Solder Paste Selection Guide

Solder paste is a mixture of alloy powder and flux, and it is the material that carries the solder to the joint. The alloy sets the melting point, the flux sets the cleaning action and the residue, and the powder size sets the finest feature that can be printed. Paste is also a perishable material with a working life, so the choice has to suit the process and the storage conditions as well as the joint.

What the Paste Has to Do

The paste has to be printed through a stencil into a defined volume, hold its shape until the parts are placed, and then melt and wet the joint during reflow.

It has to do all of that at the temperature and the time that the profile provides, and it has to leave a residue that the product can tolerate. Those three requirements are the whole specification.

The paste also acts as an adhesive before reflow, since the tackiness of the flux holds the components in place between placement and the oven. A paste with poor tack drops parts off on the conveyor.

The paste is also the material that decides how much solder reaches the joint, because the volume that is printed is the volume that melts. A pad that receives too little paste gives a starved fillet, and one that receives too much bridges to its neighbour.

The shelf life and the working life are part of the choice, because a paste that is used beyond its life prints badly and forms poor joints.

Alloy Choice

The alloy sets the melting point and therefore the reflow temperature, the strength of the joint and its behaviour at high temperature.

A tin lead alloy melts near a hundred and eighty three degrees, which gives a wide process window and a ductile joint. It is still used in products that are exempt from the restrictions on lead.

A lead free alloy based on tin silver copper melts around two hundred and seventeen degrees, which needs a higher reflow temperature and a narrower window. The joint is harder and less ductile than a tin lead one.

The comparison between the two is in the alloy guide, and the choice is usually settled by the regulations rather than by the assembly engineer.

Flux Activity and Residue

The flux removes the oxide from the pad and from the powder, and it keeps the surface clean until the alloy melts. Its activity is the measure of how aggressively it does that.

A highly active flux cleans a heavily oxidised surface and leaves a residue that has to be washed. A mildly active flux leaves a residue that can stay on the board, at the cost of a narrower process window.

The residue question decides whether the board has to be cleaned, which is a process step and a cost. A no clean paste is the usual choice for a consumer product, provided the residue is not conductive and does not absorb moisture.

The flux also has to be matched to the cleaning process, because a no clean paste cannot be washed without leaving a white residue that is difficult to remove. The combination of flux type and cleaning method is a process decision that is often made separately and should be made together.

The flux must also be compatible with the board finish, since some finishes need more activity than others to wet reliably.

Particle Size and Fine Pitch

The alloy is supplied as a powder, and the powder is classified by the size of the particles. A finer powder prints through a smaller aperture and a coarser one is cheaper.

The rule of thumb is that the largest particle should be no more than a fifth of the aperture width, and no more than a third of the stencil thickness. A paste with a particle that is too large for the aperture blocks the opening.

A finer powder has a larger surface area for the same volume, so it oxidises faster and needs a more active flux. The trade is between the print resolution and the process window.

A very fine powder also tends to form voids, because more oxide and more flux are present for the same volume of alloy. That is one of the reasons a fine pitch assembly is harder to run cleanly.

Viscosity, Printing and Storage

The viscosity of the paste is measured as it is pushed through the stencil, and it changes with the shear rate. The paste has to flow into the aperture and then hold its shape after the stencil is lifted.

A paste that is too thin slumps after printing and bridges the neighbouring pads, while one that is too thick does not fill the aperture. The correct value comes from the stencil design and the print speed.

The storage is part of the specification. Most pastes are kept refrigerated and brought to room temperature before use, because a cold paste condenses moisture and prints unevenly.

The working life on the stencil is measured in hours, and a paste that is left on the stencil overnight dries and must be discarded. That rule is often the largest single source of paste waste.

Process Compatibility

The paste has to match the stencil thickness and the aperture design, the placement pressure and the reflow profile. A change in any one of them changes the result.

It also has to match the surface finish on the pad and the finish on the component lead. A gold finish dissolves into the alloy and a thick gold layer makes the joint brittle, which is a question for the finish specification rather than for the paste.

The inspection method matters as well, since a paste with a dark flux is harder to inspect optically than a light one. The AOI process depends on the contrast between the paste and the pad.

A new paste should always be qualified with a print test and a reflow test on a real board rather than on a coupon, because the interaction with the finish and the mask is where the problems appear.

Practical Rules

Choose the alloy from the regulatory requirement, and check that the reflow profile can reach the peak that it needs without exceeding the component limits.

Choose the powder size from the smallest aperture, using the one fifth rule, and accept the shorter working life that comes with a fine powder.

Keep the paste refrigerated, bring it to room temperature before the pot is opened, and record the batch and the date on the line.

Qualify a new paste on a production board, and check the joints with the solderability method and the AOI before the whole batch is committed.

FAQ

How fine a powder is needed for a fine pitch part? The largest particle should be no more than a fifth of the smallest aperture. A finer powder prints better and oxidises faster.

Does solder paste have to be refrigerated? Most types do, and they have to reach room temperature before the container is opened. A cold paste condenses moisture and prints unevenly.

Why does a paste bridge between pads? Usually because the viscosity is too low, the stencil aperture is too large, or the paste has been on the stencil too long and has warmed up.