Cold Solder Joints: Causes and Detection
What a Cold Joint Is
A cold joint is one in which the alloy did not fully melt, or did not form a proper metallurgical bond with the surfaces, so the joint is mechanically and electrically inferior despite appearing connected. The classic visual signature is a dull, grainy, blobby fillet with an irregular surface, and a wetting angle that is too high, because the alloy never reached the temperature at which it flows and wets properly. Where the joint is only partly reflowed, the solder may have collapsed into a ball but not formed a fillet on the pad, which is the form that most often escapes a visual inspection.
Cold, Disturbed and Starved
Three defects look similar and have different causes, so they should be separated. A cold joint is one that never reached temperature or never wet. A disturbed joint is one that reached temperature and was then physically moved before it solidified, which produces a fillet with a torn or a frosted appearance and often fine cracks. A starved joint is one where the solder volume was insufficient, which shows as a thin concave fillet rather than a grainy one. The distinction matters because the correction is different: a cold joint points to the profile or the thermal mass, a disturbed joint to handling or vibration after reflow, and a starved joint to the paste deposit.
Causes in the Profile
The most common cause is a profile that does not deliver enough heat to a particular joint. On a mixed assembly the heavy thermal mass reaches a lower peak than the light parts, and the joints connected to large planes or to thick components may never exceed liquidus for long enough to wet. An excessively short soak leaves the board thermally uneven, so some joints are still heating while others are already at peak. Too fast a ramp can also cause the flux to be consumed before the alloy melts, which leaves the surfaces oxidised at the moment of melting. Each of these is diagnosed with thermocouples on the worst-case joints rather than from the oven set point.
Causes in the Paste and the Board
Paste that has passed its working life, that has been left open on the stencil, or that has been re-mixed from a dried batch will not activate properly and produces joints that look cold even on a correct profile. Paste exposed to a humid environment absorbs moisture, which consumes the flux and produces both poor wetting and spatter. On the board side, a surface finish that has oxidised during long storage, a pad contaminated by handling, and a via that wicks the solder away from the joint all produce the appearance of a cold joint without any thermal cause. Where the defect appears on one part of a panel rather than another, contamination or storage is more likely than the profile.
Why It Looks Acceptable
The dangerous cold joint is the one that passes inspection. A joint that has reflowed on one side and not on the other can show a normal-looking fillet on the visible side while the interface is unbonded, and a ball that has collapsed but not wetted shows a shiny surface with no fillet. Automated optical inspection checks for the presence of solder and for the fillet shape, and a joint with the right shape and the wrong metallurgy passes. This is why cold joints are usually found by electrical test, by thermal cycling or in the field rather than at the inspection station.
Detection
Detection starts with the process rather than the product: a profile verified with thermocouples at the worst joints, a paste that is within its working life, and a board that is clean and dry. On the product itself, X-ray can show a joint that has not wetted where the shape is wrong in projection, and oblique views help on packages with hidden joints. In-circuit test with resistance measurement finds the joint that is intermittent and might not be found by a functional test. Where a defect rate is suspected, a destructive cross section of the worst joints is the definitive check, and it will show an unbonded interface that no optical inspection can see.
Preventing It
Treat the profile as a property of the board rather than of the oven, and re-verify it whenever a large component is added or the panel layout changes. Keep the paste within its working life and its storage conditions, and control the humidity and the time on the stencil. Keep the board dry and, where it has been stored, bake it before assembly. Keep the assembly still after reflow until the joints are solid, and avoid handling a board while it is hot. Where the design has joints that are thermally difficult, address it in the layout with a thermal relief or a wider pad rather than only in the profile, since a joint that is difficult to heat is a joint that will always be marginal.
Cold Joints in Through-Hole Work
The same mechanism appears in wave and selective soldering, with a different set of causes. The wave delivers a large amount of heat in a short time, so a cold joint usually means the joint did not receive the wave at all, or received it too briefly. A barrel that is shadowed by a tall component, a pad connected to a heavy plane that draws the heat away before the wave arrives, a board that is travelling too fast, and a flux that was applied too far ahead of the wave and has already lost its activity, all produce a joint that looks cold on a board that was otherwise perfectly soldered. The diagnostic evidence is the distribution rather than the appearance: a cold profile affects a whole panel, while a shadowed or poorly preheated joint affects a specific location that can be predicted from the layout.
FAQ
What does a cold joint look like? A dull, grainy fillet with an irregular surface and a high wetting angle, or a collapsed ball that has not formed a fillet on the pad.
Is a cold joint the same as a disturbed joint? No. A disturbed joint reached temperature and was moved before solidifying, which leaves a torn or frosted appearance.
Why do cold joints pass optical inspection? Because inspection checks the shape and the presence of solder, and a joint can have the right shape with an unbonded interface underneath.
Can a profile change fix every cold joint? No. Paste that is out of life, oxidised finishes and solder wicking into a via all produce the same appearance without a thermal cause.
How do I confirm the diagnosis? With thermocouples at the worst joints and a destructive cross section showing whether the interface is bonded.
Conclusion
A cold joint is a thermal or chemical failure that often leaves no outward sign, so prevention has to come from the process rather than from inspection. Verify the profile at the worst joints, control the paste and the board, keep the assembly still after reflow, and fix thermally difficult joints in the layout. The soldering process is described in SMT PCB assembly, the electrical checks that find escapes belong to PCBA testing, and the pad geometry that avoids hard-to-heat joints is set in PCB design and layout. Joint quality is normally established during prototype PCB assembly in 2026.
