Solder Paste Powder Size And Flux Activity
Solder paste is a suspension rather than a single material. It consists of small spheres of solder alloy held in a flux vehicle, and the two halves of that mixture have separate jobs. The powder melts and forms the joint, while the flux removes the oxide and protects the metal until the alloy is molten. The size of the powder and the chemistry of the flux can be chosen independently, but the choices interact, because a finer powder carries more surface oxide and needs a more capable flux.
This article explains what the powder types mean, how the oxide content changes with particle size, how flux activity is classified, and how the pairing is matched to the aperture and the profile.
What The Powder Types Mean
The powder is graded by particle size and identified by a type number. Type 3 is the general purpose grade and spans roughly twenty five to forty five micrometres, type 4 covers twenty to thirty eight, type 5 covers fifteen to twenty five, and type 6 goes below that to around fifteen micrometres at the top of the range. The distribution is a range rather than a single diameter, and the specification also caps the proportion of particles outside the range, because a few large spheres can block a small aperture.
Printability is the reason the types exist. A finer powder flows into a smaller opening and packs more densely, so it releases from the stencil wall more completely and produces a more consistent deposit. The price of that behaviour is a larger total surface area for the same mass of metal, which means more oxide on the particles, a higher demand on the flux, and more sensitivity to moisture and to heat during storage.
Two numbers describe the powder beyond its nominal grade. The first is the particle size distribution, which states the range and the percentage of particles above and below it, and the second is the oxide content of the metal, which is quoted as a percentage by weight and rises as the powder becomes finer. Both are part of the paste specification rather than of the alloy specification, because two pastes made from the same alloy can behave very differently on the same stencil if their powders were produced and graded differently.
The metal load, which is the fraction of the paste by weight that is powder rather than vehicle, is the third variable. A higher load gives a larger solder volume for the same deposit size and reduces slump, but it also raises the viscosity and makes the paste harder to print through a small opening. The supplier balances the load against the powder type, so a paste should be treated as a single designed system rather than as a powder and a flux that can be mixed and matched at will.
How Flux Activity Is Expressed
Flux chemistry is described by its base material and by its activity level. Rosin based fluxes, water soluble fluxes, and no clean fluxes differ in what remains on the board after reflow and in how aggressive the residue is, and within each family the activity is graded from low to high. The activators are organic acids or halides that attack the oxide at the soldering temperature, and their effect is time and temperature dependent, so a flux is only as active as the profile allows it to be.
The choice is a trade between cleaning power and residue. A high activity flux removes stubborn oxide from an aged surface, but it leaves a residue that is conductive or corrosive enough to require washing, and a no clean flux that is pushed to the same activity level leaves more residue than the standard permits. The classification tests that support the choice include the copper mirror test for the aggressiveness of the flux and the silver chromate paper test for the presence of halides, and the results are quoted with the flux designation so that the assembly house knows what it is buying.
Matching Powder To The Aperture
The stencil aperture sets the powder, and it is the first thing to fix in the design. The rule of thumb is that the stencil opening should be at least five times the largest particle in the distribution, which keeps several particles across the opening and prevents bridging by a single sphere. That relationship, combined with the area ratio between the aperture and the stencil wall, decides whether the paste will transfer at all. A fine pitch design with a thin stencil therefore drives the choice towards type 4 or type 5, while a general purpose board with generous apertures is served perfectly well by type 3.
A mismatch shows up in characteristic ways. A paste that is too coarse for the aperture clogs the openings and produces deposits with missing volume, which appears as insufficient solder or as an open after reflow. A paste that is finer than necessary costs more, oxidises faster, and is more prone to slump if the flux is not formulated for it, and slump is what turns a printing problem into a bridging problem. Solder balling around the joint is another symptom of a paste whose powder and flux were not designed together.
Storage, Shelf Life And Handling
Paste is supplied refrigerated and is normally stored between two and ten degrees Celsius, and the container has to be brought to room temperature before it is opened so that condensation does not form on the surface of the paste. Once the jar is open, the working life on the stencil is measured in hours rather than days, because the vehicle absorbs moisture from the air and loses solvent, and both changes shift the rheology that the print depends on.
Age shows itself in the print before it shows itself in the joint. An old paste is tackier, slumps more, and releases from the stencil less cleanly, and the operator compensates by adjusting the squeegee pressure or the speed, which hides the drift for a while. The oxidised powder also melts less cleanly, so the deposit that prints correctly may still fail to coalesce. A defined rotation of stock, with the oldest paste used first and the date recorded, is what keeps the process inside the window the paste supplier specifies.
Process Consequences And Practical Rules
The powder size also affects the profile. Fine powder melts sooner and more uniformly, which allows a lower peak temperature and a shorter time above liquidus, and that matters for a board that carries a temperature sensitive device. The same paste, however, has less flux per unit of metal surface available, so the soak that activates the flux has to be sufficient. Where the profile is shortened for yield reasons, the flux activity and the powder type have to be reviewed together rather than one at a time.
The practical rule is to select the paste from the aperture and the stencil, not from habit, and then to confirm the choice with a print test that measures deposit volume across the panel. The flux family follows from the surface finish, the cleaning capability, and the reliability requirement. The assembly consequences of a print that is not under control are described under component shift during reflow, the alloy side under lead free and leaded soldering, and the fabrication context under PCB design and fabrication.
FAQ
Is a finer powder always better for fine pitch? It is necessary rather than sufficient. The aperture to particle ratio has to hold, but the flux also has to be formulated for the finer powder, otherwise slump and solder balling appear even when the print looks correct.
Why does paste have a working life on the stencil? Because the vehicle exchanges moisture with the air and loses solvent while it is exposed. Both changes alter the viscosity and the tack, so the deposit volume and the release from the stencil drift during the shift.
Can cold paste be used straight from the refrigerator? No. Condensation forms on the surface as it warms, and the water contaminates the paste. The container should be sealed and allowed to reach room temperature before it is opened.
