Processes involved in the production of a multilayer PCB
Chemitalic A/S, revision 4, May 2001
Below you will find an introduction as to how a multilayer PCB is produced at Chemitalic. There is a short description of each process, and to support this description, we have inserted a diagram to show the process in a simplified form. Many different processes are used in the production of a multilayer PCB, e.g.:
- Chemical cleaning processes, such as etching and plating
- Silk-screen printing to produce the solder mask, the component print and the carbon print
- Photo technical processes to produce the conductor pattern, the solder mask and the component print.
- Galvanic processes for the plating of copper, nickel and gold
- Mechanical processes for drilling, routing and scoring
- Electrical tests to check the conductor pattern, the plated through-holes and contacting pads
Though a carefully supervised production flow, these processes ensure the end-result, a product that meets the requirements of our customer, see the main processes in Fig. 1
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1: First the laminate for the inner layers are cleaned and micro etched, then a photo resist is laminated onto both sides of the laminate. Subsequently the inner layer pattern is transferred by means of a film in an automatic double-sided exposure. The photo resist is developed and the pattern is left on the laminate after the etching and stripping of the photo resist. Tooling holes are stamped into the inner layers to ensure register between each layer prior to an automatic optical inspection of the pattern. |
2: Preparatory to the pressing of the inner layers and the copper foils to the outer layers, the numbers of inner layer laminates, which will later form the build-up of the finished PCB, are welded together. In e.g. an 8-layer multilayer PCB, there are 3 inner layer laminates containing the patterns for layers 2 - 7. The 2 copper foils form the layers 1 and 2. A so-called prepreg, which is a non-hardened glass-epoxy, forms the insulation between each pattern layer.
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When the layers are laid up, they are pressed together in a high-pressure and high-temperature hot press. After pressing, X-ray registration, adjusted in accordance with the inner layer fiducial marks, is used to drill two tooling holes that will ensure register in the subsequent drilling process. |
3: Before the drilling of the PCB holes, 2 or 3 panels are pinned together to form a so-called drill stack. The PCB is drilled and X-ray inspected to ensure that the position of the drilled holes corresponds with the inner layer pattern.
The through-hole plating process leaves a palladium colloid on the glass-epoxy after drilling burrs and epoxy residue on the copper in the hole has been removed in a so-called desmearing process. This palladium colloid is electrically conductive.
Whereas copper is electrically conductive, glass-epoxy is an insulator, which must be made conductive before the deposition of galvanic copper in the pattern plating process.
5: When the panel has been laminated with photo resist on both sides, the outer layer pattern is transferred from the films onto the panel. Exposure and development follow the same procedure as that of the inner layer pattern.
In the pattern plating, which is a galvanic process, 25 microns copper nominally is plated in the panel holes to establish the electrical connections between the PCB inner and outer layers. 25 microns copper is also plated on what is to become the outer layer conductors, thus the subsequent conductor pattern thickness will be equivalent to the copper foil thickness plus 25 microns.
A thin tin layer is plated on top of the copper and in the holes to form an etching resist in the subsequent pattern plating process; the etching resist is then removed and the outer layer patterns are finished.
6: The purpose of the solder mask is to cover the entire PCB surface except for the solder pads and the contacting pads. The solder mask is a photosensitive lacquer, which is screen printed on the entire panel, a film is used to produce openings for solder pads and contacting pads. After development, the solder mask is hardened in a tunnel oven.
7: The purpose of the surface treatment is to protect the exposed copper, on which to be soldered, from corrosion.
The process used may be Hot Air Levelling, in which the copper is lowered into a eutectic tin/lead bath. After the panel has been taken out of the bath, the excess tin/lead will be blown off.
It could also be an immersion nickel/gold process, in which a thin layer of nickel and a thin layer of gold are applied chemically, and finally it could be an immersion tin process, in which a thin layer of tin is applied, also chemically.
8: The PCBs are separated from the panel in a contour routing process to match the customer drawing. An alternative process is scoring in which a V-shaped line is grooved down into the panel surface. In the centre, between the V-shaped grooves, a very thin core is left, which secures each PCB to the customer's mounting panel. When the components have been mounted, the PCBs can be cut from the panel.
9: The electrical test ensures that all connections in the PCB comply with the customer specification. Chemitalic uses two kinds of electrical testing, a fixture test, which can be performed very quickly, in which testing tools are made with pin-shaped probes in all test points.
The second testing method is a so-called flying probe test, in which a set of movable probes test one connection at a time. This latter test method is slow but makes it possible to test even very fine pattern structures.
10: The final inspection is a process in which trained staff carries out a visual inspection of each PCB. After this inspection, the PCBs are packed and sent off to the customer.