The whole board surface is coated with a thin layer of photosensitive etch resist (‘photoresist’), either by a liquid process or as a ‘dry film’. This resist layer is exposed to ultra-violet light through a photomask, so that the areas protecting the required pattern are polymerised and hardened. Unhardened photoresist is removed by ‘developing’, the remaining resist is baked to increase its etch resistance, and the board is then etched.
Terminology to describe phototools is usually ‘positive’ and ‘negative’: on a positive image, the copper features are black and the base laminate features are clear.
The most commonly used photoresists are ‘negative’-working, which means they polymerise on exposure to ultraviolet light and hence become insoluble in a direct developer (Figure 1). Non-polymerised resist is removed by ‘developing’ to expose copper areas ready for electroplating.
Volume production of conventional boards usually involves the use of wet etch resists. For higher technology requirements, almost without exception, dry film photoresists are used for outer layers. These resists are normally 35 µm thick and applied using heat and pressure in a laminator (Figure 2). Most dry films are now aqueous processable.
The stages of photoprinting
lamination, by applying pressure and heat
exposure: using film artwork which is a negative of the required pattern, exposing the photoresist to ultraviolet light to harden the resist selectively as in the pattern
developing, to remove non-hardened resist
After processing the patterned part (by etching or plating) the remaining (hardened) resist finally needs to be stripped
With correct exposure and developing parameters, resist side walls are almost perfectly vertical and will produce well-defined tracks and spaces. However, this definition can be degraded by poor copper distribution on the outer layer.
Good plating characteristics are crucially dependent on the outer-layer image quality. Imaging must be performed in temperature and humidity controlled clean rooms. A fundamental problem with the flexible photographic films used to generate the images is that their size increases with temperature, typically by 25 ppm/°C. There is also a more complex non-linear response of the film base to humidity. The dimensional stability of the phototool is a critical factor for high precision work. To hold dimensions to one part in 20,000, one needs to hold the temperature to ±1°C and humidity to ±2% RH.
For manufacturers working with narrow tracks and gaps, liquid photoresist is reported to offer a more cost effective method of processing inner layers, in terms of cost reduction, wider process latitude, better adhesion and conformance, and improved resolution. However, the surface of the panel must be free from contamination, and residual particles can also affect the quality of the coating. The photoresist used for this process can be inspected after exposure, because the polymerisation is visible even before development. Inspection at this stage is recommended to increase yield and reduce overall scrap rates.
The photoresist may either be used as the etch resist on its own, or else be used to define the pattern of an electroplated layer of resist, most commonly tin or tin-lead. Outer board layers normally use the latter method.
The main etch processes are cupric chloride and ‘ammoniacal’, the latter using a proprietary blend of chemicals. The choice between the two is influenced by the application – cupric chloride is not compatible with tin-lead used as an etch resist on outer layers, but ammoniacal etchants have a problem with over-etching for fine line circuitry. In order to ensure a uniform etch, both processes are usually applied in a conveyerised, high-pressure spray chamber that exposes the whole board to a controlled, reproducible and constantly refreshed spray of etchant.
During the etching process the etchant attacks the copper in a sideways direction as well as down, so the cross-section of the finished trace has a trapezoidal shape: depending on the conditions, the narrowest point may be at the foot, at the top, or somewhere in between (Figure 3). ‘Etch factor’ is defined as the copper thickness, divided by the horizontal distance between the foot and the line edge at the top: the higher the etch factor, the smaller the amount of sideways etch, and the closer the side wall is to being vertical. The chemistry of the bath has a major influence on etch factor, and its oxidation-reduction potential and specific gravity are more important than temperature and spray pressure.
Where impedance control is needed, the trend for thinner inner-layer materials and fine line patterning has impacted on the etching process. Tracks on inner layers must have uniform width across the panel, minimal undercut, and equivalent etching performance on both sides. Process parameters which affect this are free acid concentration and uniformity of spray application.
Line width is fairly difficult to measure, but a good way of assessing average line width is to measure the resistance of a standard trace, which can be monitored using SPC techniques.
The most commonly used photoresists are ‘negative’-working, which means they polymerise on exposure to ultraviolet light and hence become insoluble in a direct developer, so that non-polymerised resist can be removed by ‘developing’ to expose copper areas ready for electroplating or etching. An alternative approach, shown schematically in Figure 4, uses a ‘positive’ working photoresist, which starts off insoluble in direct developer, but becomes soluble once exposed to ultraviolet light. When used for etching, this means that the copper pattern is the same as the artwork, rather than being reversed.
One of the uses to which positive working photoresist may be used is indicated in Figure 12. This uses the fact that the photoresist can be exposed on multiple occasions, removing additional photoresist each time, until the final stripping operation. This progressive removal of photoresist allows complex structures to be produced of which this figure is just one example.
Positive working photoresists have distinct advantages for aligning critical features, which is why they represent 80% of the photoresist used in semiconductor manufacture, but they are the exception rather than the rule for board manufacture.
A ‘smart-power’ circuit design has some conductor tracks which must be capable of handling high currents but other areas where fine pitch control ICs are to be mounted.
What are the implications for your choice of foil thickness, pattern definition method and manufacturing sequence?
If you were to choose a selective plating approach, what might be the advantages of using a positive working resist?
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Author: Martin Tarr
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