Soldermask Lamination

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1. I am using a dry-film soldermask for my PCB boards. The cured color is yellowish-green and the finish dull. On production boards that we buy, the soldermask is dark green and has a glossy finish. Is this really the color of the dry-film soldermask or there is something wrong with my process?

The color is largely a difference between the formulations of different vendors and types of film. Generally speaking, the liquid photo-imagable (LPI) and screen printed soldermasks used on most production boards tends to be darker in color than the 8030 or 8130 series of soldermask. If your surface is rough or grainy after processing, this is a sign that the exposed (imaged) area of the film is being attacked by the developer. If this is the case, you either have your exposure time set too low or the concentration of your developer is too high. Check both and run a couple of test exposures. If both parameters are within specifications (and you film is not too old) the exposed surface will be green and glossy after the final thermal curing step.

2. I am working on a lab-scale hot roll laminator. Following the manufacturer's specs for laminating their dry film photopolymer to a circuitized board, I have observed some bubbles which look to be trapped against the copper circuitry. Have you ever observed this kind of behavior? How can I get a bubble-free lamination

I assume that you are using dry-film soldermask to encapsulate your circuit after etching. During dry-film lamination, air can become trapped between circuit elements as the board passes between the rollers. This is especially true if the circuit pattern is fairly dense, with long traces that run parallel to the lamination rollers. This trapped air manifests itself as bubbles as the film is imaged, developed, and cured.

Another source of bubbles is routinely encountered when using dry-film photopolymer. The rheology of so-called dry-films (actually super viscous liquids) during lamination prevents the perfect conformance of the photopolymer to the sharp corners that form between the copper pattern and the substrate in an etched circuit. This results in corner gaps that can run the entire length of a trace and totally surround a pad or via. This type of defect is especially troubling since it can form capillary paths that allow fluxes and other liquids to wick along circuit elements leading to shorts, leakage, and even circuit failure.

The conventional answer to both problems is to use a vacuum laminator which eliminates the trapped air that is at the heart of the problem. Being cheap, and reluctant to spend money if we do not have to, we have been reasonably successful in eliminating these defects in our own shop by using a modification of the wet lamination technique first introduced by DuPont a number of years ago. Using a "plant mister" with a 5% solution of methanol (wood alcohol):

1. evenly moisten the panel immediately prior to lamination. The panel should not be dripping wet, just moist enough to change the surface color
2. make sure that your feed table is totally free of dust and other contamination
3. if possible, orient your panel so that no long circuit features (traces or ground planes) are parallel to the lamination rollers
4. laminate your board with your laminator feed set to 0.25 to 0.50 feet per minute. I know that this seems a bit slow but it gives the photopolymer time to absorb the solution and flow around the etched circuit elements (see below).
5. let your board sit for at least 15 minutes after lamination before proceeding with further processing

The benefits of this approach are two fold. First of all. the water fills in the gaps and edges of the circuit features. If you watch closely as lamination proceeds, you will see that a small bead of water forms just before the nip region and "runs" in front of the lamination rollers. This pushes air out of areas where it would otherwise be trapped and acts as a moving seal to prevent the air from seeping back in.

The methanol acts as a wetting agent to allow the water to penetrate all of the little nooks and crannies in your circuit pattern. In low concentrations it will not affect the curing properties of the dry film and, with the water, will reduce the viscosity of a thin layer of dry film. This "thinning" allows the otherwise super viscous liquid to flow into the gaps as the water (and alcohol) are absorbed into the bulk of the photopolymer.

We are having problems with the solder resist film, the problem is that the
peel sheet is sticking very strongly to the rest. The first time we tried to
separate the peel sheet, the cover sheet got separated, and we thought the
peel sheet of this film was different from that of the photoresist film; of
course we had to remove and rotate the film roll, only to notice the error
after 2 hours and a few meters of ruined film. 

Our solution to this problem was to mount this film on the top of the
laminator (not on the multi roll adapter), hoping the hot air convecting from
the hot rollers could soften the film and help in delamination. This time the
peel sheet came out well, but the resist film delaminated from the cover
sheet also, along the way (when feed speed was higher than minimum). If we
keep at minimum feed speed, we have some success.. Is there any other
solution to this problem? I am sure the photoresist film will not be a
problem, as its peel sheet is sticking much more weakly.

The problem that you are describing with the soldermask ususally results
from the film being exposed to high temperatures for an extended period
of time (during shipment maybe?). The only way that I have ever found
to use film once it has been heat damaged is to use a technique called
sheet lamination: Do this:

1. Get your board ready to laminate
2. From the roll, cut enough soldermask to cover your board with about
3" left over. This 3" will act as a "handle" to allow you to keep
tension on the film as the board feeds through the laminator.
3. Pre-heat the laminator to 120°C and set the speed to 2 or 3.
4. Reveal the adhesive by carefully peeing the release liner off of the
film sheet (NOT the cover sheet)
5. Slightly wet a strip of adhesive 1/2" wide by the full width of the
film with a 5% solution of methanol in water. This will make the film
very tacky and easy to stick to the board.
6. Using a squeegee, seal the strip to one edge of your PCB.
7. Feed your board into the laminator, sealed edge first. As the board
is feeding, hold the soldermask off of the board and keep an even
tension on the film. This will elinimate wrinkles during lamination. Assuming
that you initially sealed the correct edge, the film should be long
enough to allow you to maintain tension until all of the board feeds
through the first set of rollers.
8. After the board exits the laminator, trim off any excess film and
repeat the process for the other side of the board.
9. Let the coated board sit for at least 15 minutes before exposure and
developing.

If you find that the film is not sticking to the board (a common problem with heat damaged film), try spraying the adhesive side of film (the
surface underneath the release liner) with a LIGHT coating of the 5%
methanol solution just prior to lamination. The methanol will slightly
dissolve the adhesive layer and make the entire film very tacky. As the
board is pulled through the laminator, any excess liquid will be
squeezed out, leaving behind a well adhered film. DuPont pioneered this
technique with their ValuMaster series of laminating machines. The
machines actually come with a spray bar that applies a mist to the film
as it is being drawn into the rollers. If you spray the entire film,
you MUST let the board set for at least 30 minutes after lamination to
insure that all of the water/methanol is absorbed into the bulk of the
soldermask.

Please give more detail on the thermal lamination film to be used as backing
material (producer, ordering code - or preferrably thickness, plastic type,
and structure) as it is not so easy to go to an office store and ask for a
certain film in this country - nothing is standard, and lamination materials
may not be readily available.. Of course paper rolls could be easier to
procure here, if the material does not have great disadvantages..

You can use wider backing material if you want but, with careful
alignment, it is really not necessary. The backing film that we use is
referred to as "thermal laminating film" or "protective film". It is
the same stuff that is used to laminate documents and labels that you
want to protect from the weather. If you have a copy center or printer
that offers lamination services, they might be able to tell you where to
purchase rolls of the film. The film that we use is 1.5 mils (37 micron)
thick and melts (laminates) at 140°C. If you want to use paper, find
40# (pound) Kraft paper. It is the brown paper that merchants use to
wrap up dishes, cups, and other items that they will be packed in a
larger box. It is reasonably free from lint and protects the rollers well.