The photopolymerization characteristic and the convenience of dry film photo resist make it useful for image transfer in PCB fabrication. Ideally, the dry film imaging process should be done in a clean room with amber light illumination. Dust is a killer in fine line construction. Hence, a clean room is needed for PCB fabrication with fine line structure. For example, a class 10,000 clean room can be used up to 5 mil (0.127mm) line width.
Dry film is available in a range of film thickness from 1.0 to 2.0 mil. For fine line resolution, thin film is desirable. Thick film is more suitable if hole tenting is needed. Dry film normally supplied in roll form on a 3-inch (I.D.) core from 6 to 27 inches in width. They are constructed in a three-layer sandwich structure. A layer of photo-polymer resists sandwiched by a bottom layer of polyethylene (PE) film act as a separator and a top layer of polyester or Mylar film act as the protector. In addition to protection, the top plastic film also function as oxygen blocking layer to limit the amount of oxygen migrating into the resist film during exposure. During lamination, the laminator will remove the bottom film. The top film should remain in contact with the photo resist for at least 15 minutes after exposure. It should only be removed just before developing. The dry film is sensitive to long wavelength UV radiation in UV-A range. Different brand name has slightly different configuration.
For production type quantity, it is common to use roller coating process or wet process to replace the dry film process. In wet process, liquid photo resist is transferred onto the copper clad by roller and dry in a clean environment. A drying temperature of 50 deg. C. for 10 minutes is sufficient. Comparing with the image transfer process using dry film, this process is inefficient for small quantity because handling and high set up cost. Dry film normally gives a better result than wet process because of a better control in the thickness of photo resist. It is more difficult to control the thickness of the photo resist using roller coating in a PCB workshop environment. Although the requirement of single-sided board is usually less stringent than double-sided board in terms of wider traces or low density, dry film process is recommended for image transfer. In a production environment, dry film is normally reserved for tighter tolerance board because the unit cost is more expensive.
Figure 1 Dry Film Lamination Process
There are also positive acting photo-resist coating available in the market. For positive resists, exposure to the UV light changes the chemical structure of the resist such that it becomes more soluble in the developer. The exposed resist can be washed away by the developer solution and exposure the underlying metal for etching. Hence, for positive resist, a positive photo tool is needed. Positive resists are available in bottle, spray can or sold as a coated PCB. Since positive photo resist remove what has been exposed, it is very easy to polymerize due to leaky light. Hence, with the same level of handling, the imaging process is more difficult to control when compares to that of the negative photo resist. If positive resist is over exposure or the exposure time is too long, fine track can be easily become fade, narrowing and some may even disappear from the PCB resulted in open circuit. Open circuit failure is difficult to detect.
Surface Preparation
After machining, the PCB must be cleaned and scrubbed. Sand paper or industry type scrub pad such as Scotch-Brite 96 scrubbing pad from 3M can be used. Alternatively, a brushing machine can be employed to remove the dirt and oxide before dry film lamination. It is desirable to immersion the PCB in a 10 – 15% sulfuric acid for 1 minute to give a better surface preparation. For proper lamination, smooth PCB edges is desirable.
Dry Film Lamination
For PCB fabrication, DuPont’s Riston and Dynachem Laminar AX from Morton International can be used. A hot roller or hot shoes laminator is needed to stick the dry film onto the PCB. The laminator will remove the protective polyethylene film from the dry film during lamination. The temperature setting depends on the type of dry film used, typically, it is around 100 deg. C. to 125 deg. C. The temperature should be set to allow the dry film to adheres to the PCB. If the temperature setting is too high, blistering may occur and you have to do it all over again. The lamination speed is around 3 to 5 fpm.
Figure 2 A Dry Film Laminator
Figure 3 PCB with Dry Film Lamination
Contact Exposure
After the dry film laminated PCB is cooled down, it is ready for imaging. Dry film polymer is sensitive to near UV radiation, around 350nm to 400nm. Hence, the resist and the dry film process is better to be handled in an area with amber or yellow lights to minimize UV radiation.
Equipment used for contact exposure is normally composed of a vacuum unit which holds the photo tool in close contact with the PCB and collimating UV-A source. In addition, a timer is needed to control the exposure time of the photo resist. Exposure time is a function of the intensity of the UV radiation that is a function of the irradiation source. For perfect exposure, industrial type exposure equipment uses high intensity UV light source composed by a number of mercury arc-lamps together with a optical system to generate an intense, collimated and uniform UV light to achieve reliable and quality image transfer. In particular, good contact between the photo tool and the PCB can be achieved by placing the work piece inside vacuum chamber or hold them together with a mechanical press. A highly collimating source is essential to achieve straight edges and fine-line formation.
