Simply defined, a Multi-LayerBoard (MLB) is an interconnected package which contains more than two layers of circuitry. Multilayer technology evolved as circuit requirements grew and the space in which to fit them shrank. Surfacemounted technology(SMT), which not only reduced the size of components but also allowed components to be attached to both sides of the PCB, further accentuated the need for MLBs of increasing complexity. Alternative via-hole technologies for MLBs are also presented at the end of this document.
The Cycle Multilayer manufacture can be broken into three parts; innerlayer fabrication, lamination and manufacture as it applies to double-sided boards. The first two steps, specific to multilayers are shown in the following flow chart:
The MLB images are prepared on CAM from the CAD data supplied. Generally some of the work done includes:
- minor design edits such as removal of unconnected pads from internal signal layers and enlargement of solder mask pads;
- step and repeating of images to fit within one of our 2 standard panels (610mm x 457mm or 457mm x 305mm);
- addition of test coupons, venting patterns to inner layers and thieving patterns to outer layers.
design rule check;
Films are then laser photoplotted using 6.35mm (¼ mil) resolution on 175mm (7 mil) thick film for stability. These films are duplicated on diazo film for use as phototools in imaging. The inner layer films are punched with registration slots.
(Refer to CID-011 for information on alternative registration systems)
The thin copper-clad laminate used for innerlayers is cut to size, baked to remove stresses and the surface cleaned to remove any contaminants from the copper surface.
In imaging, the circuitry is transferred to the thin core panels. The first step is to hot roll laminate an ultraviolet light sensitive dry photopolymer resist to the panels.
The laminated panels, which are virtually like a film waiting to be exposed, are then punched with slots for alignment of the diazo phototools.
After lamination, the panels are placed in a UV printer frame, with the diazo phototool positioned to the panel using a glass plate registration system for location. The clear areas of the phototool allows the UV light to pass through and expose the resist whilst the red areas block out the light.
Negative diazo phototools with the intended circuitry in clear are used for inner layer imaging. The panels are then placed in a conveyorised developing machine which sprays developing solution onto the panel to remove the unpolymerised resist, that is the unexposed areas.
The imaged innerlayers are then placed in a conveyorised spray etcher. The sprayed chemical etchant attacks and dissolves the uncovered copper.
The etched panels are then sprayed with a solution that strips away the photoresist.
The copper circuitry on the inspected innerlayers is then treated with a black oxide layer prior to lamination to improve adhesion to the epoxy resin.
Each multilayer stack is then laied up according to the stated requirements. Glass sheets impregnated with partially cured (B-stage) epoxy resin are used as bonding and insulating material. These sheets are called ‘prepreg’ and are available in different glass styles and thicknesses with varying resin content. Copper foils are used as outer layer conductive material. The inner layer cores, prepreg sheets and copper foil sheets are used to make up the multilayer stack.
These stacks are transferred to a multilayer book for loading in the lamination press.
Each stack is separated from another with a separator plate and release film which is a stick resistant plastic. Thicker stainless steel lamination plates are located on the top and bottom of the lamination book. Sandwiched under the lamination plates are press pads which are used to evenly distribute the pressure and temperature during the laminating process.
Up to eight 1.6mm multilayer stacks can be laid up in a single book.
(Refer to CID-011 for information on alternative construction methods & CID-012 for information on our standard multilayer buildups
)
The book is then transferred to the laminating press. The press applies heat and pressure so that the thermosetting resin in the prepreg undergoes cross-linking which bonds all layers together. The press then goes through a critical cooling cycle before the book is removed and pulled apart. The edges of the multilayer panels are then trimmed to eliminate any of the resin outflow and to smooth out uneven or rough edges.
(Refer to CID-011 for information on alternative lamination equipment)
CONTINUE THROUGH REST OF MANUFACTURING CYCLE
After lamination the pressed panels are processed in the same way as a double sided job with a couple of exceptions in drilling and electroless copper plating.
In drilling the stack height is reduced as is the maximum hole quantity per drill and the drill chip load which is the distance travelled per drill revolution.
(Refer to CID-009 for information on the manufacturing cycle)
Prior to electroless plating, the panels are desmeared and possibly etched back to remove the epoxy smear that may have been created in the drilling process. This smear prevents the electroless copper from bonding to the copper on the innerlayers, thus creating an open circuit or an unreliable plated through hole.
(CID-011 for information on alternative desmearing techniques)
VIA HOLE TECHNOLOGIES FOR MLBS
Current demands for smaller more efficient boards have led to higher layer count multilayers, smaller holes and finer lines. These techniques, however, have not always been able to satisfy designer’s often desperate needs to overcome real estate constraints whilst trying to maximise component configurations.
As a result new alternatives such as buried vias, blind vias or a mixture of both have been successfully utilised by designers trying to maximise board real estate by increasing the density of lines and pads.
However, many manufacturing and design issues must be addressed before adventuring down this path. Due to the many additional operations, the cost of a blind or buried via board is significantly higher than that of a multilayer of the same layer count.
1) Buried Vias
A buried via is a copper plated-hole, imbedded in the board, that interconnects two internal layers.
The buried via process starts by drilling holes on thin core laminate. These cores are then metallised, imaged and etched before being laminated.
Individual manufacturer’s method of fabricating multilayer boards must be understood before designing boards with buried vias. Designed and manufactured layer sequences must match.
(Refer to CID-012 for information on our Standard Multilayer Buildups)
1) Blind Vias
A blind via is a copper-plated hole that interconnects a board’s external layer to one or more internal layer(s) but not through to the other external layer.
There are two possible methods of achieving blind vias. The first and oldest is by pre drilling and metallisation the external cores before lamination. The other more recent method is the result of recent advances in drilling equipment which have controlled-depth drilling capability.
As with buried vias, individual manufacturer’s capabilities must be understood before designing multilayer boards with blind vias.
Contact our Sales staff if you require further information or if you wish to visit our manufacturing facility.
