Visual Inspection Criteria

Section Overview

The framework of the last section told you that a joint is judged against an acceptance class into target, acceptable, and defect conditions (10.1) — but it did not tell you what those conditions actually look like. This section is the yardstick itself: the specific visual criteria you hold a solder joint against. Start with the good joint: an acceptable joint is well wetted to both the pad and the lead, with a low, feathered wetting angle, a smooth surface (bright and shiny for tin-lead, more of a satin, slightly grainy look for lead-free), and a correctly sized concave solder fillet — the shape and coverage the standard illustrates as the target (6.7). Surface-mount joints are judged mostly on fillet coverage and part position: a proper side and end fillet, the part sitting centered on its pads within the allowed side and end overhang, and a minimum joint width and length (6.7). Through-hole joints add one central, measurable criterion — hole fill: the percentage of the plated barrel filled with solder, which the standard requires to be higher as the class rises, together with wetting to the annular ring on both sides. Then come the defects that fail a joint: insufficient solder, excess solder, a cold joint or disturbed joint, a solder bridge, non-wetting (solder that never bonded at all, distinct from dewetting, where it wetted and then pulled back), solder balls, excessive voids, tombstoning, and fillet lift (a fillet that has partly lifted from the pad). The single most important habit is this: every one of these criteria is applied at the product's acceptance class, so a slightly reduced fillet or a lower hole fill can be perfectly acceptable at Class 2 and a defect at Class 3 (10.1). You inspect each joint under adequate magnification and good light, in a consistent order, and grade it target, acceptable, or defect for its class. This is the standard made practical — the actual criteria you read a joint against, one joint at a time.

Why This Matters

Criteria are what make an inspection real — without them, "acceptable" is a feeling; with them, it is a decision you can make the same way twice and defend to anyone. This matters because the good-joint marks are your reference: knowing that a sound joint is well wetted, low-angled, smooth, and concave gives you the picture every real joint is compared against — you cannot spot a defect without first knowing what good looks like (6.7). It matters because through-hole quality is measurable: hole fill turns "enough solder in the hole" into a number the standard sets by class, so a barrel that is only partly filled is judged against a definite requirement, not a guess. It matters because the defects are specific: non-wetting, a cold joint, a solder bridge, excess or insufficient solder, fillet lift — each is a named condition with its own criterion, so you are looking for definite things, not a vague sense of "bad." It matters because the class changes the verdict: the same slightly-reduced fillet is fine on a Class 2 board and a defect on a Class 3 one, so the criteria only mean something once you apply them at the right class (10.1). It matters because both errors cost: accepting a real defect lets an unreliable board ship, and rejecting a joint that already meets its class wastes rework and puts an extra heat cycle through a good joint. And it matters because a criterion-based inspection is repeatable and teachable: anyone applying the same criteria to the same class reaches the same call. Learn the criteria, apply them at the class, and inspection becomes a decision instead of an opinion.

Required Prerequisites

• IPC-610 Joint Quality Standards — Overview — Section 10.1 gave you the framework: the standard, the acceptance classes, and the target/acceptable/defect conditions this section fills in with real criteria. You should also know what a good surface-mount joint looks like (6.7) and the common joint faults — cold joint, bridge, tombstone (5.4; 7.3; 7.4). This is a knowledge and judgment section — no hot work or power.

Recommended Consumables

• Sample boards with a mix of good, acceptable, and defective joints — to practice grading against real examples
• A class-criteria reference or acceptance chart — to look up the requirement for each condition and class (10.1)
• A joint-inspection log or grading sheet — to record each joint target, acceptable, or defect
• A printed good-versus-defect comparison card — to keep the reference picture in front of you

Recommended Practice Hardware

• A magnifier or stereo microscope and good, angled light (Volume 2, Chapter 9; 10.3) — to see fillet shape, wetting, and surface clearly
• A cross-sectioned or cutaway through-hole sample, or a clear diagram — to see what a given hole fill percentage looks like
• Assemblies from different product classes — to practice applying the same criteria at different classes
• No iron, hot air, or powered equipment — this section is reading and judgment, not procedure