The source of the exposure unit can be composed by a number of long-wavelength UV emitting fluorescent tubes. Some examples of these tubes are 40W/350BL from Sylvania Osram or TL K 40W/10 UV-A from Philips. These tubes emit visible light and UV from 350nm to 390nm that fall in the UV-A range of 315nm to 400nm. A 40W tube can emit around 7.4 W of energy at 350nm to 390nm. These fluorescent lamps emit very little energy in UV-B and thus are safer for workshop application. High power metal-halides lamp emits energy at 280nm to 450nm can be used for industrial application. For fine-line formation, it is desirable to use high power and short exposure time to produce quality trace.
Figure 4 A UV Dry Film Contact Exposure Unit
The amount of radiation is determined by the energy needed for polymerization. Exposure can be calibrated using an integrating UV radiometer like the Dynachem model 500UV or a step transmission sensitivity chart; Riston 25-step chart from Dupont or Stouffer 21-step transmission wedge from Stouffer Industries can be used depending on the data supplied by the dry film manufacturer.
Stouffer 21-step transmission wedge is a 21-step photo tool with increasing opacity; step 1 is the most transparent and step 21 is the most opaque. Each step increases the density by 0.15 which corresponding to an f-stop of ½; each step doubles the opacity of previous step. Both calibrated and non-calibrated versions are available. For non-critical control, you may use a simple graphic package on PC and create your own exposure chart on a transparent film. A process log should be kept so as to track the quality of imaging due to components aging; for example, the aging of lamps, worn out of glass, etc.
The exposure time can be determined experimentally. We can prepare a number of PCB strips and expose it using the step transmission wedge. The initial exposure time may also be determined using an integrating UV radiometer with a direct reading of mJ/cm2 or by estimating the intensity of the exposure lamp and illumination area. After the pattern is completely developed, we can examine the image of the step wedge. The highest step number still having some resists film is the "exposure number" and the highest number that show no removals of resist film is the "step held number". Noted that the step held number is always greater than the exposure number but the two numbers should be very close to each other. For Dynachem Laminar AX, the "step held number" is 6-8. Although the tolerance for over exposure is large, correct exposure time can provide a better resist adhesion, avoid resist brittleness and provide better protection to the underlying metal at etching.
Development and Stripping
For negative photo resist, the developer can be made up with 0.85% by wt. (8.5 g/l) of sodium carbonate anhydrous (Na2CO3, soda ash), 1% by wt. (9.9 g/l) of sodium carbonate monohydrate (Na2CO3. H2O) and 1% by wt. of potassium carbonate (K2CO3, potash). Development of the dry film can be done by immersion or spraying with a spray pressure of 1.4 bar to 2 bar (20 – 35 psig) at a solution temperature of 27 to 35 ° C. Fan nozzles can be used with cone nozzle if film tent breakage is experience on double side PTH board. Development solution with proprietary additive is normally available from the manufacturer of the dry film. Consult manufacturer’s data sheet for the best concentration of the solution and solution maintenance. After the PCB has been developed, it should be checked for defects. Oil based pen which are alkaline soluble can be used to touch up any area with missing photo resist so as to prevent the copper from etch away. Unwanted solder resist should be removed using a paper cutter or knife; this will avoid short circuit after etching.
Stripping of the dry film can be done with 3.0% by wt. of sodium hydroxide (NaOH, caustic) at 55 ° C. A spray pressure of 2 bar (30 psig) can be used to remove the dry film mask. Potassium hydroxide can also be used as a stripper. In general, KOH stripper produces smaller particle size than NaOH. The stripped dry film will not dissolve into the stripping solution. The solution life of the stripper is longer but required filtration to remove resist skins to avoid nozzle clogging. The time required to strip off the dry film increases with the amount of light exposure. Longer the PCB is left with the developed resist, more time it will need in stripping. A high spray pressure is definitely helpful in the stripping of the dry film. However, a stripping bath can also be used for small volume fabrication.
Figure 5 A Horizontal Spray Processor which can be used for Dry Film Development, Dry Film Stripping or Ferric Chloride Etching
