Why Joints Fail — Metallurgical Causes

Section Overview

This section is where Chapter 1's science pays off. You've learned what soldering is (1.1), how heat moves (1.2), what wetting is (1.3), and what the intermetallic bond is (1.4). Now we put it to work by asking the diagnostic question: how do solder joints go bad, and why? A joint must be two things at once — a sound electrical connection and a mechanical bond — and it fails when the metallurgy goes wrong, either at soldering time or slowly over years in service. The causes group into a handful of families, and you already understand every one. The solder may have never wetted — a cold joint (too little heat or dirty metal), or non-wetting/dewetting (1.3), or a disturbed joint. There may be too little or too much solder — a starved or excess joint. The intermetallic may have grown thick and brittle (1.4) — from overheating, over-reflow, or aging — or suffered gold embrittlement or Kirkendall voids. The joint may have fatigued and cracked from thermal cycling — solder fatigue driven by CTE mismatch (the component, solder, and board expanding at different rates), the leading long-term killer. It may be hollowed by a void, or degraded by contamination and corrosion. Each failure has a visual signature, and — because you know the science — each has a prevention this chapter has already taught: good wetting, the right heat for the right time, the right amount of solder, clean boards, and designing for thermal stress. This section closes the science of soldering and hands you the eyes of a diagnostician.

Why This Matters

Repair is diagnosis, and half of diagnosis is reading solder joints. A technician who understands why joints fail can look at a board and see its history and its faults — and fix the right thing. Without this understanding, a bad joint is just a mystery; with it, every failure is a clue that points to a cause and a cure. It matters because the failures have wildly different causes and cures: a dull, grainy joint (a cold joint) needs more heat and flux; a cracked ring around a lead (solder fatigue) points to thermal stress and design, not your technique; a green, crusty joint means corrosion and cleaning, not resoldering. Confusing them wastes time and fixes nothing. It matters for reliability engineering: knowing that CTE mismatch fatigues joints under thermal cycling is why big components and BGAs get underfill or compliant mounting, and why power-cycled, hot-running products fail at the solder joints first. It matters for your own work: every failure mode in this section traces back to a principle you can control — wetting, heat, time, quantity, cleanliness — so understanding the failures is understanding how to avoid them. And it matters because a failed joint can be a safety issue: an intermittent or cracked joint in a live product can arc, overheat, or drop a safety-critical connection. This section is the capstone of the chapter: it takes the abstract science — wetting, intermetallics, heat — and turns it into the practical, visual skill of knowing a good joint from a bad one and why. When you can name why a joint failed, you can make one that won't.

Required Prerequisites

• Intermetallic Compounds and Joint Strength — several failure modes here (brittle joints, gold embrittlement, Kirkendall voids, thermal aging) are the intermetallic story from that section; and this synthesis draws on all of Chapter 1 (wetting in 1.3, heat in 1.2, the joint in 1.1). Read 1.4 (and ideally the whole chapter) first.

Recommended Consumables

• A collection of scrap boards with real failures — cold joints, cracked joints, corroded joints, bridges — to practice reading (a "museum of bad joints" is a great learning tool)
• Flux and fresh solder (Volume 2) — because most bench-time failures are prevented by good wetting
• IPA and lint-free wipes (Volume 2 10.3) — because contamination and corrosion are cleaning failures
• Notes from Sections 1.1 through 1.4 — this section ties them all together

Recommended Practice Hardware

• A magnifier or microscope (Volume 2, Chapter 9) — reading failures is a visual skill; you need to see the fillet, the surface, and the cracks
• Dead boards from scrapped equipment — especially thermally-stressed ones (power supplies, LED drivers, automotive modules) to find fatigue cracks
• A temperature-controlled iron to practice the preventions (good wetting, right heat/time)
• Fume extraction, eye protection, and the usual soldering safety