Real-World Applications

Visual criteria are applied on every board that is inspected, and getting them right is the difference between a reliable repair and a false pass or a needless rework. An inspector checking a reworked chip resistor confirms a good side and end fillet, the part centered within the allowed overhang, and a smooth concave surface — and passes it (6.7). A technician judging a through-hole connector sees the barrel is only partly filled, checks the hole fill requirement for the board's class, and reworks the joints that fall short. A repairer who spots a dull, beaded pad where solder never took identifies non-wetting — not merely a cold joint — and re-cleans, re-fluxes, and re-solders to get true wetting. An inspector on a lead-free board recognizes the normal satin, slightly grainy surface as acceptable rather than mistaking it for a cold joint, and separately catches a genuine fillet lift at a pad edge. And a shop inspecting the same joint on two products passes the slightly reduced fillet on the Class 2 unit and rejects it on the Class 3 one, correctly applying the criterion at each class (10.1). The failures good criteria prevent: a partly-filled barrel passed by guess, a non-wetted pad mistaken for acceptable, a normal lead-free surface needlessly reworked, and a critical joint judged by the wrong class — all avoided by reading each joint against the right criterion for its class.

Common Challenges

• Mistaking a normal lead-free surface for a defect. Lead-free joints look satin and slightly grainy, not bright and shiny — judge them by wetting and fillet shape, not by gloss, or you will reject good joints (5.4).
• Guessing at through-hole fill. "Looks like enough" is not a criterion — hole fill is a percentage set by the class, so judge the barrel against that requirement.
• Applying one personal standard to every board. The same joint is graded differently by class — inspect to the product's acceptance class, not to your own fixed idea of good (10.1).

Safety Notes

Risk Level: Low. This is a knowledge and judgment section — no hot work, no power, no chemicals. The real risk is a wrong verdict, in either direction.

Professional Tips Before Starting

• Fix the class, then read the criteria. Every verdict depends on the acceptance class, so establish it first and apply each criterion at that class — good looks different at Class 2 and Class 3 (10.1).
• Judge lead-free by shape, not shine. A satin, slightly grainy surface is normal for lead-free — look at wetting and fillet shape to tell a good joint from a genuine cold joint, not at gloss (5.4).
• Treat hole fill as a number. Do not eyeball "enough solder in the hole" — know the fill percentage the class requires and judge the barrel against it.

Reading a Joint Against the Criteria

What a Good Joint Looks Like

Before you can name a defect, you need a clear picture of a sound joint — the reference every real joint is compared against. A good solder joint has three visible marks. First, wetting: the solder has bonded to and spread smoothly across both the pad and the lead, meeting each at a low, feathered wetting angle rather than balling up on a surface it did not take to (6.7). A low wetting angle is the single strongest sign the joint actually bonded. Second, the fillet: the solder forms a smooth, correctly sized concave solder fillet — a gentle hollow curve sweeping from the pad up to the lead, neither starved and thin nor bulged and excessive (heel fillet). The fillet's shape and size are what the standard illustrates as the target condition (10.1). Third, the surface: a sound joint is smooth and even. For tin-lead solder that means bright and shiny; for lead-free it means a duller satin finish that is often slightly grainy — and that lead-free texture is normal, not a cold joint, so you judge a lead-free joint by its wetting and fillet, not by its shine (5.4). Put together, the target is easy to picture: well wetted at a low angle, a clean concave fillet, a smooth surface. Everything else in this section is a departure from that picture — either a still-acceptable variation or a defect — so fix this reference firmly in mind first. Know what good looks like, and the defects announce themselves.

SMD Joint Criteria

Surface-mount joints are judged mostly on two things: how well the fillet covers the connection, and where the part sits on its pads. For a chip component such as a resistor or capacitor, the criteria look at the fillet at each end of the part and at the part's position (6.7). There should be a proper end fillet where the termination meets the pad, with solder wetted up the end of the part, and the joint should meet a minimum height and width — enough solder, well wetted, not a thin smear. The part should sit centered on its pads, and for a chip component the standard allows only a limited side overhang (the termination hanging off the side of the pad, a class-dependent fraction of the width) — but no end overhang at all: the termination must not hang off the end of the pad, which is a defect at every class. For a gull-wing lead, such as on a quad flat package, the criteria look at the toe (the tip of the lead, which may extend slightly past the pad within the allowed limit), the heel (heel fillet, the bend where the lead rises from the pad — often the most important fillet), and the side, each needing proper wetting and a minimum fillet, with limits on how far the lead may sit off its pad. The recurring theme is coverage and position: enough well-wetted solder forming a real fillet, and the part aligned within the allowed overhang. These are exactly the marks you learned to read on a surface-mount joint (6.7), now expressed as pass/fail criteria — and, as always, the exact limits tighten with the acceptance class. Judge an SMD joint by its fillets and its alignment, at the class the product requires.

