Solder Paste Printing Defects And How To Clear Them

Solder paste printing is the step that decides how much of the rest of the assembly process will go smoothly. The stencil deposits paste on every pad at once, and if the deposit is wrong on one pad, no amount of care at placement or reflow will produce a good joint. The defects that appear are predictable, and so are the parameters that cause them.

This article works through the printing problems that occur most often, the cause behind each, and the adjustment that clears it.

Almost all of them come down to three things: the paste, the stencil, and the machine settings. A defect that survives a change to all three is usually a footprint problem rather than a printing problem.

Uneven Deposit Thickness

Paste that is thick in one area and thin in another produces joints that range from too much to too little across the same board. The usual causes are uneven squeegee pressure, a stencil that has lost tension and no longer sits flat, insufficient support under the board so that it flexes as the blade passes, and paste whose rheology has changed because it was not brought to room temperature or was not mixed.

The order of investigation matters. Support comes first, because a board that moves under the blade will produce variation no matter how the other settings are adjusted. Stencil tension and aperture condition come next, and the paste is checked last, by measuring its viscosity rather than by judging it by eye. The pad and aperture geometry sets the target deposit in the first place.

Bridging Between Adjacent Pads

Bridging is a solder paste printing problem with more than one cause, and the printed deposit is only the first of them. Excess paste is the obvious culprit, and it arises when the aperture is too large for the pad or when the stencil does not release the paste cleanly, so part of the deposit stays in the wall of the aperture and is smeared onto the board as the stencil lifts.

The geometric quantity that governs release is the area ratio, the area of the aperture opening divided by the area of its walls. Below roughly two thirds, paste release becomes unreliable, which is why fine pitch apertures are made in a thinner stencil rather than made smaller in a thick one. A trapezoidal or rounded aperture wall, and a stencil with a coating that reduces friction, both improve release. Separating the board from the stencil at the right speed, rather than too quickly or too slowly, completes the fix.

Slump And Loss Of Edge Definition

Slump is the paste spreading after it has been printed, so that the crisp deposit becomes a rounded mound that reaches toward its neighbour. It is a matter of rheology: a paste with insufficient yield stress flows under its own weight, and the effect gets worse as the temperature rises. A long delay between printing and reflow gives it more time to happen.

The remedies are to keep the room within the paste supplier’s temperature range, to use a paste formulated against slump, and to keep the interval between printing and reflow short. The environment matters as well, since high humidity affects both the paste and the board. Where a design has narrow gaps between pads, the combination of a low metal load paste and a warm room is enough to produce slump on its own, regardless of the stencil.

Stencil Clogging That Survives Cleaning

A stencil that does not fully release its paste becomes clogged over time, and the first sign is a deposit that is short of paste on a small aperture. The direct cause is usually an area ratio that is marginal for the paste being used, combined with a cleaning cycle that does not remove what is left in the walls.

Cleaning has to match the paste. The solvent, the dwell time, the wiping action and, where available, vacuum assistance all affect whether the aperture is truly empty. Increasing the cleaning frequency helps when the paste dries quickly, and a paste with a longer open time reduces the problem at its source. Where the aperture is genuinely below what the process can handle, no cleaning routine will compensate, and the stencil design has to change.

Squeegee Sticking And Stringing

A squeegee that leaves strings of paste behind, or that lifts paste out of an aperture it has just filled, is usually a rheology problem rather than a blade problem. Paste that is too tacky, or a separation speed that is too slow, allows the paste to adhere to the blade rather than to the pad. Worn or damaged blades, and a blade angle that is not as specified, produce the same symptom.

The adjustment sequence is to verify the blade condition and angle, then the separation speed, then the paste. Metal blades hold their edge better than polymer ones and are preferred for fine pitch work, while polymer blades conform better on an uneven surface. The choice is a process decision and should be recorded, because a change of blade type changes the deposit volume.

Fine Pitch Below Four Tenths Of A Millimetre

Below a lead pitch of four tenths of a millimetre the process window narrows sharply. The aperture is small, so the area ratio is the limiting factor; the paste volume required is small, so the tolerance on thickness becomes proportionally large; and the gap between adjacent deposits is short, so bridging is easy. The standard measures are a thinner stencil with apertures designed for release, a stencil coating that reduces friction in the walls, better board support, and a paste with a fine particle size that suits the aperture.

It also helps to check the pads themselves. A pad that is larger than the component lead allows the deposit to spread further than the joint requires, and one that is smaller than the lead starves the joint. Where the printing process has been optimised and the defect remains, the footprint is normally the thing to change, and the position and size of the pads is a design decision rather than a machine setting. The mechanisms behind components moving during reflow are often decided at this stage as well, because an unbalanced deposit pulls a part to one side.

Keeping The Process In Control

Printing is verified by measurement, not by appearance. Deposit volume or height in a few representative locations, checked at a defined frequency, catches drift before it becomes a defect. The stencil is inspected for tension, damage and cleanliness at the same time, and the paste is checked for viscosity and for its age since opening.

The data is most useful when it is compared over time. A slow rise in the number of short deposits points to a stencil that is wearing or a paste that is losing its properties, and both can be addressed before the yield falls. gopcb builds boards with footprints and paste apertures designed for the process that will assemble them, and reviews pad geometry against stencil capability as part of the design release.

FAQ

Is bridging always caused by too much paste? Not always. It can also come from poor release, from slump, or from a pad that is larger than the component requires.

Why is a thin stencil used for fine pitch work? Because the area ratio depends on the thickness. A thinner stencil raises the ratio for a given aperture size, which improves paste release.

Does the paste have to be at room temperature before use? Yes. Cold paste does not mix properly and its viscosity is not predictable, which shows up as inconsistent deposit volume.