Generation of the primary image

There are two types of photoresist (or resist) used to produce the track image including the reference marks on the laminates from the phototools (see Figure 2.27):

• liquid resist applied by roller coating

• dry resist applied by lamination.

Inner layers for MLBs may be either structured by screen printing or by dry resist (dry film).

The pattern of the outer layers is generally structured by dry resist. In isolated cases, a third process, screen printing, may be used. For photo-printing the three process steps are coating (liquid or dry), exposure and developing. These process steps may be automated and conducted in a continuous process.

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PT/EIPPCB/STM_BREF_FINAL September 2005 117 Coating

During the first step, the pretreated surface is coated with a photosensitive layer (photo resist) which is available either as liquid resist or as dry resist. For the coating, two different important processes are available: roller coating for liquid resists and lamination for dry resists. Table 2.3 shows the advantages and the disadvantages of both systems

Process Advantages Disadvantages

Low film thickness (approximately

2 - 12 µm) Risk of pinhole formation

Lower pollution in effluents due to

lower material usage Contain more than 50 % VOC-based solvents which may require reduction under Solvent Emissions Directive

High yield from material used Potential problems during ultrafiltration of effluents from developer and stripper

Lower investment Higher energy requirement

Roller-coating (liquid resist)

Low under-etching of copper Risk of involuntary bonding of inner layers Constant coating thickness because

of dry film Film thickness is generally 37.5 µm, more resist

removed creating more effluents and resist waste

No pinholes Higher investment

Low energy consumption as no drier is needed

Lamination (dry resist)

No VOC emissions

Table 2.3: Comparison of resist layer types for primary imaging [122, UBA, 2003]

• Roller coating (liquid resist). The liquid resist consists of acrylates, photo-initiators and fillers and is dispensed through a metering gap and spread under pressure by a corrugated rubber roller onto the surface of the laminate. Because of the corrugation of the roller, the contact pressure and the solid content of the liquid resist, the coating thickness is adjusted to a thickness of between 2 to 12 µm. The resist is continuously filtered and monitored in its viscosity while being circulated through a storage tank. The condition of the rollers is important as damaged rollers produce defective coatings. The inner layers are coated on both sides and then dried with on-line infrared driers. The conveyor speed is about 2 – 4 m/min with a high yield from the liquid resist. Material losses occur only during the cleaning of the equipment or when changing the resist

• Lamination (dry resist) (Figure 2.30). Dry resists are structured in several layers. In clean room conditions with only yellow light, the photo sensible emulsion is applied by curtain coating on a polyester carrier foil of 25 µm thickness, dried and covered with a polyethylene protective layer. This sandwich can be rolled onto a paper or a plastic roll core from which it is laminated to the surface of the inner layer. Either manual or fully automatic laminators are used, the centre core of which is the roller system. Principal factors influencing the process are: laminating rollers with an aluminium core to maintain dimensional stability, temperature, contact pressure, waviness of the laminate surface, laminating speed and the pretreatment method of the copper surface

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118 September 2005 PT/EIPPCB/STM_BREF_FINAL Figure 2.30: Application of dry resist

Exposure

The image is photo-printed onto either type of photoresist– either manually or automatically - in imaging equipment in clean rooms under amber light conditions. In Figure 2.30, the phototool (see Section 2.11.2.3) is exposed to UV light which passes through the transparent areas of the phototool, and polymerises the monomer components of the resist by cross-linking. The resulting polymer is resistant to the following development process.

Figure 2.31: Exposure of resist

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PT/EIPPCB/STM_BREF_FINAL September 2005 119 Developing

The laminate (resist) is developed by spraying with a sodium carbonate solution while passing along horizontal continuous line. This step exposes the final track pattern (tracks, pads with holes, pads for SMDs (surface mount device) and ground planes for the following process (Figure 2.32).

Figure 2.32: Development of resist

The typical development line consists of the main development chamber, followed by a smaller chamber(s) with fresh sodium carbonate supply. This enables the panel surface to be rinsed with clean development solution, reducing the drag-out of contaminated solution into the following fresh water rinsing zone. The development chamber may be equipped for the first 60 % with cone nozzles and for the remainder with fan jet nozzles. The manifolds are equipped with an oscillation to bridge the mechanical effect of the spray nozzles and a better exchange of solution. The fan jet nozzles are to be installed at an angle of approximately 30 - 40° transverse to the direction of the process. In case of multiple nozzle bars, these may be installed against each other.

Screen printing process

In isolated cases non-pth PCBs are still manufactured using screen printing technology. The copper surface of the panel is coated with a special ink applied through a screen, described in Section 2.11.1.3. The exposed copper surfaces will be removed during the following etching process. The pattern is finished; although further processing with solder mask can follow (see Section 2.11.2.11). Screen printing inks may be removed either by solvents or sodium hydroxide solution.

Environmental considerations

Table 2.3 sets out the advantages and disadvantages of the two layer coating techniques.

Liquid resists contains more than 50 % solvents that evaporate causing VOC emissions, which may require reduction by the Solvent Emission Directive [97, EC, 1999, TWG, 2004 #159]

The exposure stage generates no waste or waste water. Cooling water can be recirculated. The development stage can use successive static rinses of developer to reduce drag-out, and use of fresh rinsing water (see Section 4.7).

Removed resist can be filtered from the effluent from the developer stage.

Screen printing inks that have to be removed (stripped) by halogenated solvents can now be substituted by alternatives.

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120 September 2005 PT/EIPPCB/STM_BREF_FINAL Process tanks are usually equipped with fume extraction and scrubbing to remove generated

aerosols.

Waste water s may need treatment including neutralisation and settlement.

Packing and protection of dry resists generates significant volumes of waste. Different types of plastic are used, making separation and return difficult. Some material may be separated and returned for recycling. Due to the elaborate packing requirements, a return to the suppliers is generally not possible. It is possible to minimise the incurring wastes by using liquid resists for inner layers.