11.5.1 General Process
Multi-layer boards are produced by bonding together inner layers and outer layers with prepreg.
Prepreg, as explained earlier, is fibre-glass fabric impregnated with partially hardened resin. The individual layers are arranged and bonded by placing them in a pressing tool to prevent misalignment of the layers.
After bonding, the bonded layers are further processed as double-sided through-plated circuit boards. Due to smearing of the hole with epoxy that takes place during drilling, through-hole wall cleaning is required before through-hole plating can take place.
In a multi-layer board, the outer layers may consist of either copper foil and prepreg or of single- sided or double-sided copper clad laminates. The inner layers consist of double-sided copper clad, etched (with structured conductor tracks created) and through-plated board material.
The inner layer etching is done by standard printed circuit techniques. Before bonding, it is important to make a very careful layout for each one of the layers in order to prevent masking of the desired holes. Each layer of B-stage and board substrate requires a different hole arrangement. In order to prevent the flow of resin into aligning pins, the tooling and aligning holes on the prepregs must be 1.25 mm larger in diameter than those of the conductor pads. On the other hand, the holes in the laminates must be 1.25 mm smaller than the pads over which they are to be placed.
In order to illustrate the process, let us take the construction of a four-layer board. The process steps are shown in Figure 11.4. They basically consist of two single-sided laminates and one double- sided laminate with two sheets of prepreg. The process starts by making a sandwich of all the required panels, stacked in the following order from bottom to the top:
a Thermal insulation material (a) to control rate of temperature rise;
a Bottom laminate fixture or caul plate (b);
a Sheet of release material (c) such as Teflon glass cloth;
a Bottom circuit panel (d);
a Prepreg (e);
a Inner circuit Panel (f);
a Prepreg (g);
a Top circuit panel (h as d);
a Sheet of release material (i);
a Top lamination fixture; and a Thermal insulation material.
Press platen
Toplamination fixture(k) Releasematerial(j) Top circuit(i)
Innercircuit(f)
Bottom circuit(d) Releasematerial(c) Registration pins
Bottomlamination fixture(b) (l) Thermal
insulation material (h) Additionalcircuits
&prepreg (g)Prepreg
(e)Prepreg
(a) Thermal insulation material
Press platen
Fig. 11.4 Typical multi-layer board (MLB) lamination process lay-up
11.5.2 Lamination
The bonding is done in a laminating press, which is similar to those used in the manufacture of copper clad laminates. After the different layers are arranged, the sandwich is inserted between the plates of the press. Lamination requires a specific time/temperature/pressure cycle, which depends upon the properties of the prepreg.
During lamination, the resin gets softened due to high temperature when the pressure is applied;
it causes it to flow to fill all the voids between the panels. Meanwhile, the material hardens due to a polymerization reaction that takes place, resulting in a single strong panel obtained with the two internal copper layers perfectly embedded in the resin.
The bonding pressure is 150–300 N/cm2. The curing temperature and the time must be selected according to the type of prepreg used, the number of layers and the thickness of the press stack.
Multi-layer Boards 423
In the case of over-pressure chambers (autoclaves), gas or oil is used to convey the compression force and heat to the press stack. The press stacks are placed on platforms in the tiered stand that has a vacuum connection and vacuum-sealed temperature and pressure-resistant foil.
Once the pressing chamber is loaded, it is closed and the inert gas or oil is introduced into the chamber. The isostatic pressure (pressure exerted evenly in all directions) for bonding is 80 to 200 Newton/cm2. Figure 11.5 shows a typical lamination press for multi-layer prototyping.
In contrast to hydraulic press, different press sizes may be bonding simultaneously in an over-pressure chamber.
The advantages of this method of bonding are improved heat transmission and a more favourable thermal time gradient. The all-round application of pressure has a particularly positive effect on the multi-layer stack. It
prevents resin flow, which is the main cause of stresses in the fibre-glass fabric. Dimensional stability, torsion/warping and thickness tolerance are significantly improved if stresses of this nature are not generated. Furthermore, no resin deficiencies will be found within the board. A lower bonding pressure is required for vacuum bonding (vacuum chamber press, vacuum frame or vacuum autoclave). Fewer stresses are generated in the multi-layer with lower bonding pressure. This gives considerably better dimensional stability of the inner layers, improved thickness tolerance and reduced inner layer misalignment. Since the melting point is lowered in a vacuum, volatile components, including void-free multi-layers can be achieved.
