Inner Layer Registration In Multilayer Boards

Layer registration is the alignment of the copper patterns on the different layers of a multilayer board, one to another. It is not the same as the alignment of the artwork to the drill, although the two are related: registration describes how the inner layers sit relative to each other after lamination, and the drill is positioned relative to the result.

This article covers why the layers move, how targets and scaling are used, and how the error reaches the annular ring.

Why Layers Move

Each inner layer is imaged and etched separately, and each is subject to its own tolerance in that step. The layers are then stacked and pressed, and during the press the resin flows and the laminate shrinks. The shrinkage is not the same in both directions, because the glass weave of the laminate has a direction, and it is not the same at the centre of the panel as at the edge.

The result is that a layer that was accurate when it was etched is displaced by the time it is bonded. The displacement has two components: a translation, which moves the whole pattern, and a scale, which changes its size. The scale component is usually the larger of the two on a large panel, and it is the one that cannot be corrected by moving the drill. The way the laminate behaves as it is heated and pressed is described under PCB dimensional stability and expansion.

Targets And Their Design

Registration is measured and corrected using targets, small copper features placed on the panel at defined positions. Each layer carries its own target pattern, and after lamination an X-ray system looks through the panel and measures the position of each layer’s targets relative to the others. The measurement is used to decide whether the panel is within tolerance and, in the case of the drill, to position the drill programme.

The targets themselves have to be designed so that they can be seen through the laminate and distinguished from each other. They are placed in the border or in areas that will be removed, in a pattern that surrounds the product, so that a translation and a scale can be separated from the measurements. A panel with targets on one side only cannot distinguish a scale from a translation, which is why at least two targets on each side, ideally three, are used. The way such features are laid out on a panel is described under layer stackup from one to eight layers.

Scaling And Pre-Compensation

Because the laminate shrinks in a known and repeatable way, the artwork can be pre-scaled so that the finished layer is the right size. The scale factor is measured on a test panel and applied to the artwork, and it differs between laminate types and even between batches. The correction is a scale in both axes, and where the shrinkage is anisotropic, a different factor for each axis.

The correction is applied at the panel level rather than at the board level, which means the scale is right at the centre of the panel and slightly wrong at the edges if the shrinkage is not uniform. On a large panel with a tight requirement, the residual error at the edge is one of the terms in the tolerance stack, and it is the reason a large panel is harder to hold than a small one. The design rules that absorb that error at the pad are treated under PCB pad design standards.

The Tolerance Stack

The error that matters at the end is the distance between the wall of a drilled hole and the edge of an inner layer pad. It is the sum of the imaging tolerance of the inner layer, the shrinkage from the lamination, the registration of the layers to each other, the drill position tolerance, and the hole size tolerance. Each of those is a number the fabricator knows, and their sum is what the annular ring has to accommodate.

On a fine pitch inner layer the sum can approach the pad size, which is why the inner layer annular ring is often specified looser than the outer. The drill can also be positioned using the measured targets of the panel it is drilling, rather than from a nominal position, which removes the panel to panel component of the error and leaves the within panel variation. That correction is standard on modern equipment, and its absence is a common reason for poor registration on older lines.

Measuring Registration

The primary measurement is the X-ray inspection of the targets after lamination, which gives a position for each layer. The second is a microsection, which shows the actual offset at a hole and the resulting annular ring. The two are complementary: the target measurement covers the whole panel quickly and shows the pattern of the error, while the section shows what it means at a specific hole.

A third measurement is a check of a coupon with a deliberately small ring, which is designed so that it opens or shorts if the registration exceeds a limit. This is a go or no go check that can be done on every panel rather than on a sample, and it is useful as a process monitor between the periodic sectioning.

Designing For The Available Capability

The designer’s contribution to registration is to specify an annular ring that the intended fabricator can hold, and to place the vias so that the tolerance is not the limiting factor. Where the ring is marginal, a smaller hole is better than a larger pad if the routing space is fixed, because the hole size tolerance and the drill position tolerance scale with the hole. Where the pad must grow, the space has to come from somewhere.

The capability also depends on the panel size and the layer count, because more lamination cycles and a larger panel both increase the error. A design that is comfortable on a small panel at six layers may be marginal on a large panel at twelve, which is a reason to confirm the panel arrangement with the fabricator before the stack is fixed. The qualification that establishes the capability is described under multilayer prototype requirements.

Process Control and Verification

On a design of this kind, tolerance stack is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

FAQ

What is the difference between registration and the annular ring? Registration is the misalignment of the layers and the drill; the annular ring is the copper that remains around the hole after that misalignment. The ring is the consequence and the criterion, while registration is the cause.

Can registration be corrected after lamination? The scale cannot, because the layer is bonded. The drill position can be adjusted to the measured position of the panel, which corrects translation and part of the scale error.

Why is a large panel harder to register? Because the shrinkage and the imaging error accumulate with distance, so the scale error at the edge of a large panel is larger than at the centre. A small panel keeps the whole product inside the region where the error is smallest.