating and drawing the tip away cleanly is enough. Lesson 10 covers both.

How can I tell if a solder joint is good without a multimeter? Look along the board at a shallow angle with a light behind it, which shows the silhouette of each joint. Check for the concave flank, check nothing bridges its neighbor, and give each part a very light wiggle. A meter turns opinion into fact, but the low-angle look finds most of it.

Why did the pad come off my circuit board? Too much heat for too long, usually from reheating the same joint repeatedly. Pads are bonded to the board and that bond fails with prolonged heat. Two attempts, then stop and diagnose.

Does a joint that holds firmly mean it is electrically good? No. A solder joint does two jobs, mechanical and electrical, and they can come apart. A cold joint can grip a component perfectly while conducting badly. That is why you look at the shape rather than trusting the feel.

Next

Lesson 10: Fixing mistakes, desoldering and rework. Braid, pumps, bridges and lifted pads. Nothing you have done is permanent.

Soldering Fundamentals is free and always will be. No sign-up, no gate, no catch.