Solder Paste Metal Load Control in Stencil Printing

Metal load is the share of a solder paste by mass that is solder alloy rather than flux, and it is the number that links the chemistry of the paste to the volume of the joint it forms. A no-clean paste for stencil printing typically sits between 85 and 88 percent by weight, and roughly 48 to 52 percent by volume, because the alloy is much denser than the flux vehicle. The two figures are not interchangeable, and confusing them is the most common source of error when a print process is being dialled in.

Weight Percent and Volume Percent

The weight figure is the one a supplier states on the certificate, and it is easy to verify by burning the flux off a small sample. The volume figure is the one that governs the printed deposit, because the stencil aperture defines a volume and the paste has to fill it. A paste at 87 percent metal by weight may be close to 50 percent by volume, and the conversion depends on the densities of the alloy and of the flux vehicle.

The practical consequence is that a change of one percent by weight moves the volume fraction by about one percent as well, so the deposit volume tracks the stated figure closely. Two pastes with the same weight percent but a different alloy density, such as a lead-free SAC alloy compared with a tin-lead alloy, will have different volume fractions and different slump behaviour on the same stencil.

How Metal Load Sets Viscosity

Viscosity in a solder paste is largely a consequence of the solids content of the suspension rather than a property of the flux alone. As the metal load rises the particles crowd, the flow resistance increases and the paste becomes stiffer. Below about 84 percent by weight the paste tends to slump and to bridge between adjacent apertures, while above about 89 percent it becomes difficult to roll across the stencil and does not fill the aperture completely.

The measurement is made with a rotational viscometer at a defined shear rate, commonly a spindle at 10 rpm and 25 degrees Celsius, and a standard stencil paste should land between about 150 and 250 pascal-seconds. The figure is strongly temperature sensitive, so paste has to be brought to room temperature before it is printed, and a jar taken straight from a refrigerator will read several times higher than its rated value.

Deposit Volume and Transfer Efficiency

Because the aperture volume is fixed by the stencil, the alloy delivered to a pad is the aperture volume multiplied by the volume fraction of metal and by the transfer efficiency of the release. A 50 percent volume fraction and a 90 percent transfer efficiency on an aperture of 1000 cubic micrometres therefore deliver about 450 cubic micrometres of alloy to the joint.

Transfer efficiency falls as the aperture area ratio falls, and it is the metal content that decides how forgiving the paste is at small apertures. A high metal load releases cleanly from the aperture wall but fills poorly, while a lower one fills well and leaves a tacky tail on release. The balance is struck for the smallest aperture on the board rather than for the average, and the geometry rules are set out in the guidance on aperture area ratio.

Measuring Metal Content in Production

The reference method is gravimetric. A sample is weighed, the flux is washed out in a solvent, the alloy is dried and weighed again, and the ratio is reported to two decimal places against the certificate. The test takes about thirty minutes and it is the only method that gives an absolute figure for the batch.

Faster methods are used for trend monitoring rather than for acceptance. Thermogravimetric analysis heats a milligram sample and records the mass lost as the flux volatilises, which gives a metal fraction in a few minutes. A deposit weight check on a printed coupon is a cheaper proxy still, and it catches a drifting process sooner than a periodic gravimetric test because it measures what actually reached the board.

Working Life and Drift on the Stencil

Once a jar is opened the metal load of the material inside it does not change, but the distribution of what is printed does. Solvent evaporates from the paste that sits on the stencil, the flux fraction at the top of the roll falls, and the deposit arrives drier and less tacky than the certificate describes. Slow printers see the effect first at the ends of a long board, where the roll has been standing longest.

A practical window for a no-clean or water-soluble paste is two to four hours of continuous printing, with a top-up of fresh material at a defined interval rather than a complete change. Paste left on the stencil at the end of a shift should be removed and discarded instead of being mixed back into the jar, because dried material carries an unknown metal fraction and an unknown particle distribution.

Stirring, Reuse and Top-Up Rules

Stirring a jar before use redistributes the powder, which settles during storage. A slow, gentle stir for thirty to sixty seconds is enough. High-shear mixing drives air into the paste, lowers the effective density of what is printed and shows up later as voiding in the reflowed joint, which is then misread as a profile problem.

Reuse is where the metal load is most often changed without anyone noticing. Paste recovered from the stencil and the squeegee carries flux that has lost solvent, and blending it back into fresh material produces a mixture whose behaviour no longer matches the certificate. A workable rule is that recovered paste is used within the same shift and never added to a sealed jar.

Print Parameter Compensation

When a paste sits at the low end of the metal range the printer compensates with a slower squeegee speed, a higher print pressure and a shorter snap-off distance, all of which raise the volume pushed into the aperture. When it sits at the high end, the opposite adjustments keep the deposit from smearing across the stencil and contaminating the underside.

These changes have to be recorded with the paste batch, because a parameter set that works for one lot can scrape the stencil for another. The verification is a print check on the smallest and on the densest aperture of the panel, measured by solder paste inspection instead of by eye, so that the volume trend becomes visible before a defect appears.

Acceptance Limits and Records

A workable incoming specification is a weight percent within one point of the certificate, a viscosity within twenty percent of nominal at the reference shear rate, and a print volume distribution whose standard deviation stays below ten percent of the mean on the smallest aperture. Each of those limits is tied to a measurement that can be repeated on the line.

The defects that follow from a metal load outside the window are characteristic. A low metal load produces slump, paste bridging and solder balling after reflow, while a high one produces incomplete filling, missing paste and a dry, crusted roll. The aperture clogging pattern that develops on a warm stencil is a second indicator, and it is worth recording as a traceable field alongside the print parameters for every lot.

FAQ

What is a normal metal load for solder paste? About 85 to 88 percent by weight for a standard stencil printing paste, which is roughly 48 to 52 percent by volume because the alloy is denser than the flux vehicle.

Does metal load change during a print run? The alloy fraction in the jar does not, but the fraction that reaches the board drifts as solvent evaporates from the paste standing on the stencil, which is why a top-up interval has to be defined.

How is metal content measured? By burning off the flux and weighing the alloy, or by thermogravimetric analysis for a faster result. A deposit weight check on a coupon is used as a production trend rather than as an absolute figure.