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Solder mask is a green lacquer that is spread out over a PCB to insulate traces and prevent solder flow outide of the pads intended for soldering. A board treated with solder mask is shown in Figure 6.4.1.

Solder mask is desireable for the aesthetic and insulating properties it confers to the circuit, but it can be very difficult to apply. Practice and experimentation will be required to develop a manual technique that produces consistent results. The following section will outline the procedure and describe the manual techniques that have produced the best results so far.

Figure 6.4.1: Board Treated with Solder Mask
Solder is repelled by the green surfaces.

! Safety Precautions ! Read Before Attempting Solder Mask Application !

The following rules should be obeyed at all times to prevent property damage and serious personal injury.

• Wear gloves and eye protection when handling solder mask and acetone.
• Wear UV-blocking eye protection!!! before operating the UV lamp. Failure to do so will almost inevitably result in Ultraviolet Keratitis, the symptoms of which will not be noticed until many hours after exposure.
• Do not solder in the same room where acetone is being used to clean PCBs. Acetone is extremely flammable.
• Lay down paper towels or paper to prevent solder mask from staining work spaces.

Overview and compatibility

Solder mask is applied to the PCB after a side of the material had been milled, but before any drilling or milling. Because of this, a modified phase processing order must be employed. See the Reprocessing section for more information on processing phases in custom orders. The best phase order for solder mask application is shown in Table 6.4.1.

There are some design constraints that must be obeyed if a board is to be coated with solder mask. The most important constraint is that the majority of the copper from the material be left unmilled. This is because the solder mask adheres to copper after it is cured, but it does not adhere to the bare FR4 substrate. If too much copper is removed in a particular area, the solder mask might peel or flake off, especially near the boundaries of the board. An easy way to accomodate this constraint is to not select “Complete Rubout” method of insulation in the Technology Dialog when generating toolpaths. Another way is to define “copper pour” regions in the board design so that most of the board is covered in copper. The second approach allows for more customization in clearance and appearance.

A second design constraint appears if rivets are used. The automatic punch has a head diameter of about 1.6 mm. This means that for the 0.6 mm rivets set in a default sized pad, the automatic punch will crush the solder mask surrounding the rivet unless extra solder mask clearance is given. An easy way to achieve this is to increase the pad size of the through-hole so that only bare copper is exposed to the automatic punch. Alternatively, the negative mask can be adjusted to not apply solder mask so close to the rivet.

Aside from the two aforementioned constraints, solder mask is quite versatile and it very accurate if good technique is practiced.

Table 6.4.1: Production Phases Orderer for Solder Mask.
The first four phases should always be processed before the others.

Production Phase	Tools Used	Description
Mount Material		Secure the FR4 copper-clad material on the moving table
Material Settings		Specify the thickness and dimensions of the material
Placement		Select location, orientation of board. Select number of copies
Drill Fiducial	Spiral Drill – 1.5 mm	Drill holes for board alignment
Milling Bottom	Micro Cutter Universal Milling Tool End Mill – All Sizes	Insulate traces on the bottom side by milling away copper
Apply Solder Mask		Remove the material and apply solder mask to the bottom side CircuitPro does not process this phase; it is performed manually.
Milling Top	Micro Cutter Universal Milling Tool End Mill – All Sizes	Insulate traces on the top side by milling away copper. Read Fiducials Top will be processed automatically.
Apply Solder Mask		Remove the material and apply solder mask to the top side CircuitPro does not process this phase; it is performed manually.
Marking Drills	Universal Milling Tool	Mark the location of through-holes to be drilled. The Read Fiducials Bottom phase will be processed automatically.
Drill Unplated	Spiral Drill – All Sizes Long End Mill – Both Sizes Contour Router – Both Sizes	Drill through-holes with the spiral drill and widen them if necessary with long end mills and contour routers.
Contour Routing	Spiral Drill – 1.0 mm Contour Rounter – 1.0 mm Spiral Drill – 2.0 mm Contour Router – 2.0 mm	Cut out the board outline, leaving small breakout tabs of substrate where requested. The Read Fiducials Top phase will be processed automatically.

Materials

There are several unique materials and tools that must be gathered before solder mask application is possible. These include:

•disposable gloves
•UV-blocking eye protection
•printer
•transparency paper that is compatible with the printer
•tape
•scissors
•sheet of plexiglass
•UV source
•metal needle
•bottle of acetone
•lots of paper towels
•abrasive pad or sand paper
•transparent sheets of cellophane

Designing Negative Mask

To control which parts of the PCB are covered in solder mask and which are not, a negative photolithography mask is used. The mask is made out of two face-to-face transparency sheets taped together.

To create the mask, export a bitmap or .png file of both the top and bottom solder mask patterns from the PCB editor. These designs should have black pixels where solder mask will not be applied, and white pixels where solder mask is applied. The black pixels will block UV light and prevent the solder mask underneath it from curing. An example pattern for the top layer is shown in Figure 6.4.2. A resolution of 600 dots per inch (dpi) is known to give good results.

Figure 6.4.2: Solder Mask Design for Top Layer

Without scaling or stretching the images, compose them with their mirror images and print them on transparency paper to create a pattern like the one shown in Figure 6.4.3. The pattern includes the top layer mask, its mirror image, the bottom layer mask, and its mirror image.