Real-World Applications

Failure reading is the daily work of repair. A technician handed an intermittent device inspects the joints first: a dull, grainy joint (cold joint — poor wetting) gets reflowed with flux; a hairline crack circling a heavy component's leads (solder fatigue from CTE mismatch and thermal cycling) tells them the device died of thermal stress, common in power supplies and anything that runs hot and cycles. A repairer of a liquid-damaged board sees green corrosion and knows the failure is contamination/corrosion, not a bad solder joint per se — the cure is cleaning, not resoldering. A reflow/BGA specialist X-rays a joint and finds a void — a gas cavity that weakened it and blocked heat flow. A reliability engineer who understands CTE mismatch specifies underfill under BGAs and compliant leads on big parts precisely to fight fatigue cracking. And a good solderer, at every joint, is quietly preventing this whole catalog: clean and flux for wetting, right heat for the right time for a thin intermetallic, the right amount of solder, and a clean board — so the joint doesn't become one of these failures. The value of this knowledge is enormous: it turns "this board is broken" into "this joint failed by fatigue, so I'll remake it and consider the thermal design," and it turns your own soldering from hoping into knowing what makes a joint last. Every repair tech is, in part, a solder-joint failure analyst.

Common Challenges

• Confusing failure modes. A dull joint (cold), a cracked joint (fatigue), and a green joint (corrosion) have totally different cures — reading them correctly is the whole skill; resoldering a corroded joint fixes nothing.
• Blaming technique for a design failure. A fatigue crack from CTE mismatch is thermal stress over time, not your soldering — recognize it so you address the real cause.
• Missing hidden failures. Voids and thin cracks can hide inside or under a joint that looks fine — some failures need magnification or X-ray, and knowing they exist is half the battle.

Safety Notes

Risk Level: Low. This is failure analysis, so the primary "hazard" is joint reliability — but the usual soldering safety applies, and a failed joint is itself a safety matter.

Professional Tips Before Starting

• Read the joint before you touch it. The look tells you the cause: dull/grainy (cold/disturbed), balled/bare (non-wetting), cracked ring (fatigue), green/crusty (corrosion). Diagnose first, then fix the right thing.
• Match the cure to the cause. A cold joint wants heat and flux; a fatigue crack wants a remake and a look at thermal design; corrosion wants cleaning. Resoldering everything is not a diagnosis.
• Suspect thermal stress on the hot, heavy, and cycled parts. Big components, BGAs, and anything that runs hot and power-cycles are where fatigue cracks live — look there first on a device that failed after long service.

How Solder Joints Go Bad — and How to Prevent Each

Poor-Wetting Failures — Cold, Non-Wetted, and Disturbed Joints

The first family is failures of wetting — the solder never formed a proper bond — and it's the most common on the bench. A cold joint is the classic: the joint didn't get hot enough (too little heat, or heat lost to thermal mass — Section 1.2), and/or the metal was dirty/oxidized, so the solder never properly wetted (Section 1.3). It looks dull, grainy, and lumpy rather than smooth and shiny, often balled or blobby, and it's mechanically weak and electrically high-resistance or intermittent. Closely related are non-wetting (solder balls up on bare metal — never adhered) and dewetting (solder wet then pulled back), both from Section 1.3, and both wetting failures at root. A fourth is the disturbed joint: a non-eutectic alloy (with a plastic range — Volume 2, Section 10.1) that was moved while cooling through that slushy range, fracturing the solidifying metal into a grainy, cracked joint (a eutectic alloy avoids this, and holding the joint still prevents it). The common root of this whole family is that the solder never made a sound wetted bond — and the cure is the wetting recipe: clean, flux, enough heat, and hold still.

Solder-Quantity Failures — Starved and Excess

The second family is the wrong amount of solder. A starved (insufficient) joint has too little solder — a thin, incomplete fillet that may not fully bridge the pad and lead, giving a weak, low-reliability connection that cracks or opens easily. An excess (overloaded) joint has too much solder — a bulging blob that, while it looks "safe," actually causes problems: it hides whether the joint actually wetted underneath (a ball of solder can sit over a non-wetted pad), it can bridge to a neighboring pad or pin (a short), and its extra mass adds stress and can stress the pad. The right amount of solder is enough to form a full, concave, feathered fillet — no more. Both too little and too much are defects; aim for a proper, complete fillet — the shape you learned to recognize in Section 1.1.

