Deaeration: 5 Rules for Solder Paste Handling

Deaeration is the removal of the air that gets into solder paste during storage, transfer and printing. On an SMT line the word usually covers the steps that stop that air from reaching the stencil at all, because air in the paste becomes entrapped air in the printed deposit, and that becomes voiding under the joint after reflow. Paste handling therefore decides the void level long before the printer is switched on.

Air gets in by simple routes. Opening a jar that has been chilled draws air into the surface, stirring with a spatula folds more in, and scooping paste out of a jar drags a layer of air down with it. Printing adds more when the squeegee roll moves too fast or the stencil separates too quickly, so paste that was clean when the jar was opened can still print with voids.

Where Air Enters Solder Paste

Manufacturers deaerate paste during mixing, so a fresh sealed jar is as free of air as the process allows. Everything after that adds air back. A jar that is opened warm, a jar that is left open while the operator does something else, and a jar that is returned to the shelf with a skin on top all carry air into the next print.

Transfer is the second source. A metal spatula that is stirred through the paste traps pockets as it folds the material. A scoop that is pushed into the jar compresses the paste ahead of it and then releases it, which leaves a cavity behind. Both are habits that can be changed without any equipment purchase.

Jar Handling and Mixing Practice

The rule that matters most is to keep the jar closed except when paste is being taken out. A jar that is open on the bench for a whole shift absorbs air and moisture, and the surface skins over. Skinned paste that is mixed back into the body carries the dried fragments into the print as hard particles.

When mixing is required, it should be slow and in one direction with a clean, flat tool, and it should stop as soon as the paste is uniform. Long, fast stirring does not improve the paste and does add air. Where a jar is nearly empty, it is better to use it up on a coarse print than to scrape the last paste into a fine pitch job.

Deaeration by Vacuum

Vacuum deaeration is used mainly for paste that will be dispensed rather than printed, because a dispensing needle cannot tolerate a pocket of air. The paste is held under reduced pressure so that the trapped bubbles rise and break at the surface. The cycle has to be gentle, since a rapid pull down can boil the solvent and change the rheology.

The same idea appears in syringe filling, where the supplier fills from the bottom and the plunger is fitted without trapping air above the paste. Where a shop fills its own syringes, the filling station should be checked for the same reason, and the syringe should be stood tip down for a short time before use so that any remaining pocket rises away from the needle.

Printing Parameters That Release Trapped Air

Printing cannot remove air that is already in the paste, but it can decide whether the air ends up inside the deposit or escapes at the aperture wall. A slow squeegee stroke gives the paste time to roll and to fill the aperture from the bottom. A fast stroke skims over the opening and leaves the corners unfilled.

Separation speed is the other half. If the stencil lifts away from the board faster than the paste can follow, the deposit tears and a void is left behind at the pad edge. Setting the separation speed from the printed result, rather than from a machine default, is the practical way to control it, and the paste volume measurement confirms whether the change worked.

Voids Under Thermal Pads and BGAs

A void under a thermal pad matters more than the same void elsewhere, because the pad exists to carry heat. A gap in the solder layer raises the thermal resistance of the joint, and the component runs hotter than the design assumed. Voiding under a large pad is normally assessed as a percentage of the joint area rather than as a count.

Ball grid arrays have their own acceptance limits, and the measurement is made by X-ray rather than by optical inspection. Where the void percentage is consistently high, the cause is usually the paste deposit rather than the reflow profile, and the print should be checked before the oven is adjusted. Standard acceptance limits for these joints are described in the IPC assembly standards.

Paste Warming, Thaw and Handling Windows

Paste that is taken straight from the fridge and opened immediately will condense moisture on its surface. That water is mixed into the paste on the first stir and appears later as spatter or as a poor print. The correct routine is to leave the sealed jar to reach room temperature first, then open it, then use it.

The time a jar may stand open, the time it may be out of the fridge in total, and the number of times it may be returned to storage all need limits. A paste that has been through several warm and cold cycles separates, and separated paste prints unevenly no matter how carefully the printer is set. The solder paste thaw routine should be written and logged rather than left to habit.

Recognizing Air Defects on the Line

Air in the paste shows up in a few recognisable ways. Deposits may have a pitted surface like a sponge, corners may be rounded or missing, and small craters may appear at random positions across the panel. Random positions are the clue that the fault is in the material rather than in the stencil.

Repeatable positions point somewhere else entirely. A missing corner in the same place on every panel is a stencil or support problem, and a void under one specific component is usually a thermal or a pad geometry problem. Separating random from repeatable defects is the fastest way to decide whether deaeration and handling are the right place to look.

Inspecting Prints for Air Defects

Paste inspection equipment measures height and volume at every deposit, and a deposit that contains a large pocket of air often shows a volume that is correct while the shape is irregular. Where the equipment reports a height map, a crater or a dimple across the deposit is visible as a local depression rather than as a missing area.

Where inspection is optical only, the inspection should include a low angle light view, which makes surface pitting much easier to see. The results should be compared with the volume record from the previous build, because a rise in variation across the panel is often the first evidence that the paste has been over handled. The SPI inspection setup determines whether those differences are visible at all.

Records and Supplier Requirements

The paste log should record the lot number, the date the jar was opened, the time out of storage and the panel numbers printed from it. When a void problem appears, that log is what allows the affected boards to be traced, and it also shows whether a particular lot or a particular shift is involved.

Where voids persist, the supplier should be asked for the deaeration data on the paste and for the recommended handling window. Asking for that information before a problem appears is cheaper, and it turns a future argument about responsibility into a comparison of two sets of numbers. Reflow defects such as graping can be caused by the same handling faults, so the two should be investigated together.

FAQ

Should solder paste be stirred before use? Only if the supplier says so. Most modern pastes are designed to be used straight from the jar after thawing, and stirring adds air that the manufacturer removed. Where the paste has separated, a slow fold with a clean flat tool is enough, and it should stop as soon as the texture is even.

Does vacuum deaeration help printed paste? It helps most where paste is dispensed through a needle, because a trapped pocket interrupts the flow. For stencil printing the greater gains come from handling discipline and from printer settings, since the roll of paste at the squeegee is constantly exposed to air in any case.

How much voiding is acceptable under a BGA? The limit depends on the joint and on the product class, and it is normally expressed as a percentage of the ball or pad area measured by X-ray. Where a design has a thermal requirement, ask the customer for the number rather than applying a generic figure, because the same void that is acceptable for a signal ball may fail a thermal pad.