Figure 6.4.3: Negative Mask Printout

Cut out the four images as shown in Figure 6.4.4, and align each image with its mirror. Tape the mirroring images together as shown in Figure 6.4.5 so the transparency papers are facing each other and their ink is touching. Be very careful to align the images perfectly to provide maximum contrast and no shadowing. Ensure that no hairs, particles, scratches, fingerprints, or other optical contaminants are present in the design.

Figure 6.4.4: Negative Mask Cutouts

Figure 6.4.5: Top and Bottom Negative Masks
The tape that holds the two transparency cutouts together is outside of the pattern region.

Preparing the Copper

To improve the solder mask’s adhesion to the copper, contaminants such as grease, smudges, fingerprints, and oxidation need to be removed. This is done by using an abrasive material such as sandpaper or steel wool. While wearing gloves, rub the region of copper where the circuit board will be milled until it is shiny and uniform as shown in Figure 6.4.6. This may be done after milling if care is given to not break the traces with too much pressure.

After removing contaminants, wash the board with a paper towel wetted with acetone. Dry the board and repeat the wash until the copper is shiny.

A slight drawback to scratching contaminants off the copper is that the scratch marks will appear under the fiducial recognition microscope and might make fiducial location slightly more difficult. Another drawback is that the thickness of the copper is slightly decreased. Repeating the cleaning procedure excessively may result in conductivity issues.

Figure 6.4.6: Oxidation and Impurities Removed with High Grit Sand Paper
The dark powder left behind by the scratching can be removed with acetone and paper towels.

Spreading the Solder Mask

Spreading the solder mask is the most difficult part of the application and will require practice to yield consistent results. Before beginning, cover the fiducials on the material with painter’s tape as shown in Figure 6.4.7

Figure 6.4.7: Fiducials Covered by Tape

Next, squirt a few blobs of fresh solder mask onto the PCB material. After that, gently place a sheet of cellophane over the blobs as shown in Figure 6.4.8. Avoid making air bubbles.

Figure 6.4.8: Fresh Solder Mask

Then use an old card to gently push the solder mask outward, thinning it and spreading it across the design. The card should be able to press the cellophane flush against the solder masked copper to spread out the green gel as thin as possible. Push excess gel out of the board outline. A decent job is shown in Figure 6.4.9.

Ensure that no thick puddles of gel stand on the copper underneath the cellophane. The board should be coated very thin and the cellophane should be completely flat. Push air bubbles outside of the board boundary, as they will hinder the curing process and leave dots behind.

Figure 6.4.9: Spread Solder Mask

Aligning the Negative Mask

After the solder mask has been spread even and thin, align the photolithography mask over the PCB, and then press it down with a sheet of plexiglass, as shown in Figure 6.4.10. Shift the plexiglass to perfectly align the mask with the board. A good test for alignment is to inspect soldering pads at four corners of the board to ensure that they are covered by black ink.

Figure 6.4.10: Plexiglass Photolithography assembly.

Curing the Solder Mask

To cure the solder mask, align the UV source above the plexiglass assembly without bumping or moving the glass. If bumping the glass cannot be avoided, tape it down to the table before proceeding.

Before turning on the UV source, wear UV-blocking eye protection! UV rays, either direct or reflected, cause cumulative damage to the eye. The painful and frustrating symptoms from UV eye exposure are usually experienced roughly 8 hours later and can last for 2 days. If other people are in the lab room with the UV source, cover it completely with an opaque lid or bin and do not allow UV rays to escape.

After the UV source is in place and safety precautions have been taken, put a weighted object on top of the source turn on the source for 15 minutes. After the 15 minutes have elapsed, turn off the source and remove the plexiglass and negative lithography mask. Slowly and gently peel off the cellophane. There should be a wet pattern of uncured solder mask on the cellophane as shown in Figure 6.4.11, indicating that curing process was a success.

Figure 6.4.11: Cellophane Being Peeled

With the cellophane removed, wipe the board gently with a paper towel dipped in acetone. The acetone will dissolve the uncured solder mask while leaving the cured solder mask behind. Repeat the wash until the traces are clean and shiny was shown in FIgure 6.4.12. Be careful not to push or press too hard, as pressure and friction can break off the fragile solder mask.

Figure 6.4.12: Successfully Applied Solder Mask

Expose the board to about 10 more minutes of UV light to finish hardening the remaining solder mask. The board should be dry to the touch after the hardening process and touching the board with a paper towel should not result in green residue. If any of the green solder mask residue comes off the board, continue UV exposure. Take all UV safety precautions like before.

Inspecting the PCB

After the board is hardened, inspect the board carefully to ensure that all of the copper pads are exposed as intended. If there are any small pads covered in solder mask, they can be cleared by scraping away solder mask with a small metal needle.

Remounting the Base Material

After finishing solder mask application, remove the painter’s tape covering the fiducials and ensure that the fiducials are optically clean before remounting the board. Solder mask appears black underneath the fiducial recognition microscope and can make fiducial location significantly more difficult if the fiducials are surrounded by solder mask. Use sand paper to remove waste solder mask outside of the board outline if fiducial recognition becomes too difficult.

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