Brittle-Intermetallic Failures

The third family comes straight from Section 1.4: the joint is brittle because of its intermetallic. A thin intermetallic is the bond; but a thick intermetallic — grown by overheating, soldering too long, repeated reflow/rework, or thermal aging in service — is brittle, and the joint cracks under mechanical or thermal stress. Gold embrittlement (soldering onto heavy gold, forming brittle gold-tin intermetallic) is a specific case, as are Kirkendall voids (aging voids along the intermetallic interface). These failures often look fine on the surface — the brittleness is at the interface — and they show up as cracking under stress or over time. The prevention is the Section 1.4 discipline: the right heat for the right time, minimal reflows, and keeping gold out of the joint — so the intermetallic stays thin.

Fatigue and Thermal-Cycling Failures

The fourth family is the leading long-term killer of solder joints in real products: solder fatigue from thermal cycling. Here's the mechanism. Electronics heat up when powered and cool when off — they thermal-cycle. The component, the solder, and the board are different materials that expand and contract by different amounts when heated — a CTE mismatch (a mismatch in coefficient of thermal expansion). So every heat-cool cycle strains the solder joint (the solder is squeezed and stretched as the parts grow and shrink differently), and — cycle after cycle, over months and years — the solder fatigues (like a paperclip bent back and forth) and eventually cracks. The crack often rings the joint (a hairline crack around a lead or under a component), giving an intermittent or open connection. Solder also creeps — slowly deforming under sustained stress at operating temperature — which compounds the fatigue. Fatigue is worst where thermal stress is highest: large components (more expansion difference across their size), BGAs (rigid, no compliant leads), and hot, power-cycled parts (regulators, power devices). This is why big and hot parts fail at the solder first, why BGAs get underfill, and why "it worked for two years then went intermittent" so often means a fatigue crack. Thermal cycling plus CTE mismatch fatigues and cracks solder over time — the dominant long-term failure.

Voids

The fifth family is voids — empty cavities inside the solder. A void is a pocket of trapped gas or flux (from outgassing flux, entrapped air, or contamination) that forms inside a joint as it solidifies, most notoriously in reflowed and BGA joints (where you can't see under the part). Voids reduce the joint's cross-section, so they lower its mechanical strength, raise its electrical resistance, and — importantly for power and thermal parts — impede heat transfer (a voided joint under a power device runs hotter). Voids also concentrate stress, giving fatigue cracks a place to start. A small amount of voiding is usually acceptable; large or numerous voids are a reliability problem. Voids are hidden — they need X-ray (or a cross-section) to see — which is why BGA rework is X-ray-inspected. (The Kirkendall voids of Section 1.4 are a related, aging-driven kind at the intermetallic interface.) Voids hollow out a joint's strength, conductivity, and heat path — and hide where you can't see them.

Contamination and Corrosion

The sixth family fails the joint chemically, over time: contamination and corrosion. Left-on corrosive flux residue — especially unwashed water-soluble/activated flux (Volume 2, Sections 10.2 and 10.3) — is ionic and hygroscopic: it holds moisture and conducts, causing current leakage, electrochemical migration and dendrites (Volume 2, Section 10.6), and corrosion of the metal over weeks to years. Other ionic contamination (fingerprints, salts, handling) does the same. Corrosion — from moisture, contaminants, and voltage — eats the metal and degrades the connection. These failures often look green, crusty, or filmy, and they develop after the board leaves the bench, which is why they're so insidious. The cure is cleanliness: clean off corrosive residues (right cleaner for the flux — Section 10.3), keep boards clean and dry, and conformal-coat for harsh environments (Section 10.6). Contamination and corrosion kill joints slowly and chemically — and the prevention is a clean, dry, well-finished board.

