Etch Line Conveyor Speed: 4 Checks for Dwell Control

Etch line conveyor speed is the parameter that converts chemistry into geometry. It sets how long a panel spends under the spray, and therefore how much copper is removed and how much the side wall is undercut while it happens.

It is also the easiest parameter to change, which is why it is so often used to compensate for a chemistry problem. That works for a shift and fails over a month, because the side wall and the uniformity move with the dwell.

What Conveyor Speed Controls

Speed sets the time a point on the panel spends inside the etch chamber. At a given rate of attack, that time decides the depth of copper removed, so speed and etch rate are two expressions of the same result.

It also sets the time in every other zone of the machine, including the rinse and the pre-heat where the machine has one. A change in speed therefore changes more than the etch.

Finally, speed affects the way the spray behaves relative to the panel. A panel that moves quickly through a fixed spray pattern receives less solution per unit area than one that moves slowly.

Dwell Time and Etch Rate

Dwell time is the controlled variable, and speed is the way it is set. The route card states a dwell that the process was qualified with, and the speed follows from the length of the chamber. A chamber that has been lengthened or shortened changes that relationship immediately.

Etch rate is measured in microns per minute, and it is confirmed on a coupon or by the weight of copper removed from a test panel. When rate and dwell are both known, the expected result can be calculated rather than guessed.

That calculation is the basis of process control. It shows at once whether a change in the result came from the dwell or from the chemistry, which is the question that has to be answered before anything is adjusted.

Spray Pressure and Solution Exchange

A spray delivers fresh etchant to the surface and carries the reaction products away. If either side of that exchange is weak, the etch slows regardless of what the chemistry would allow.

Spray pressure is what drives solution into holes and along the surface between closely spaced tracks. Partly blocked nozzles reduce it locally, so a stripe of under-etched copper can appear across a panel that was etched at the correct dwell. Spray pressure should be measured at the header rather than assumed from the pump setting.

Oscillating bars and overlapping fans exist to make the delivery uniform, and the pattern should be checked on a card as part of routine maintenance.

Temperature and Chemistry Interaction

The etch rate responds strongly to temperature, so a bath that warms through a shift will etch faster at the same speed. If the temperature controller drifts, the operator sees it as a thickness change that the speed appears to explain.

Chemistry acts in the same direction. A bath that is loading slows down, and one that has been overdosed speeds up, so the two effects can cancel or reinforce depending on what has happened.

Measuring temperature at the tank and the rate on a coupon separates the two. The control of plating bath temperature follows the same logic and is described in plating bath temperature control.

Setting Speed for a New Job

A new job starts from the qualified rate for the chemistry, the copper thickness to be removed and the chamber length. That gives a first estimate of the speed, which is then confirmed by etching a test panel.

The confirmation is done on a coupon that carries the finest feature on the product, because that is where undercut shows first. A result that is correct on a solid area and wrong on a fine gap has not confirmed the setting.

Where the product has thick copper, the speed may have to be reduced enough that the machine becomes the constraint. That is a planning decision rather than a line adjustment, and it should be recorded for the next run.

Tracking, Rollers and Panel Handling

The panel has to be transported without slipping or twisting. Roller condition, drive tension and the state of the guides all affect whether the panel moves at the speed the controller displays.

A worn roller that slips produces the same effect as a faster conveyor, and the effect is uneven across the panel. Panels that skew can also spend different times in the chamber along their length, which shows as a tapered etch. Guides and side rails should be checked at the same time as the rollers, because a rail that has been knocked out of line steers the panel into the wall of the chamber.

Thin panels are the hardest case, because they flex and can be drawn into a roller. The handling rules for panels on a conveyor are similar to those in other wet lines, and the rinse that follows is described in spray rinse optimisation.

Undercut, Taper and Edge Effects

Undercut is the sideways removal of copper beneath the resist, and it grows with dwell. Some undercut is unavoidable in any wet etch, because the attack is not perfectly directional.

Longer dwell in a slower line therefore costs line width as well as time. The width control that the chemistry provides is described in alkaline etch bath control, and it is only effective if the dwell is stable.

Edge effects are the other geometry consequence. Panel edges and isolated traces see more solution than the middle of a dense pattern, so they etch faster and the width tolerance has to allow for the difference.

Symptoms of a Speed Fault

An etch that is uniformly shallow points at too high a speed, poor spray or a weak bath. One that is uniformly deep points the other way, and the same three factors are the candidates in each case.

A result that varies along the panel, rather than across it, usually means the transport is at fault: a slipping roller, a jammed section or a speed that changes as the drive warms.

Where the fault follows a shift rather than a product, the temperature and chemistry records for that shift are the fastest way to separate a machine problem from a bath problem.

Records and Verification

The record should carry the speed, the corresponding dwell, the spray pressure, the temperature and the analysis for the same lot, together with the measured etch depth and line width.

Verification is the coupon result rather than the controller display. The dial can be correct while the panel moves at a different rate, and only the measurement on the product shows which happened.

Where the chamber has been modified or a roller section replaced, the relationship between speed and dwell should be re-established, and reference methods are published by IPC.

FAQ

Is it acceptable to slow the line to correct a weak bath? Only as a short-term measure. The extra dwell increases undercut and changes the width, so the bath should be corrected rather than compensated for.

How often should the actual speed be measured? At commissioning, after any transport repair, and periodically thereafter. The controller reading is not evidence that the panel moves at that speed.

Does conveyor speed affect the rinse? Yes. The panel spends less time in the rinse zones at higher speed, so rinse flow and stage length have to be adequate for the fastest setting the line uses.