Electroless Accelerator: Strength, Rinse and Control

The accelerator is the step that follows the palladium catalyst in an electroless plating line, and its job is to remove the tin that surrounds the palladium so that the catalyst can actually work. Without it, the surface carries palladium that is buried inside a tin shell. The tin shell is a few atoms thick, so the accelerator does not have to remove metal, only a layer that was never meant to stay.

It is a short immersion in a mild chemical, and it is one of the most easily neglected steps on the line. A weak accelerator produces a panel that catalyses slowly, and the copper that follows is thin, patchy or missing in the holes. Coverage faults that follow an accelerator problem are always worse in the holes than on the surface, because the chemistry has further to travel there.

What the Accelerator Does

The catalyst deposits a tin-palladium colloid on the surface, and the tin is there to keep the palladium particles dispersed. Once the colloid has attached, the tin has done its job and becomes a barrier that stops the palladium from catalysing. The sequence is fixed by the supplier and should not be shortened at the bench, because each step prepares the surface for the one that follows it.

The accelerator removes that tin shell and exposes the active palladium. The reaction is selective, which is why the step is called an accelerator by some suppliers and an activator by others. A tank that has turned dark or cloudy has usually taken up tin to the point where it can no longer strip, whatever the titration says.

Position in the PTH Line

The sequence runs through conditioner, microetch, catalyst and accelerator before electroless copper. Each step has to be finished before the next begins, and each is separated from the next by a rinse. Titration should be done on a sample taken from working depth, because the surface of a still tank is not representative of the bulk.

Because the accelerator sits near the middle of the sequence, a fault there is easily confused with a conditioner or a catalyst problem. Those steps are described in PTH conditioner control and palladium catalyst control work. Make-up chemistry should be added slowly with the tank circulating, so that the strength is even before the next load goes in.

Strength and Make-Up

The accelerator is normally a dilute acid or a mild complexing solution, and its strength is measured by titration. It is consumed by the tin it removes, so the working life is set by throughput rather than by time alone. Where a line runs several hole sizes, the immersion time is set by the smallest hole, since that is where the chemistry arrives last.

Make-up should be to the analysed strength rather than by adding a fixed volume per shift. Adding chemistry to a tank already at strength is one of the common ways to push a bath out of its window. Heating should be controlled to a narrow band, because a bath run at the top of its range will strip tin and begin to attack the laminate.

Immersion Time and Temperature

Immersion time has to be long enough to strip the tin from every surface, including the walls of a small hole. Time is normally set with a margin over the minimum, because the concentration falls through the shift. Racks should be drained over the tank for a fixed time, and that time should be written into the work instruction.

Temperature affects the rate in the same way, so a tank without temperature control has an immersion time set for its coldest condition. If the time is set for the warm condition, a cold tank leaves tin behind all day. Rinse tanks should be changed on a throughput basis rather than on appearance, because a rinse that looks clear can carry a great deal of tin.

Rinse Between Stages

The rinse after the accelerator has to remove the stripped tin before the panel reaches the electroless copper. Tin carried forward contaminates the copper bath and stabilises it in the wrong direction, which is why the rinse is a process step rather than a formality. Spray rinses between the stages improve carry-over control, but only where the nozzles are kept clear and aimed correctly.

Rinse water should be checked for carry-over, and the rinse refreshed at a rate set by the drag-out from the accelerator stage. Electroless bath control is described in electroless copper bath control notes. A catalyst rack with blocked holes in the frame will hold solution and carry it forward, which shows up as an unstable accelerator.

Contamination and Drag-In

The accelerator is sensitive to drag-in from the catalyst and from the rinses, because both of them bring in species that consume its active chemistry. Carry-over from a poorly drained catalyst rack is a common cause of a bath that will not hold its analysis. Backlight panels should be taken at a fixed point in each shift, so that a drift is visible before a lot is affected.

Drag-out from the accelerator into the next stage matters just as much, since the rinse is the only thing standing between it and the copper bath. Drainage time on the rack is the cheapest control available for both problems. A hole wall that is dark and complete under backlight indicates good coverage, while a light or patchy result means the sequence did not complete.

Failure Symptoms

A weak accelerator shows up as voids and skips in the electroless copper, particularly in the holes and on the inner layer edges. The surface may look catalysed because the palladium is present, but it is buried and does not initiate plating. The accelerator is also a good indicator of line hygiene, because it reacts to whatever the previous stages leave behind.

Backlight testing is the usual check, because it shows coverage through the hole wall rather than only at the surface. A panel that passes a surface inspection and fails a backlight test is a classic accelerator or catalyst fault. Bath replacement should be planned against the titration trend rather than against a breakdown, because a spent accelerator gives no visible warning.

Records and Verification

The record should carry the strength by titration, the temperature, the immersion time, the throughput since the last make-up and the rinse condition. Trended over weeks, that set shows the consumption rate and the point at which the bath needs replacement. The rinse before electroless copper should be sampled for tin, since that is the measurement that predicts copper bath contamination.

Acceptance of the plated panel follows the criteria in the published IPC documents. A panel released without a backlight check is a panel whose hole wall coverage has never been confirmed. A line that is running well will hold its accelerator strength for weeks, while a line that is not will consume it in days.

FAQ

What does the accelerator do? It strips the tin that surrounds the palladium catalyst so that the palladium is exposed and can initiate electroless plating.

How is accelerator strength controlled? By titration against a specification, with make-up added to the analysed value rather than as a fixed volume per shift.

Why does a rinse matter after the accelerator? Because stripped tin carried into the electroless copper bath contaminates it and changes the way the bath deposits.