Blowhole Defects in Wave Soldering: Causes and Cures

A blowhole is a void that has pushed a bubble of gas up through the solder fillet and left an opening at the surface of the joint. It is not the same defect as a pin hole caused by shrinkage, and it is not a void trapped inside the barrel, although all three start from gas that could not escape before the alloy froze.

In wave soldering the gas has a short window in which to leave, and anything that delays it leaves a marker on the joint. Reading that marker correctly is the whole diagnosis, because the fix differs depending on whether the gas came from the flux, from moisture in the laminate or from the plating in the barrel.

What a Blowhole Looks Like on a Joint

The classic signature is an irregular opening, often with a raised lip where the bubble broke through, and frequently more than one per joint along the same barrel. The inner surface can show a darker oxide colour because the escaping gas was hot and reducing conditions were lost as the bubble formed.

Position is informative. A blowhole near the solder entry side points to gas that formed late in the wave contact, while one at the top of a barrel points to gas travelling ahead of the rising alloy. Photographing the defect at magnification, with the board orientation marked, makes the pattern visible across a batch.

Where the Gas Comes From

Four sources dominate: volatiles released by the flux as it activates, water absorbed in the laminate and driven out by heat, plating chemistry residues left in a poorly rinsed barrel, and air displaced from the barrel as alloy rises through it. Each produces a similar looking opening but responds to a different control.

Distinguishing them is a matter of timing and of what the parts have in common. If every board from one supplier of laminate shows the defect, moisture is the likely source. If the defect follows a change of flux lot but appears across several laminate suppliers, the chemistry of the flux is the place to look.

Flux Chemistry and Blowhole Formation

Flux has to be volatile enough to carry its activators into the joint and stable enough not to boil violently when it meets the alloy. A flux with too much solvent, or one applied in a thickness beyond the recommended film, releases a large volume of vapour exactly when the wave makes contact and the barrel is closing.

Application control is therefore more important than the choice of chemistry. Foam, spray and jet applicators all deliver different amounts, and a spray nozzle that has partly blocked can deposit a heavy patch on one area of the panel. Weighing a test coupon before and after fluxing converts that judgement into a number, and the usual target is a flux film near the low end of the supplier range, not the high end. The methods behind this are set out in the solderability test literature.

Preheat and Its Effect on Outgassing

Preheat exists to bring the board and the flux to a temperature where the flux activates and most of its solvent has already evaporated before the wave arrives. If the top side of the board is too cold, the solvent flashes off under the alloy and the vapour has nowhere to go except through the joint.

A practical window is a top side board temperature of 90 to 110 C at the moment of wave contact for a no clean flux, with the laminate supplier limit treated as the ceiling. Measuring that temperature with a contact probe on the top surface, on the real board with its real copper distribution, is the only reliable way to set the preheat zones.

<img src="https://www.gopcba.com/wp-content/uploads/2026/09/270-2-scaled.jpg" alt="Blowhole defect opening on a wave soldered through-hole joint” />

Barrel Geometry and Plating Effects

A barrel that is narrower than the component lead by a small margin fills quickly, and the gas inside it has to pass the alloy on the way out. The same barrel with a generous clearance vents more easily, because the alloy rises around the lead rather than sealing the hole in one movement.

Plating quality sets the other half of the problem. A barrel with thin or discontinuous copper in the hole heats unevenly and can release trapped chemistry from the laminate interface, and a poorly rinsed hole after desmear carries residues that turn to vapour. Barrel inspection before assembly is what makes this deterministic rather than a matter of luck.

Solder Pot Condition and Wave Dynamics

Contamination in the pot changes the surface behaviour of the alloy. Copper, iron and organic residues raise the oxide level on the wave, and a dirty wave transfers that oxide to the joint, where it can be trapped under the fillet and later appear as an opening after the surface is disturbed.

Wave height and contact length also matter. A wave that is too high floods the board and lets flux escape where it should not, while a short contact time does not give the gas time to leave. Contact lengths of two to four seconds with the wave crest just touching the board are the normal starting point.

Panel Handling, Moisture and Storage

Boards that have absorbed water show blowholes that cluster in areas of high copper density and fine geometry, because those areas heat fastest and release vapour first. Storage in a controlled environment and a documented drying step before assembly remove most of this class of defect before it can occur. Where the panels have already absorbed moisture, the drying profile described for moisture sensitive boards gives the starting point for the bake step.

The same applies to the parts. A through-hole connector that has been stored in a humid warehouse carries water in its moulding compound, and that water becomes gas in the barrel. Placing both board and components under the same moisture control avoids a defect that no adjustment of the wave can correct.

Inspection and Sectioning Evidence

Visual inspection finds the openings, but it does not tell the difference between a blowhole that breached the surface and a void hiding just below it. Sectioning a known bad joint and a known good one from the same panel gives the comparison that identifies which layer the gas came from.

When a barrel is sectioned, look at the interface between the plating and the solder, the condition of the plated copper, and the amount of residue in the laminate. Dark, ring shaped defects around the barrel are the signature of outgassing at the hole wall, while clean plating with a single void points to flux or contamination from the wave itself.

Process Fixes That Actually Work

Order the fixes by cost. Confirm the flux film weight, then confirm the top side preheat temperature, then confirm the board and component moisture history. Only after those three are proven should the pot analysis and the wave parameters be adjusted, because changing them first makes the earlier numbers meaningless. The wider family of joint defects that share these mechanisms is summarised under solder defects and board failures.

When the cause has been found, change one thing and re-measure. The defect is eliminated by removing the gas source or by giving the gas more time to escape, and the two strategies show up differently in the data: a source fix removes the defect from all joints at once, while a timing fix usually improves the worst joints first.

FAQ

What is the difference between a blowhole and a pin hole in wave soldering? A blowhole is formed by gas escaping through the joint before the alloy freezes, while a pin hole is a shrinkage or surface tension effect and has a smoother internal wall. Sectioning shows the difference clearly.

Does preheat always prevent blowholes? Preheat removes solvent from the flux and reduces the vapour that forms at the wave, but it cannot remove water that is sealed inside the laminate or the component body. Those sources need controlled storage and drying instead.

Should the flux film be thicker for boards with heavy copper? No. A thicker film releases more vapour at the wave and is a common cause of the defect itself. Heavy copper asks for more preheat energy and a longer contact time, not a heavier flux application.