Type 4 Paste: 6 Rules for Fine Pitch Printing

Type 4 paste is a solder paste classification that describes the powder size inside it, and the number is printed on the jar because it decides what the paste can print. A type 4 powder has particles small enough to pass through a fine aperture, but not so small that the surface area of the alloy changes the flux chemistry.

The practical question on a mixed board is which powder size the smallest aperture actually needs. Choosing finer powder than the design requires costs money and can create new problems, while choosing coarser powder for a fine pitch pad produces skipped deposits that no amount of squeegee tuning will fix.

What Type 4 Paste Means in Powder Size Terms

The classification counts how many particles of a stated diameter make up the bulk of the powder. A type 4 paste has almost all of its particles below 38 microns, with a small tail allowed above that figure, and the distribution is what the supplier certifies on the certificate of analysis.

The tail matters as much as the average. A few oversized particles in an otherwise fine powder can bridge a narrow aperture or block a stencil opening, which is why the specification is written as a distribution with a permitted maximum rather than as a single number.

Powder Size Against Aperture Width

A common working rule is that the largest particle should be no more than one third of the smallest aperture dimension. That keeps several particles across the opening so the paste behaves as a bulk material rather than as a queue of individual spheres.

Applying the rule to a 0.3 mm aperture gives a maximum particle size of about 100 microns, which type 3 paste satisfies on paper. In practice the wall friction and the release behaviour push the choice finer, and type 4 becomes the standard answer for apertures below 0.4 mm.

Area Ratio Limits for Fine Pitch Printing

Area ratio is the aperture opening divided by the area of the aperture walls, and it is the number that predicts transfer more reliably than any other. Below about 0.66 the paste tends to stay in the aperture, and the deposit comes out short even with a good powder size.

Powder size and area ratio interact. A finer powder releases from a low ratio aperture more easily because fewer particles touch the wall, while a coarse powder in the same aperture locks up. The snap off setting then has to be tuned around the powder that was chosen.

Print Definition, Slump and Bridging

Print definition is the sharpness of the deposit edge, and it improves as the powder gets finer because the particles pack more densely and the paste flows less at rest. Better definition shows up as deposits that hold their shape instead of slumping across the mask.

The benefit is not unlimited. Very fine powders have more oxide on the particle surfaces and more flux per unit weight, so the paste is more sensitive to open time and to humidity. A jar that sits on the stencil too long dries at the edges and the definition falls away again.

When Type 5 or Type 6 Paste Is Justified

Type 5 paste pushes the particle distribution down again, and it is used where apertures fall below about 0.15 mm or where the area ratio cannot be raised by thinning the foil. Type 6 goes further still and is normally reserved for specialised work.

The justification is a specific aperture that fails with type 4 paste, demonstrated on a print trial rather than assumed from a datasheet. Where the failure is caused by a low area ratio, thinning the stencil or enlarging the pad is usually cheaper than moving to a finer powder.

Paste Handling Differences for Finer Powders

Finer powder ages faster. The larger surface area means the flux reacts with more oxide, so the working window on the stencil is shorter and the response to a warm print room is stronger. A roll that looks acceptable with type 3 may dry visibly with type 4 in the same conditions.

Warm up, open time and stencil cleaning therefore have to be respected more tightly than on a coarse paste line. The general rules around viscosity drift and paste condition apply, and the appearance of graping after reflow is often the first visible sign that the powder and the process are mismatched.

Reflow Behaviour and Voiding by Powder Type

Finer powder melts slightly faster because the particles are smaller and the surface energy is higher, so the alloy coalesces earlier in the profile. On a joint with a lot of paste, that can change the point at which flux is trapped and therefore the void result.

The effect is modest, but it appears when a paste type changes on a product that was qualified with another. Where voiding matters, the profile should be rechecked with thermocouples on the same joints as before, and the paste lot recorded with the result.

Cost, Availability and Supply Considerations

Choosing a type 4 paste or something finer costs more, because the atomisation yield is lower and the classification step is slower. Availability is also narrower, so a line that standardises on type 5 can find itself short of material while a type 4 equivalent sits on the shelf.

The sensible approach is to use the coarsest powder that prints the board reliably, and to keep one finer grade qualified as a fallback. That way a supply problem changes the paste rather than stopping the line.

Qualification, Print Trials and Records

Qualification should be a print trial on the real stencil with the real apertures, measuring deposit height on the smallest and largest pads. Volume calculations from the aperture geometry give the target, and the measured deposit is what confirms the powder is fine enough.

Record the powder type of the type 4 paste, the lot, the stencil thickness and the print settings together, and keep a sample of the deposit data. Where a customer specifies a paste classification, the standard quoted on the drawing should be filed with that record so the next build can be compared against it, and the IPC definitions give the language for both.

FAQ

Is finer powder always better for fine pitch work? No. Finer powder helps release from small apertures, but it also ages faster, costs more and is harder to supply. The right grade is the coarsest one that prints the smallest aperture on the board within the deposit tolerance.

What powder size is type 4 paste? Type 4 is defined so that the great majority of particles are below 38 microns, with a small permitted tail above that. The exact distribution, including the maximum particle size, is stated on the supplier certificate of analysis.

Can I mix two powder types on one board? Not in a single print. Two pastes on one stencil would need separate print steps and separate profiles, and the stencil would have to be cleaned between them. Where a board has both coarse and fine apertures, pick the grade that serves the smallest one, which on most fine pitch assemblies is a type 4 paste.