Solder Paste Metal Content And Powder Size Explained

Solder paste is an alloy powder suspended in a flux vehicle, and the ratio between the two is the single number that most influences how the paste prints. Suppliers quote that ratio as metal content by weight, which is convenient for them and slightly misleading for everyone else, because printing responds to how much of the volume the powder occupies rather than how much of the mass.

What Metal Content Means By Weight And Volume

A jar marked 88.5 percent describes the weight fraction of alloy. Because a tin-silver-copper alloy is roughly seven times denser than the flux vehicle, that weight figure corresponds to a volume fraction near fifty percent. Two pastes can share the same weight figure and still print differently, because the volume fraction shifts when the flux density or the powder packing changes.

Suppliers hold the weight fraction inside a band of about one percent for a stencil grade, typically 87 to 89 percent, and adjust the flux to reach the target viscosity. The band is tight because a tenth of a percent already changes the deposited volume measurably once the aperture is small, and that change applies to every joint on the board at once.

Powder Size Class And Its Practical Effect

Powder is classified by the upper end of its particle distribution. A Type 3 powder spans roughly 25 to 45 micrometres, Type 4 spans 20 to 38, Type 5 spans 10 to 25, and Type 6 falls below 15. The usual rule is that at least five particles must fit across the narrowest aperture dimension, which is what pushes fine apertures toward a finer class.

Finer powder carries more surface area for the same mass, so the oxide load rises and the flux has to work harder to displace it. A Type 5 paste is therefore not automatically better than Type 4. It buys printability on small apertures and pays for it with a shorter working life and a greater tendency to form solder balls.

How Metal Content Changes Viscosity

Viscosity climbs steeply as the volume fraction of powder rises, because the particles crowd and resist flow. A paste at 88 percent by weight may measure near 180 pascal-seconds at a low shear rate, while a small increase in metal load can add tens of pascal-seconds. The relationship is not linear, which is exactly why the weight fraction is controlled so tightly.

Paste is also thixotropic: it thins while the squeegee shears it and thickens again once the deposit is at rest. The recovery rate decides whether a printed deposit holds its shape during the pause before placement. A paste can therefore show the correct steady viscosity and still slump if its recovery is slow.

Print Behaviour At Different Metal Loads

A higher metal load produces a deposit that holds its shape and resists slump, which helps on wide pads and thicker deposits. A lower load flows more readily and releases from small apertures with less residue left behind in the stencil. The practical window for stencil printing runs from about 85 to 90 percent by weight, and the smallest aperture on the board normally sets the choice, which is why paste grade and paste inspection limits are usually reviewed together.

Print speed and squeegee pressure interact with the load. A high-metal paste wants a slightly slower stroke and adequate pressure to fill the aperture, while a low-metal paste prints faster but risks a thinner deposit. Both settings belong in the process record next to the paste lot, since a supplier change invalidates them.

Reflow Consequences And Joint Volume

Only the alloy survives reflow, so the deposited volume minus the flux residue fixes the joint. A paste sitting one percent below its nominal metal content deposits about one percent less alloy for the same print, which lowers fillet height across the whole assembly. That is a systematic shift rather than a random defect, and it is easy to mistake for a stencil problem when the visible symptoms are solder defects at the joint.

Metal load also influences voiding. More flux means more volatiles that must escape, and a thick deposit of a low-metal paste can leave voids under a large thermal pad. Where a void limit applies, the paste grade and the stencil thickness should be chosen together instead of being specified by different people.

Measuring Metal Content On Incoming Lots

The classic check is a burn-off test: weigh a sample, heat it until the flux is fully driven off, and weigh the residue. The result is the weight fraction directly, and the test is simple enough to run on every incoming lot. The sample should come from the container that will actually go to the line, not from a separate qualification jar kept on a shelf, and the result should be filed the way any other quality judgement is filed.

Powder size is verified by sieve analysis or laser diffraction, and the distribution carries more information than the single upper bound. A paste can satisfy its declared class and still print badly if the distribution has a long tail of coarse particles. Requesting the distribution rather than the class alone is the more useful specification.

Powder Oxidation And Shelf Behaviour

Oxide on the powder is typically limited to about 0.15 percent by weight in a stencil grade. Oxide does not melt with the alloy; the flux has to displace it, so a higher oxide load consumes flux activity and degrades wetting. Storage temperature and repeated freeze-thaw cycles both raise the oxide level in service.

This explains how a paste can measure the correct metal content and still perform poorly. The weight fraction is unchanged while the surface condition of the powder has moved. Recording the storage history alongside the lot number is the only practical way to separate the two effects when a defect appears.

Adjusting For Fine Pitch And Small Apertures

Below an aperture of roughly 0.3 millimetres, the powder class becomes the limiting factor rather than the metal load. The area ratio of the aperture governs transfer efficiency, and a powder whose coarse particles approach the aperture width produces ragged, incomplete deposits. For a 0.25 millimetre aperture, Type 5 or finer is the practical choice.

Raising the metal load can partly compensate by stiffening the deposit, but it also raises the viscosity and reduces aperture fill at higher print speeds. The combination that usually works is a finer powder with a slightly lower load, which keeps transfer efficient while still holding the printed shape through placement.

Lot To Lot Variation And Specification

A workable specification states the weight fraction with a tolerance near half a percent, the powder type, the maximum oxide content, and the viscosity measured by a defined method at a defined shear rate. It also states the storage and thaw requirements, because both shift the values that a later measurement will produce.

Sampling frequency matters as much as the limits, and it belongs in the same document as the rest of the production process flow. Testing one lot in ten will not reveal a slow drift, while testing every lot catches a single bad delivery at the cost of time. A control chart on the burn-off result, drawn from the same container, shows drift long before the paste leaves its usable window.

FAQ

Is metal content a weight or a volume figure? Suppliers publish weight fraction, but printing follows volume fraction. At a typical 88 percent by weight, the powder occupies roughly half the volume.

Should fine pitch work always use the finest powder? No. Use the coarsest class that still places five particles across the smallest aperture, and keep the finer classes for apertures that genuinely need them.

Can metal content be adjusted on the production line? No. It is fixed by the supplier, and any attempt to thin or thicken the paste changes the flux chemistry as well as the load.