Liquid Photo-Imageable Solder Mask

If you have ever undertaken the task of PCB Assembly on a board without Solder Resist, then you are likely well-acquainted with its importance. In addition to giving the PCB its distinctive colour, solder masks also perform the much more important job of preventing soldering from flowing onto undesirable areas of the board, facilitating a High Quality PCB Assembly. Given the importance of this aspect of your board, it is only natural to wonder how one type of solder mask might differ from another. Once again, Solder Mask Colour is not the only variable at play here; there are three main types of solder resist that exist on the market today: Silkscreened Epoxy, Dry-Film, and Liquid Photo-Imageable (LPI).

At Bittele Electronics, we provide LPI Solder Mask as one of our Standard Price PCB Options, at no additional cost. We encourage our clients to use this form of mask since it is not only the most convenient in terms of the PCB Assembly Process, but more importantly it is the most robust and high-quality option for your PCBs. The only case where we do not regularly use LPI solder mask is for Flexible PCB Assembly, which requires a special coverlay solution.

LPI solder masks are composed of an ink compound that can be either silkscreened or spray-coated onto the PCB. The LPI solder mask technique is commonly used with the Hot Air Solder Leveling (HASL) and Electro-less Nickel, Immersion Gold (ENIG) surface finishes, and its application process requires a clean environment, free of particles and contaminants. After an LPI solder mask is applied, and the PCB is completely covered on both sides with the solder mask, the next stage in the process is curing.

Unlike other types of solder mask, LPI inks are sensitive to UV light, and can be cured after a short “tack cure cycle”, making use of UV light exposure. This curing process cements the solder mask in place permanently, and the durable material is extremely difficult to remove from the board afterward, lending to long shelf life.

To ensure that the LPI mask is cured in the proper locations, negative film stencils of the top and bottom solder mask Gerber Files are printed using a contact printer. The film sheets are printed with black sections corresponding to any areas of the PCB that are to be left uncoated for soldering, or otherwise free of solder mask. For Complete PCB Assembly projects, this is very efficient, since we already have to create the positive-image stencils for paste application. The entire PCB is then exposed to a UV light, which causes the solder mask to cure and harden in any exposed areas, but has no effect of those areas shielded by the black film. After the curing process is complete, any excess solder resist can be washed off of the film-shielded sections, leaving only the desired areas covered. This is generally one of the final steps in PCB Fabrication, proceeded only by the application of surface finish and PCB Silkscreen.

Our high quality LPI solder mask is durable and long lasting. Usually, we consider that a 1 μm solder mask layer can withstand 100 VDC. We can ensure the solder mask thickness will be minimum 5 μm on the conductor corner, and minimum 10 μm on the conductor surface. These thicknesses ensure that, for the vast majority of designs, breakdown of solder mask will never be an issue.

If you still find yourself with any questions about LPI Solder Masks, or to get started on an official PCB Assembly Quote for your design today, feel free to Contact Us any time! You can reach us over email at sales@7pcb.com, or call us toll-free at 1-888-812-1949.

Bittele is a full-service provider of low-cost Canada-Based PCB Assembly solutions. In business since 2003, we have established ourselves as a leader in the PCB production industry, and we offer a variety of tailored solutions to meet your specific needs.