Recognizing and Preventing Failures

Pulling it together, here is the diagnostic-and-prevention synthesis of the whole chapter. Recognize by the look: dull, grainy, lumpy → cold or disturbed (poor wetting); balled up on bare metal → non-wetting; thin, incomplete fillet → starved; bulging blob → excess; cracked ring around a lead or under a part → fatigue; hidden cavity (X-ray) → void; green, crusty, filmy → contamination/corrosion; cracks with no surface cause → brittle intermetallic. Prevent by the principles this chapter taught: for wetting failures, clean, flux, and enough heat (1.2, 1.3); for quantity, the right amount for a full fillet; for brittle intermetallics, the right heat for the right time and few reflows (1.4); for fatigue, manage thermal stress — design (underfill, compliant mounting), derating, and sound joints to resist it; for voids, good process (proper flux, heat profile); and for contamination, clean boards (Volume 2, 10.3/10.6). Almost every failure traces to a principle you now control. Read the joint, name the cause, and fix — or prevent — the right thing. That is the science of soldering made practical.

Common Mistakes

• Resoldering a joint that failed from corrosion. Corrosion is a cleaning failure — reflowing it fixes nothing; clean the contamination instead.
• Treating a fatigue crack as a soldering error. Fatigue is thermal stress over time (CTE mismatch) — remake the joint, but address the thermal design/derating, not just your technique.
• Trusting a shiny blob. Excess solder hides whether the joint wetted — a ball can sit over a non-wetted pad; aim for a readable, feathered fillet, not a blob.
• Ignoring hidden voids. Voids and interface cracks hide — power/BGA joints may need X-ray; don't assume "looks fine" means "is fine" on critical work.
• Leaving corrosive flux residue on. Unwashed water-soluble residue corrodes over time — clean it (Section 10.3).

Troubleshooting Guidance

Match the symptom to the family, then the cure. If a joint is dull, grainy, and weak: it's a cold joint or disturbed joint (poor wetting) — reflow with flux and enough heat, and hold it still while it cools. If solder balled up on bare metal: non-wetting — clean, flux, heat, and check solderability (Section 1.3). If a heavy or hot component is intermittent after long service: suspect solder fatigue from thermal cycling (CTE mismatch) — look for a crack ringing the leads, remake the joint, and consider the thermal design. If a joint looks perfect but cracks under stress: suspect a thick/brittle intermetallic or gold embrittlement (Section 1.4) — remake it with the right heat/time, removing gold if present. If a power device runs hot or a BGA is flaky: suspect voids — X-ray if you can, and rework with a good heat profile and flux. If the board is green, crusty, or leaky: contamination/corrosion — clean thoroughly (Section 10.3), don't just resolder. If joints fail across a whole board over time: look for a systemic cause — unwashed corrosive flux, a hot environment (thermal aging/fatigue), or poor process. The throughline: the look and the history name the failure family, and each family has a specific cure — reading correctly is the whole skill.

Verification & Testing Methods

Use this as a failure-diagnosis checklist:

• [ ] I can identify a cold joint (dull, grainy, weak — poor wetting) and its cure (flux + enough heat; hold still).
• [ ] I can tell non-wetting/dewetting (balled/bare) and starved/excess solder from a good, feathered fillet.
• [ ] I can recognize brittle-intermetallic failures (thick IMC, gold embrittlement, Kirkendall voids) and their cure (right heat/time, few reflows).
• [ ] I understand solder fatigue from thermal cycling and CTE mismatch as the leading long-term failure, and where it strikes (big/hot/cycled parts, BGAs).
• [ ] I know a void hollows out strength, conductivity, and the heat path, and is often hidden (needs X-ray).
• [ ] I can spot contamination/corrosion (green/crusty) and know the cure is cleaning, not resoldering — and I can name the prevention for each failure family.

Then try the practice exercises below — failure diagnosis; scenarios differ from the quiz.

Practice Exercises

1. Name the failure by its look (5 minutes, reasoning). For each appearance — dull and grainy; balled up on bare metal; a bulging blob; a hairline crack ringing a heavy component's leads; green and crusty — name the likely failure family and its most probable cause.
2. The two-year intermittent (5 minutes, applied). A power supply that ran hot goes intermittent after two years, and you find a crack around a large component's leads. Explain, using CTE mismatch and thermal cycling, what happened and what you'd do (repair and design).
3. Match cure to cause (5 minutes, reasoning). For a cold joint, a corroded joint, and a gold-embrittled joint, state the correct cure for each — and explain why "just resolder it" is wrong for at least one.
4. Prevent the catalog (5 minutes, applied). Pick three failure families and, for each, name the Chapter 1 principle (wetting, heat/time, quantity, cleanliness, thermal design) that prevents it.