Through-Hole Criteria and Hole Fill

Through-hole joints add one criterion that surface-mount does not have, and it is the most important one for them: how much of the hole is actually filled with solder. That criterion is hole fill — the percentage of the plated barrel (the metal-lined hole the lead passes through) that is filled with solder, measured up the height of the barrel and often called vertical fill. It matters because a through-hole joint's strength and reliability come from solder filling the barrel and wetting its full length, not just from a bead on the surface, so a partly-filled barrel is a weak joint even if the top looks fine. The standard sets a minimum vertical fill — commonly at least about 75 percent of the barrel — and that same 75 percent minimum applies to both Class 2 and Class 3, while Class 1 is permitted less. What actually tightens as the class rises is not a higher headline fill percentage but the circumferential wetting the joint must show (on both the solder side and the component side) and how little reduced fill — for a heavy thermal or ground-plane connection that pulls heat from the joint — is allowed: reduced-fill exceptions permitted for a fraction of holes at Class 2 are restricted at Class 3. Alongside fill, the through-hole criteria look at wetting to the annular ring (the ring of pad around the hole) on both the sides of the board, and a proper solder fillet on the solder side and, where required, evidence of solder having reached the component side. The practical consequence is that you cannot judge a through-hole joint from one side alone or by "it looks full" — you assess the fill against the class requirement. This is why fill is treated as a number, not an impression: hole fill is measurable, class-dependent, and central to a through-hole joint's acceptance. Judge a through-hole joint first on its fill, against the minimum and the circumferential wetting its class requires.

The Defects That Fail a Joint

A defect is any condition that fails the criterion for the joint's class — and the visual defects fall into a recognizable set. Too little solder is insufficient solder*: a thin, starved joint with too small a fillet, not enough to make a reliable connection.* Too much is excess solder: a bulged, convex blob that hides the joint and can bridge — the fillet should be concave, not ballooned. A dull, grainy, or ball-shaped joint that never properly flowed is a cold joint, and one made dull or fractured by movement while it solidified is a disturbed joint (5.4). Unwanted solder joining two points that should be separate is a solder bridge*, a short* (7.3). Then there are the wetting failures. Non-wetting is solder that never bonded to a surface at all — it beads up and leaves the metal bare or barely coated, because the surface was contaminated, oxidized, or unfluxed; it is distinct from dewetting, where the solder did wet the surface and then drew back into irregular mounds, leaving thin and thick patches. Both mean the joint did not form properly. Small stray spheres of solder around the joint are solder balls, and gas pockets trapped inside the joint are voids — a few small ones may be acceptable, but excessive voiding is a defect (note that voids buried inside a joint or barrel usually need X-ray or a cross-section to see, not visual inspection — only surface voids show to the eye, 10.5). A chip stood up on one end is tombstoning (7.4). And fillet lift is a fillet that has partly lifted or separated from the pad at its edge — a condition seen more often with lead-free solder — which can reduce the joint's integrity. Each of these is a named condition with its own criterion, and each is judged against the class — the next idea. Learn the set, and you are looking for definite defects, not a vague sense of wrong.

Grading Against the Class

The step that ties everything together is the one from the last section: every criterion is applied at the product's acceptance class, so the class decides the verdict. This is not a detail — it is the whole reason the criteria have to be learned alongside the classes (10.1). Take a joint with a slightly reduced but still concave fillet, well wetted, just not the full target size. On a Class 1 or Class 2 board, that is very likely an acceptable condition — it meets the requirement and needs no rework. On a Class 3 board, the same joint may fall short of the stricter criterion and be a defect that must be reworked. The same is true of hole fill*: a fill that satisfies Class 2 can be below the Class 3 minimum.* And it runs the other way too — a condition that is a clear defect at every class, like a solder bridge or non-wetting*, fails regardless.* So grading is always a two-part act: identify the condition (what is it — a reduced fillet, a partial fill, a non-wetted pad?), then apply the criterion for the product's class (is that condition target, acceptable, or a defect here?). Skipping the class turns inspection back into personal opinion; applying it makes the verdict defensible. There is also the process indicator to remember (10.1): a condition that is not a defect but that, seen repeatedly, warns the process is drifting. Always ask not just "what is this joint?" but "what is this joint, for this class?" — that question is the heart of criteria-based inspection.

