Solder Paste Powder Size: 6 Rules for Choosing Type 4 or Type 5
Solder paste powder size is the classification that describes how large the alloy particles are, and it is written as a type number from 1 to 8. The number rises as the particles get smaller, so a Type 4 paste has a finer powder than a Type 3, and a Type 5 finer still.
The choice matters because the powder has to pass through an aperture and then release from it. A powder that is too coarse for the aperture clogs the stencil and prints inconsistently, while a powder that is finer than necessary costs more, oxidises faster and brings its own handling problems.
What Powder Type Means
The type number is defined by a maximum particle size and by the proportion of particles that must fall below a given diameter. A Type 4 paste, for example, has a maximum particle size around 38 micrometres with the bulk of the distribution well below it, while a Type 5 maximum sits near 25 micrometres.
The distribution matters as much as the maximum. A paste with a few oversized particles will block a small aperture even if its average size is fine, which is why the standard includes limits on both the top end and the fines content. Purchasing on the type number alone is only safe when the supplier controls that distribution. That is why a supplier change should be treated as a process change even when the type number on the label stays the same.
Powder Size and Aperture Size
The rule of thumb is that the aperture should be several times the largest particle. Below about five particles across the aperture width, printing becomes erratic, because bridging between particles blocks the opening and the deposit is short or irregular. The effect appears first on the smallest aperture and then spreads to the rest of the panel as the aperture clogs.
This is why fine pitch work drives the powder choice rather than the reverse. A 0.4 mm pitch device may print well with a Type 3 powder, while a 0.3 mm pitch or a small microvia usually needs Type 4 or finer to fill the aperture reliably on every print stroke.
Area Ratio as the Practical Limit
Area ratio is the aperture opening area divided by the wall area of the aperture. It combines the pad size with the stencil thickness, and it is a better predictor of print performance than either dimension alone. Below roughly 0.6, the paste clings to the aperture walls and release becomes the limiting factor.
Powder size and area ratio work together. A finer powder improves the aperture design margin because each particle is smaller relative to the wall, so it can transfer more easily. Reducing stencil thickness raises the area ratio and is often the cheaper fix, since it does not change the paste.
Oxide Content and Fine Powders
Finer powder has more surface area for the same mass, so it carries more oxide and more flux per unit weight. That is why a Type 5 paste typically has a higher metal load in percentage terms but a lower metal content by volume of usable alloy than a Type 3.
The oxide also affects flux chemistry. A finer powder consumes flux activator faster, so a fine paste has a shorter working life and is more sensitive to time on the stencil. The handling rules that apply to any paste become tighter as the powder gets finer.
Paste Release and Slump
Paste release depends on the powder and on the stencil together. A fine powder fills a small aperture more completely, and it also leaves a deposit with a larger surface area for the same volume, which makes it slightly more prone to spreading before reflow.
Where slump is a concern, the response is not to move to a coarser powder, because that would compromise aperture fill. The better levers are the stencil surface finish, the squeegee settings and the hold time between printing and reflow, all of which are covered in the usual paste volume and print control work. Replacing a powder to fix slump usually trades one problem for a worse one at the aperture.
Handling and Shelf Life of Fine Powders
Fine powders are more sensitive to storage and to temperature. Because the flux reacts more quickly with the larger surface area, a Type 5 paste has a shorter shelf life and a shorter time on the stencil than a Type 3, and it degrades faster when the room is warm.
The solder paste thaw routine matters more, not less, as the powder gets finer. Cold paste with a fine powder is stiff and prints badly, and paste left on the stencil for an hour is already different from the paste in the jar. Closing the container between uses and controlling room temperature both extend the usable period without changing the paste.
Reflow Behaviour and Voiding
Fine powders melt more uniformly because the particles are smaller and the alloy is more evenly distributed, which generally helps fine pitch joints form. The same property means more total oxide to be reduced by the flux, so a paste with a weak activator may leave more residue or form more voids.
Voiding also depends on the deposit volume and the profile rather than on the powder alone. A change of powder type should therefore be evaluated on the same board and the same profile, with X-ray data before and after, so the effect can be attributed rather than assumed. Two changes made at once, such as a new powder and a thinner stencil, will leave the line unable to say which one delivered the improvement.
Choosing With the Stencil, Not Against It
The paste is only one part of the printing system. Stencil thickness, aperture design and surface finish set the stencil side of the equation, and the powder has to be matched to them rather than chosen independently.
A common and effective combination is a thinner foil with a Type 4 powder, which raises the area ratio and keeps the powder within a reasonable cost band. Moving to a finer powder without addressing the stencil is the more expensive route and often delivers less improvement.
Qualifying a Powder Type Change
A change of powder type is a process change. Print the same board with the same stencil and paste chemistry, measure the deposit volume with the inspection system, and compare the fill rate on the smallest aperture rather than the average.
The qualification should also cover the working life, since that is often the real difference between two powder types. Printing the first board well is easy; printing the hundredth board at the end of a shift is what the powder size and the handling routine have to deliver.
FAQ
Do I always need Type 4 or Type 5 paste? No. Coarser powders print reliably on larger apertures and are cheaper and more robust in handling. The powder should be matched to the smallest aperture on the board rather than chosen for its own sake.
Can a finer powder fix a poor stencil design? It improves the margin but does not repair a low area ratio or an aperture that is badly shaped. Correcting the stencil thickness or the aperture dimensions usually gives a larger improvement for less cost.
Does powder size affect reflow profile? It affects how the paste behaves during the ramp and how much flux is available, so the profile may need adjustment after a change. The safe approach is to keep the profile within the paste specification and verify the joints rather than assuming the old profile still applies.