A registration system is required to achieve precise alignment of the several copper layers bearing a layout in a multi-layer during bonding. This registration is done by using locator holes drilled in the production board or in the individual layers. An exception to this is the floating bonding process used for four-layer multi-layers. This involves bonding the inner layer with prepregs and copper foil in the same way as for an outer layer. The locator holes for drilling the multi-layer are then obtained by milling and drilling the targets (registration marks) on the inner layer. Each manufacturing step affects the inner layer registration of multi-layer PCBs. As the number of layers increases and pad sizes decrease, the probability of mis-registration increases dramatically. Hinton (1992) explains the various steps for solving the problems of internal layer registration in multi-layer boards.
The cooling rate for bonding multi-layers must be as slow as possible as too great a temperature gradient within the press stacks gives rise to varying rates of shrinkage between the outermost and innermost layers in the press stack, thus causing distortion in the multi-layers. In extreme circumstances, the press cooling system may be switched off so that the multi-layers take twelve or more hours to cool down.
11.5.3 Post-lamination Process
After removal from the mould, the laminate is inspected for insulation resistance as per the design requirements. The board can also be inspected by radiography. The board is then trimmed of excess
Fig. 11.5 Typical lamination press for multi- layer boards (Muller, 2000)
material and drilled. The feed and speed of drilling are adjusted so as to minimize burring and epoxy smear.
Before the drilled multi-layers can be through-hole plated, hole wall cleaning must be performed as the action of drilling can heat the resin to above the glass transition temperature, allowing the resin to soften and be smeared over the end face of the inner layer copper by the drill bit. This smear layer must be removed so that copper is present only on the wall faces so that contact between the inner layers is not impeded in any way. This thickness of the smear is generally 2–6mm; however, it may be as thick as 12 mm if the drilling parameters are not selected properly. Chemical processes or plasma de-smearing may be used to perform hole wall cleaning. Three-stage cleaning with permanganate is the most suitable and widespread among the various chemical processes available.
The use of direct metallization to create the electrical connection between the various conductor track layers in the multi-layer through-holes is an environment-friendly process. Once the holes have been cleaned and coated with carbon particles or palladium (which have no impact on the environment), metallic copper is deposited from a solution of copper salts in sulphuric acid, to which an electrical current is applied. This copper acts as a connector element between the various conductor track layers and as a reinforcement of the external conductor tracks. In the case of some substrate types, through-hole plating for microwave engineering can be performed by using the standard process of direct metallization. Some types of substrate require an additional etching process as part of the standard direct metallization process. Using a similar process, it is possible to fabricate multi-layer boards with as many internal layers as required. However, the production yield and cost considerations become a major limiting factor for higher number of layers.
11.5.4 Multi-layer Drilling
The techniques for drilling copper clad for double-sided and multi-layer PCBs with automated equipment are identical, with the exception that multiple drilling steps will be needed if your multi- layer design includes buried or blind vias.
11.5.5 Schematic Key for Multi-layer Built-Ups
Multi-layer built-ups are designated as per the following (Table 11.1) schematic key: (courtesy, Printed Circuit Boards, GmbH)
Table 11.1 Multi-layer Built-ups
04_188_FR4_L41.35_71.18_p10_20_v1.99_2-3_4-5_6-7_s0
a b c d e f g + h + i
04 188 FR4 55 L41.35_71.18 P10_20 V1.99_2–3_4–5_6–7_s0
(Contd.)
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Table 11.1 (Contd.)
Parameter: Examples Explanation: Units:
a Number of layers: Core-layer 04 Four-layer MLBs Numeric
Sequential (1– 4 –1) Two-outer layer of sequential built-ups built-up and four-core layer
b Total thickness after the built-up and final 188 1880 m Per 10 m plating:
c Type of material: FR4 Quality of material
d Copper thickness of the outer layer after
the built up and final plating: 55 55 m Per 1 m
e Different kinds of core material and co- L41.35_ L = core material (prefix): core pper foils on both sides: L73.18 thickness 410 m + cu foil 2 ¥
35m + core thickness 730 m + Per 10 m. cu foil 2 ¥ 18 m Per 1 m
f Number and thickness of the prepregs: p10_p20 P = prepregs (prefix)Core thic-
kness 410 m + core thickness Per 10 m 730m
g Buried vias: v2 – 3 V = buried via (prefix):conne- Inner layers
+ cts inner layer 2 to inner layer3
h Blind vias: v1.v99 V = blind via (prefix)Connects Outer layers:
+ outer layer 01 to inner layer 2/3 top outer is
/4 etc and outer layer 99 to layer 1 and
inner 7/8 etc bottom outer
is always layer 99 i Special code number of the assembly s0 none
It may be noted that tolerance on total thickness = ± 5%.