A Systematic Inspection Method

Criteria only help if you apply them the same way every time, so the last piece is a repeatable method for working through a board. Set up to see clearly first: adequate magnification and good, angled light, because most criteria — fillet shape, wetting, a fine bridge, a partial fill — cannot be judged with the naked eye (10.3; 6.7). Establish the acceptance class before you start, so every verdict is made at the right bar (10.1). Then inspect in a consistent order: work across the board methodically — by region, by row, or component by component — so no joint is skipped and none is judged twice. For each joint, compare it against the criteria for its type and class: for surface-mount, the fillets and the part's position; for through-hole, the fill and the annular-ring wetting; for every joint, the wetting, the fillet shape, and the surface, and a scan for the defect set. Grade each joint and record it — target, acceptable, or defect — and note any process indicator pattern you see repeating. Route the defects to rework and re-inspect them afterward, to the same class. The discipline is what makes it reliable: good light and magnification, the class fixed up front, a consistent path, each joint against its criteria, a recorded verdict. Done this way, inspection is a repeatable procedure whose result does not depend on who did it or what mood they were in — which is exactly what a standard is for.

Common Mistakes

• Judging a lead-free joint by its shine. A satin, grainy surface is normal for lead-free — reject it as a cold joint and you throw out good work; judge by wetting and fillet instead (5.4).
• Eyeballing through-hole fill. "Looks full" is not a criterion — hole fill is a class-set percentage of the barrel; judge the fill against that number.
• Confusing non-wetting with dewetting — or with a cold joint. They are different conditions — non-wetting never bonded, dewetting bonded then receded; naming it right guides the fix.
• Inspecting to a personal standard. The same joint grades differently by class — apply the criterion at the product's acceptance class, not your own fixed idea (10.1).
• Skipping magnification. Fine bridges, partial fills, and fillet shape hide from the naked eye — inspect under adequate magnification and good light (10.3).

Troubleshooting Guidance

Grading problems usually trace to the reference, the class, or naming the condition. If you cannot tell good from bad: fix the good-joint picture first — well wetted, low angle, concave fillet, smooth surface (6.7). If a lead-free joint looks dull: that satin, grainy surface is normal — check wetting and fillet before calling it a cold joint (5.4). If a through-hole joint looks full on top but you are unsure: judge hole fill against the class percentage, not the surface bead. If a pad is bare or beaded where solder should be: distinguish non-wetting (never bonded) from dewetting (bonded then receded), then clean, flux, and resolder. If two inspectors grade the same joint differently: they are probably applying different classes — agree the acceptance class and re-grade (10.1). If you keep seeing the same minor condition: treat it as a process indicator and look at the process (10.1). If a fillet edge has lifted: that is fillet lift — judge it against the class and rework if it fails. The throughline: know good, fix the class, name the condition, and grade it against the class.

Verification & Testing Methods

Use this as a check that you can apply the criteria, not a hot procedure:

• [ ] I can describe a good joint — proper wetting to pad and lead, a low wetting angle, a concave fillet, and a smooth (shiny or satin) surface (6.7).
• [ ] I can judge a surface-mount joint by its side and end fillets, the allowed overhang, and the part's position on the pads (6.7).
• [ ] I can judge a through-hole joint by hole fill against the class requirement and wetting to the annular ring on both sides.
• [ ] I can identify the defects — insufficient and excess solder, cold and disturbed joints, bridges, non-wetting and dewetting, solder balls, voids, tombstones, and fillet lift.
• [ ] I grade each joint against the product's acceptance class, knowing the same condition can be acceptable at Class 2 and a defect at Class 3 (10.1).
• [ ] I inspect systematically — class fixed first, adequate magnification and light, a consistent order, each joint recorded target, acceptable, or defect (10.3).

Then try the practice exercises below — inspection and classification practice; scenarios differ from the quiz.