These core ideas — the metallurgical failure families (poor wetting, wrong quantity, brittle intermetallics, thermal-cycling fatigue, voids, contamination), their visual signatures, and how each is prevented — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• Solder joints fail metallurgically in a few families, each with a visual signature and a prevention this chapter taught.
• Poor-wetting failures: a cold joint (too little heat/dirty metal — dull, grainy, weak, intermittent), non-wetting/dewetting (1.3), and the disturbed joint (moved in its plastic range) — cure: clean, flux, enough heat, hold still.
• Solder-quantity failures: starved (too little — weak fillet) and excess (too much — hides defects, bridges, adds stress) — cure: a full, feathered fillet, no more.
• Brittle-intermetallic failures (1.4): a thick IMC (overheating/aging), gold embrittlement, and Kirkendall voids — often invisible until they crack — cure: right heat for the right time, few reflows.
• Solder fatigue from thermal cycling and CTE mismatch (component, solder, and board expanding differently) is the leading long-term failure — it cracks the solder over years, worst under large, hot, and power-cycled parts and BGAs — cure: manage thermal stress (design, derating, sound joints).
• Voids (trapped gas/flux cavities) hollow out strength, conductivity, and heat flow and hide (X-ray); contamination/corrosion (left-on corrosive residue) degrades joints chemically over time — cure: good process and clean, dry boards.

Skills Learned

• You can now name the metallurgical failure modes and their root causes.
• You can now recognize poor-wetting failures and solder-quantity failures.
• You can now explain brittle-intermetallic failures and thermal-cycling fatigue.
• You can now recognize voids and contamination or corrosion failures.
• You can now prevent each failure by applying this chapter's principles.

Glossary Additions

• cold joint — a defective solder joint in which the metal did not get hot enough (too little heat, or heat lost to thermal mass) and/or was too dirty or oxidized, so the solder never properly wetted; it looks dull, grainy, and lumpy rather than smooth and shiny, is mechanically weak, and makes a high-resistance or intermittent electrical connection. The cure is the wetting recipe: clean metal, flux, enough heat, and holding the joint still while it solidifies.
• solder fatigue — the progressive cracking of a solder joint caused by repeated mechanical strain, most commonly from thermal cycling: as a device heats and cools, its component, solder, and board expand and contract by different amounts (a CTE mismatch), straining the solder each cycle until, over months or years, it fatigues and cracks (often in a ring around a lead or under a component). It is the leading long-term failure of solder joints and is worst under large components, BGAs, and hot, power-cycled parts.
• CTE mismatch — a difference in the coefficient of thermal expansion (how much a material expands per degree of temperature) between the parts of a solder joint — the component, the solder, and the circuit board — so that they grow and shrink by different amounts as temperature changes; under thermal cycling this mismatch repeatedly strains the solder and drives solder fatigue, and it is managed by design measures such as underfill under BGAs, compliant leads on large parts, and derating.
• void — an empty cavity of trapped gas or flux inside a solder joint, formed as the joint solidifies (from outgassing flux, entrapped air, or contamination) and common in reflowed and BGA joints where it cannot be seen from outside; a void reduces the joint's cross-section, lowering its mechanical strength and raising its resistance, impedes heat transfer (a concern under power devices), and concentrates stress that can start a fatigue crack. Large or numerous voids are a reliability problem, and they are detected by X-ray or cross-section.

Suggested Next Sections

Must read next:

• Tin-Lead (SnPb) Solder — this section completes Chapter 1, "The Science of Soldering." Chapter 2, "Solder Chemistry and Selection," turns from the science to the materials in depth, starting with tin-lead solder: its metallurgy, the eutectic, its properties, and why it remains the benchmark for hand soldering.

Recommended:

• Intermetallic Compounds and Joint Strength — the source of the brittle-joint, gold-embrittlement, and Kirkendall-void failures covered here.
• Wetting — The Key to a Good Joint — the source of the cold-joint, non-wetting, and dewetting failures, and the recipe that prevents them.