Bookbinder Lamination: Keeping the Bend Area Free

Bookbinder lamination is the rigid-flex technique that keeps the bend area free of prepreg and of the resin that flows out of it during the press cycle. The rigid sections are bonded, the flexible sections are protected, and the result is a board that can be folded without cracking. The process is used wherever a finished board has to fold in service, and the tooling is what makes that fold possible.

The name comes from the way the lay-up is assembled, with the rigid sections on either side of the flex tongue. It is a lamination process with a temporary structure built into it, and that structure has to be removed cleanly after the press. The temporary structure is removed after the press, and the release film has to leave no trace on the flex surface.

What Bookbinder Lamination Does

A rigid-flex board has rigid sections that carry most of the circuitry and flexible sections that fold. The flexible sections have no prepreg around them, so the copper and the coverlay remain free to bend rather than being locked into cured resin. The rigid sections carry the connectors and the dense routing, while the flex section carries only what has to cross the fold.

The lamination has to bond the rigid areas completely while keeping resin out of the bend. Those two requirements pull against each other, and the whole tooling and lay-up design exists to satisfy both of them at once. The drawing should show the free zone and the bond zone as separate dimensions, so that the two are not confused at lay-up.

The Bend Area and Why It Must Stay Free

A bend area that has been infiltrated with resin loses its flexibility and will crack along the resin boundary when it is folded. The failure appears at the edge of the bond, which is exactly where the stress concentrates. Release film should be cut cleanly and be free of creases, because a crease presses into the coverlay and leaves a mark.

The extent of the free zone is defined on the drawing, and it normally includes a margin on either side of the actual bend. That margin covers the resin flow which will occur, since no release film stops flow perfectly. Shim thickness is set by the difference between the rigid and flexible stack heights, and it should be measured rather than assumed.

Release Film and Shims

Release film is placed over the bend area so that the prepreg cannot bond to it, and it has to be a material that will not leave a residue and will not deform under press pressure. The film thickness also sets a clearance for the flex tongue. Datum holes in the tooling plate are what keep the film and the shims in the right place from one panel to the next.

Shims or filler plates make up the thickness difference between the rigid and flexible sections, so that the press applies even pressure across the whole panel. A stack without shims will be over-compressed at the flex and under-compressed at the rigid ends. Separator plates should be flat and clean, because a dent in a separator is transferred straight into the panel surface.

Lay-Up Sequence

The lay-up is built as a book: a tooling plate, a separator, the stack, more separators and then the next panel. Each rigid-flex panel carries its own release film and shims, and they must be positioned to the same datum every time. Pressure should be applied slowly at the start of the cycle, so that the resin has time to flow before the stack is compacted.

Registration of the film and the shims is as important as registration of the inner layers, because a shim displaced by a millimetre will press on the bend it was placed to protect. A vacuum stage before the full pressure helps remove the air that is trapped between the separate pieces of the stack.

Press Cycle for a Bookbinder Stack

The cycle has to bring the stack to temperature evenly, hold it long enough for the resin to flow and cure, and then cool it under pressure. Uneven heating is more of a problem here than on a plain multilayer, because the stack has more separate pieces in it. Resin content is set by the prepreg grade, so a change of grade should be treated as a change to the whole lamination recipe.

Cooling under pressure is what fixes the flatness of the rigid sections. A stack released while still hot will move, and the rigid sections can bow after the tooling is removed. Press parameters are described in lamination process control notes. Flow that reaches the release film edge is a warning that the free zone margin is too small for the material being used.

Resin Flow Control

Prepreg flows during the heat-up, and the flow has to fill the rigid areas without travelling into the bend. Resin content, press pressure and the rate of temperature rise all influence how far it travels. A stiff bend should be checked for resin bleed before the design is changed, because the cause is usually in the press cycle.

A high flow prepreg in a thin rigid section will travel further than a low flow grade, so the material choice belongs in the same discussion as the tooling. The free zone margin is the buffer that absorbs whatever flow does occur. Bend coupons should be folded to the specified radius and inspected for cracks, since a flat panel can look perfect.

Common Defects

Resin bleed into the bend is the classic defect, and it shows up as a stiff or discoloured area that cracks on folding. Voids and delamination in the rigid sections come from incomplete flow or from moisture in the prepreg. The lay-up drawing should be re-issued whenever the tooling changes, because an outdated drawing is a defect waiting to happen.

Shifted shims and wrinkled release film produce local pressure marks that are visible after the press and are difficult to correct afterwards. Flex design guidance is covered in rigid-flex bend area design notes. The press profile, the materials and the bend result belong in one record, so that a single coupon can answer a customer question.

Records and Verification

The record should carry the lay-up drawing used, the prepreg grade and lot, the release film and shim materials, the press profile and the result of the bend test. The build sequence itself is described in rigid-flex manufacturing work. Where a stack produces good panels the lay-up should be photographed, because a picture settles more arguments than a description.

Acceptance of the finished board follows the criteria in the published IPC documents. A bend test on a coupon from the same stack is the only evidence that the free zone survived the press.

FAQ

Why does the bend area need release film? So that the prepreg cannot bond across the bend, which would lock the flex section and cause it to crack when it is folded.

What are the shims for? They make up the thickness difference between the rigid and flexible sections so the press applies even pressure across the whole panel.

What happens if resin flows into the bend? The flex section becomes stiff and the resin boundary becomes a stress riser, so the board cracks on folding rather than flexing.