Practice Exercises

1. Read the good joint (5 minutes, reasoning). From a described or pictured joint, list the marks that make it acceptable — wetting, wetting angle, fillet shape, surface — and note whether a lead-free surface changes your call.
2. Grade through-hole fill (6 minutes, classification). Given several through-hole joints with stated fill percentages and a stated class, decide which pass and which fail on hole fill, and explain each.
3. Name the defect (5 minutes, classification). Given short descriptions — a beaded bare pad, a bulged blob, a dull grainy joint, a lifted fillet edge — name each defect and distinguish non-wetting from dewetting.
4. Grade the same joint at two classes (4 minutes, reasoning). For one slightly-reduced-fillet joint, decide the verdict at Class 2 and at Class 3, and explain why the class changes the outcome.

These core ideas — the good-joint marks, the surface-mount and through-hole criteria, hole fill, the defect set including non-wetting and fillet lift, and grading each joint against its class — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• A good joint is well wetted to pad and lead at a low wetting angle, with a correctly sized concave solder fillet and a smooth surface — bright for tin-lead, a normal satin/grainy finish for lead-free, which you judge by wetting and shape, not shine (6.7; 5.4).
• Surface-mount joints are graded on fillet coverage and part position — proper side and end fillets, minimum joint width and length, and the part centered within the allowed side and end overhang (6.7).
• Hole fill — the percentage of the plated barrel filled with solder — is the primary through-hole criterion: about 75 percent vertical fill is the common minimum for both Class 2 and Class 3 (Class 1 allows less), and the class tightens the circumferential wetting and reduced-fill allowances rather than the headline percentage, plus wetting to the annular ring on both sides.
• The defects that fail a joint are specific: insufficient and excess solder, cold joint and disturbed joint, solder bridge, non-wetting (never bonded) or dewetting (bonded then receded), solder balls, excessive voids, tombstoning, and fillet lift.
• Every criterion is applied at the product's acceptance class*, so the same joint can be acceptable at Class 2 and a defect at Class 3* (10.1) — inspect systematically under magnification, class fixed first, and grade each joint target, acceptable, or defect.

Skills Learned

• You can now recognize a good joint by its wetting, fillet, and surface.
• You can now judge a surface-mount joint by fillet coverage, overhang, and position.
• You can now judge through-hole hole fill against the requirement for a class.
• You can now identify the defects that fail a joint, including non-wetting and fillet lift.
• You can now grade a joint target, acceptable, or defect against its class.

Glossary Additions

• hole fill — for a through-hole solder joint, the percentage of the plated barrel (the metal-lined hole the lead passes through) that is filled with solder, measured up the height of the barrel and also called vertical fill; it is the primary acceptance criterion for through-hole joints because a joint's strength and reliability come from solder filling and wetting the barrel, not just from a surface bead. A vertical fill of about 75 percent is the common minimum for both Class 2 and Class 3, with Class 1 permitted less; what tightens as the class rises is the circumferential wetting the joint must show and how little reduced fill (for a heavy thermal or ground-plane connection) is allowed, not a higher headline fill percentage. Fill is assessed alongside wetting to the annular ring on both sides of the board.
• non-wetting — a condition in which molten solder fails to bond to and spread across a metal surface at all, beading up and leaving the surface bare or only thinly coated, typically because the surface was contaminated, oxidized, or not properly fluxed; it is a defect because the joint never truly formed. Non-wetting is distinct from dewetting, in which the solder does initially wet the surface and then draws back into irregular thick and thin mounds — non-wetting means the bond never happened, while dewetting means it happened and then receded.
• fillet lift — a solder-joint condition in which the fillet has partly lifted or separated from the pad or land at its edge, most often seen with lead-free solder as it solidifies and shrinks; it can reduce the integrity of the joint and is graded against the acceptance class, where its allowability depends on the class and the extent of the lifting. It is distinct from a lifted pad, where the copper pad itself separates from the board — in fillet lift the pad stays down and the solder fillet lifts from it.

Suggested Next Sections

Must read next:

• Using Microscopy for Inspection — this section assumed you can see the joint clearly; the next one is the tool that makes it possible — choosing and using a stereo microscope and lighting to resolve fillet shape, wetting, fine bridges, and the fill and defects these criteria depend on.

Recommended:

• IPC-610 Joint Quality Standards — Overview — the framework these criteria live inside: the standard, the acceptance classes, and the target/acceptable/defect conditions you grade each joint into.
• SMD Joint Inspection — the practical surface-mount inspection skill these criteria formalize, joint by joint.