Solder Paste Powder Size Selection: Type 3 to Type 6

Solder paste is a suspension of alloy spheres in flux, and the size of those spheres is specified as a type number with a corresponding particle range. The choice looks like a detail, but it decides how the paste behaves in the stencil aperture: too coarse and the aperture clogs, too fine and the paste oxidises faster, slumps more and produces solder balls. Selecting a powder size for a new board is a matter of matching the sphere range to the smallest aperture the stencil will carry.

How Paste Powder Size Is Described

Pastes are described as Type 3, Type 4 and so on, following the classification used across the industry. The type number corresponds to a nominal sphere diameter, and each type also defines the maximum permitted fraction of particles above and below the nominal range, because a paste containing a few oversized spheres behaves very differently from one that is uniformly fine.

The relevant specification is usually quoted as a mesh range, which is a historical way of describing the sieves used to grade the powder. It is worth reading the particle size distribution rather than only the type number, since two Type 4 pastes from different suppliers can differ in the tail of coarse particles.

Type Numbers and Particle Diameters

The types in current use run from Type 3 at roughly 25 to 45 micrometres down to Type 6 at roughly 5 to 15 micrometres and finer. Type 3 suits coarse apertures and general work, Type 4 is the workhorse for fine pitch, and Types 5 and 6 are used where the aperture is very small or where the printed wall must be very thin.

Finer powder costs more and behaves differently. The surface area of the same mass of alloy increases sharply as the spheres get smaller, so there is more oxide to reduce and more flux is needed to coat the powder. This is why a Type 6 paste has a different flux-to-metal ratio from a Type 3 paste and why it does not simply substitute for one.

Matching Powder to Stencil Aperture

The working rule is that the smallest dimension of the aperture should be at least five times the diameter of the largest spheres, and preferably more. Fewer than five spheres across the aperture makes the flow erratic, and a single oversized particle can block a small opening completely.

For a 0.4 mm pitch device with a 0.20 mm aperture, that rule points to a maximum sphere of roughly 0.04 mm, which places the paste in the Type 4 range. Dropping to a 0.15 mm aperture with the same rule leads to Type 5. The calculation should be done for the smallest aperture on the board rather than the average.

Area Ratio and Printability

The area ratio of the aperture sets how readily the paste releases, and the powder size interacts with it. An aperture with a marginal area ratio can often be printed successfully with a finer powder, because the finer spheres flow into the corners more easily and detach from the wall more cleanly.

The interaction is why a change of paste type is a process change. A board that prints acceptably with Type 3 paste may print better or worse with Type 4, and the deposit volume, the slump behaviour and the residue all change at the same time. Any such change should be verified with a print study rather than assumed to be an improvement.

Powder Size, Oxidation and Solder Balling

Finer powder oxidises faster because it has more surface for the same mass, and oxidation is the source of solder balling. Small spheres that have oxidised do not coalesce with the main deposit and remain as separate balls beside the joint after reflow.

The effect is partly offset by the higher flux content of fine pastes, but not entirely. In practice a fine powder paste is more sensitive to the atmosphere in the oven, to the time spent on the stencil and to the paste bridging behaviour at the edges of the deposit, so the process window narrows as the powder gets finer.

Flux Ratio and Metal Load

Metal load is the percentage of the paste by weight that is alloy rather than flux, and it falls as the powder gets finer because more flux is needed to wet the larger surface area. The volume of alloy deposited for the same printed volume therefore changes with the paste type, so the stencil aperture has to be reviewed whenever the type changes.

The flux fraction also determines how much residue is left and how much the deposit slumps before reflow. A paste with a low metal load slumps more, which is one reason a fine powder paste produces more solder balls at the edges of a pad than a coarse one on the same footprint.

Storage, Thawing and Shelf Effects

Finer powders are more sensitive to storage. Refrigerated storage slows the reaction between flux and powder, and the paste must be allowed to reach room temperature in its sealed container before it is opened, or condensation will add water to the mixture.

Once opened, the paste has a working life on the stencil and a shelf life in the jar, and the finer types are usually given shorter figures. A paste that has been through several warm-up and cool-down cycles shows its age as a change in viscosity and in slump, and the effect is visible in the paste volume measurements before it is visible in the joints.

Choosing a Type for a New Board

The starting point is the smallest aperture on the board. Apply the five-particle rule, check that the resulting type is available in the alloy and flux chemistry the product needs, and then confirm the choice by printing a test board and measuring the deposits.

Where a board carries both fine-pitch and coarse features, the choice is a compromise or a stepped stencil. The finer type is usually chosen, because the coarse apertures print adequately with a fine powder while the fine apertures will not print with a coarse one. Apertures that clog repeatedly during a print run are the first sign that the powder is too coarse for the stencil.

Verification and Records

The record should carry the paste type, the mesh range, the metal load, the alloy, the flux classification and the storage history. Those fields make it possible to explain a change in print quality without a trial, and they are needed if a lot has to be traced after a field failure.

Verification of a new paste is a print study: the smallest aperture printed across a speed and separation range, with deposit volume measured for each combination. Comparing the resulting window with the window of the incumbent paste shows whether the change is an improvement for that specific board.

FAQ

What paste type should 0.4 mm pitch use? Type 4 is the usual choice, because the five-particle rule places the maximum sphere size in that range for apertures around 0.20 mm. Type 5 is used when the apertures are smaller still.

Does a finer powder always print better? It releases more easily from small apertures, but it also oxidises faster, slumps more and produces more solder balls. The improvement is only real if the process can control the additional sensitivity.

Can one paste serve a whole product range? Often yes, if the finest aperture on the range is used to set the type. The compromise is that coarse work is printed with a finer paste than it needs, which costs money but is usually harmless.