Non Wet Open Defects Explained

A non wet open is a joint where the solder never bonded to the pad at all. The solder may sit on the surface, it may have formed a ball beside the pad or it may appear to have covered it, but the interface is a separation rather than a bond. It is different from a cold joint, in which a bond exists but the grain structure is poor, and the difference matters because the two have different causes and different corrections.

How It Differs from a Cold Joint

In a cold joint the solder has wetted and then cooled without forming the proper intermetallic layer, so the joint is continuous but weak. Reheating it usually restores the joint, which is why a cold joint is a process problem.

In a non wet open there is no bond to restore. The solder never came into intimate contact with a clean, wettable surface, and the interface is a physical separation that a second reflow will simply reproduce unless the cause is removed.

The visual appearance helps to separate them. A cold joint is dull and grainy with a rounded, convex fillet, while a non wet open shows solder pulled back with an exposed pad edge, or a solder ball sitting on a surface it refused to wet.

The Surface Is the Usual Cause

Solder wets a metal, not an oxide or an organic film. Anything between the molten solder and the copper prevents the bond, and the wettable window is created by the surface finish and by the flux rather than by the solder itself.

Surface finish is the first suspect. An organic coating that has been stored too long, a gold layer that is too thin and has diffused, an immersion silver that has sulphided in the air and a tin layer whose oxide has grown all present a surface that the flux may not be able to reduce at the profile used.

Oxidation is a function of time and of environment. The shelf life of a finish is quoted for a defined storage condition, and a panel that has been stored in a warm humid room will oxidise faster than the specification assumes. Recording the receipt date and the storage condition is part of the control rather than paperwork.

Moisture and Contamination

Moisture in the board turns to steam in the reflow oven and expands out through the pad, and each escaping jet of vapour is a place where the flux is displaced and the solder cannot wet. This is why a board that has been exposed to humidity behaves differently from one that has just been baked.

Contamination from handling is the second source. Skin oils, residues from a wipe that was used for cleaning, flux that has been applied and left to dry and particles from the environment all change the surface energy and produce the defect on a small number of pads rather than across the board.

The pattern of the defect carries the diagnosis. A defect that follows the board position, the panel location or the direction of the conveyor points at a process or a machine; a defect that follows a component, a lot or a date points at a material.

Process Causes

Insufficient flux activity or an unevenly applied flux produces a non wet open that tends to appear where the spray coverage was weakest. The flux quantity should be measured by weight, not inferred from the appearance of the board.

A profile that is too short or too cool does not give the flux time to activate and to reduce the oxide. The window between activation and burn off is narrow, and a board with a large thermal mass can enter the reflow zone with the surface still below the activation temperature.

Paste and its storage belong in the same list. A paste that has been left open, that has passed its shelf life or that has been warmed and cooled repeatedly will not perform to specification, and the failure appears as a random distribution of unwetted joints that no machine adjustment corrects.

Detection and Inspection

An optical inspection finds the gross cases but cannot see under a package, and a joint that looks covered from above may be a non wet open at the interface. X ray shows the shape of the solder and reveals voids and missing volume rather than the bond itself.

The most reliable detection is a destructive test on a sample. A pull test or a dye and pry test separates the interface and shows exactly where the failure occurred, whether at the pad, at the component terminal or within the solder.

In production the practical approach is to combine automated inspection with a periodic destructive check on the first article of a run. A non wet open that passes optical inspection and only appears in a pull test is precisely the case that a first article check catches.

Root Cause and Correction

The correction follows from the pattern. Where the defect is spread across a board, the surface finish, the storage or the profile is the place to look. Where it follows one component or one area, the paste, the component lot or the local thermal mass is the likely cause.

The solderability of incoming boards should be tested before they enter production rather than after a failure. A wetting balance test on a coupon gives a number that can be compared with the specification and with previous lots, which turns a subjective judgement into a trend.

Where a cause is identified, the change should be verified on the same test that revealed it. Applying several corrections at once leaves the process in a state that nobody can reproduce, and the defect returns when the next lot arrives.

Prevention in the Flow

Control the finish specification at purchase, control the storage and the shelf life of the panels, and control the exposure between the bake and the reflow. Those three items remove most of the surface causes before the board reaches the printer.

On the line, measure the flux quantity, verify the profile on a representative board and hold the paste under the specified conditions. The checks are simple, and their value is that each one isolates a variable rather than the process as a whole.

Finally, keep the records. A non wet open is a defect that is easy to dismiss as a one off, and the only way to see that a batch of panels, a supplier or a shift has a problem is to have the data that links the defect to the boards it appeared on.

Practical Rules

Treat the surface as the primary variable: specify the finish, store the panels within the shelf life, bake before reflow when moisture is a risk and keep the handling clean. Measure the flux and the profile rather than assuming them.

Record the defect pattern with the build records and the defect history, and review it against the incoming material inspection results and the shelf life control whenever the same pattern repeats.

FAQ

How is a non wet open different from a cold joint? A cold joint has wetted and then cooled badly, so reheating can restore it. A non wet open never bonded at all, and a second reflow reproduces it unless the cause is removed.

What is the most common cause? The surface. An oxidised or contaminated finish, moisture escaping through the pad or contamination from handling all prevent the solder from making intimate contact.

How can it be detected under a package? A pull test or a dye and pry test on a sample separates the interface and shows where the failure occurred. Optical inspection cannot see the bond under a package.