Solder Powder: 6 Particle Size Rules for Fine Pitch
Solder powder is the metal fraction of a solder paste, and its size distribution is described by a type number that runs from type 3 down to type 7. The finer the powder, the smaller the aperture it can fill, and the more the paste costs and the faster it ages. Choosing the type is therefore a balance between print capability and process stability.
The choice is not free, and it is not reversible by changing the printing parameters. A paste with the wrong powder for the stencil will either clog the apertures or deposit far more oxide than the flux can clean, and both faults appear after reflow rather than at the printer.
What the Powder Type Number Means
Each type defines a range of particle diameters. Type 3 covers roughly 25 to 45 micrometres, type 4 covers 20 to 38, type 5 covers 15 to 25, and types 6 and 7 go finer still. The standard also limits the proportion of particles larger than the top of the range, because a few oversized grains can block a small aperture on their own.
The number is a purchasing and process decision, not a quality ranking. Finer is not better in general. A type 4 paste on a coarse stencil prints well and tolerates storage, while a type 6 paste on the same stencil brings oxide and shelf life problems with no gain in print quality.
Aperture Size Sets the Powder Type
The working rule is that the smallest aperture dimension should span at least five particles across, and six or more is safer. Below that ratio the powder bridges in the aperture, the deposit is short, and the stencil cleans poorly because paste stays in the opening between prints.
Fine pitch work therefore drives the choice. A 0.4 millimetre pitch stencil can run on type 4, a micro BGA at 0.3 millimetre usually needs type 5, and anything finer calls for type 6 or a different deposition method. The stencil aperture design rules give the area ratios that go with each class.
Oxide Content and Surface Area
Halving the diameter of the solder powder particles multiplies the surface area of the same mass of metal. Every square metre of that surface carries an oxide layer, and the oxygen is present before the paste is even opened. The flux has to reduce that oxide before it can clean the pad, so a fine powder consumes flux activity faster.
The visible consequence is a paste that loses tack quickly and needs a more active chemistry. Where the flux cannot keep up, the result is non wetting at the edges of the deposit, more solder balls and a joint that looks grainy. Metal content and powder type have to be read together for this reason, and the metal content specification is where that relationship is written down.
Print Release and Paste Rheology
Fine solder powder raises viscosity and yield stress for the same flux system, which helps a deposit hold its shape after release. That is an advantage for fine pitch, where a deposit must stand up without spreading towards the mask, and it is a disadvantage for coarse apertures where the paste must flow into a large opening.
Rheology is also set by the flux, so powder type alone does not predict printing. Two pastes of the same type and different flux can print very differently. The practical test remains the same: print the production stencil, measure the deposits with the inspection system, and compare volume against the paste volume target.
Voiding and Fine Powder
A solder powder that is too fine tends to produce more voids, for two reasons. The first is the extra oxide, which releases gas as it is reduced. The second is the larger amount of flux trapped under a deposit that melts quickly at its outer edge and seals the volatile path.
The effect is strongest on thermal pads and ground planes, where a large area has to vent through a narrow gap. Where void limits apply, a coarser powder with a well matched flux often beats a finer one, provided the stencil still fills. Voids should be measured rather than assumed, using X ray on a representative sample.
Storage, Shelf Life and Handling
Fine solder powder ages faster because it oxidises faster. Types 5, 6 and 7 should be stored cold, brought to room temperature in a sealed container, and used within the working life printed on the label. Warm up time is not optional, because condensation inside an opened jar adds water to the flux.
Handling matters as much as storage. Paste that is left on the stencil for a full shift loses solvent and gains viscosity, and a fine powder paste changes sooner. The paste working life for the type in use should be posted at the printer so that the change point is a decision rather than a surprise.
Solder Balls and Fines
Every solder powder batch contains a small fraction of very fine particles, and those fines are the first to oxidise and the last to coalesce. They are also the material that ends up as solder balls beside a deposit when the flux is exhausted, or as a haze of tiny spheres around a joint after reflow.
Reducing the fine fraction is a supplier question, and it is usually answered by a tighter cut of the distribution rather than by a different type. Where solder balls are a recurring defect, the paste specification, the stencil wipe interval and the reflow soak should be reviewed together, because each of them can leave fines behind on the mask.
Cost and Supply Decisions
Finer powder costs more per kilogram, and the premium is real rather than a marketing step. A type 6 paste can cost noticeably more than a type 4 of the same alloy, and the extra cost has to be justified by a product that cannot be built otherwise.
Supply is the second consideration. A fine type may be stocked in fewer alloys and fewer package sizes, which lengthens lead time and makes a second source harder to qualify. Where the design allows a coarser powder, choosing it keeps the material available and the process easier to control.
Verifying Powder Type on the Line
Verification starts with the label and the certificate that arrives with the jar, because solder powder cannot be identified by eye once it is mixed into paste. The batch number, the type and the alloy should be recorded against the product, so that a later defect can be traced back to the material used that day.
The second check is the print itself. Deposits that are short, ragged or full of pinholes point at solder powder that is too coarse for the aperture, while deposits that dry quickly and hold a peaked profile point at powder that is too fine for the flux. Both are visible at the printer, before reflow, which makes the powder type one of the few material choices that can be judged early.
FAQ
Can a finer solder powder be used to fix a bridging problem? Not as a first move. Bridging usually comes from stencil geometry, print pressure or slump rather than from particle size, and a finer paste can reduce bridging by stiffening the deposit. Where the aperture is already marginal, correcting the stencil is the cheaper route.
Is type 4 powder suitable for a 0.3 millimetre pitch? It can be, where the area ratio is healthy, the stencil is thin and the aperture walls release cleanly. Where any of those conditions is missing, type 5 gives more margin, and the paste should be qualified on the production stencil rather than on a test pattern.
Does a finer solder powder improve joint strength? Not directly. Joint strength follows the alloy, the void content and the wetting, and a finer powder can raise voiding while leaving strength unchanged. Selection should be driven by print capability, not by strength claims.
