Chemask LF - Lead-Free Solder Mask

The main difference between temporary solder mask and solder resist (overcoat) lies in their purposes and longevity. Temporary solder mask is used for selective soldering and rework processes and is removed after its intended use, while solder resist is a permanent coating applied to the entire PCB surface to provide long-term protection and prevent solder bridges. Chemtronics Solder Mask Products Learn more in our Blog Follow our Linkedin

Solder mask is engineered to withstand molten solder temperature for the very brief time (about a second) it is exposed in a typical wave soldering process. Most commercially available masks should be able to withstand melting point temperatures of lead-based and lead-free solder, up to 650°F (343°C). As exposure time is lengthened, like in a reflow oven, there is more potential the mask will bubble, char, and become brittle. How long a mask can withstand extreme temperatures depends on the thickness of the bead, and the type of mask.

Temporary solder mask is intended to be just that – temporary. Manufacturers of solder masks, like Chemtronics, always recommend removing the mask because there is no testing to support doing otherwise. If you decide to leave the solder mask on as a permanent part of the PCB, you will have to do your own functional and reliability testing to ensure it does not cause a problem.

No, temporary solder mask will not leave behind ionic residue that can lead to dendritic grown and board failure.

Peelable mask is usually removed by hand or with tweezers. Resourceful engineers have come up with devices with rotating brushes and other creative methods, but there is no commercially available equipment to speed this process along. 

Believe it or not, the way you hold the bottle can make a big difference on how easily the cured mask is removed. Many operators hold the bottle at a 90° angle as they drag the dispensing tip across the area they are masking. This has a tendency to force the mask through the vias and causes 3 potential problems: 1) The mask tends to stick to the inside of the thru-holes, and may push out the other side and mushroom out. This creates a plug on the other side, which increases the chance of breakage when peeling off the cured mask. 2) The material inside the thru-holes are less accessible if washing off mask in an aqueous cleaning system. 3) The bead of mask is thinner when holding the bottle at 90°. When using a peelable mask, this bead acts as a draw-string, so a thinner strip has more of a tendency to become brittle in high soldering temperatures, and is more likely to break when removing the cured mask.

If you think of spot mask as basically liquid masking tape, the application become more intuitive: 1) Apply the mask to the contact areas to protect. Allow mask to fully cure. 2) Apply the conformal coating. 3) After the coating is dry to the touch, but before it is fully cured, peel off the mask. Otherwise it could create a cracked, ragged edge, or even pull up the coating.

Spot mask can be hand applied, dispensed with a pneumatic system, or stencil printed. To hand apply using a squeeze dispensing bottle: 1) hold the mask bottle upright, but at an angle (around 60°), 2) squeeze the bottle as you drag is across the areas to be masked, 3) for best results, allow mask to fully cure before running through extreme soldering temperature.

The shelf life of a product can be found on either the technical data sheet (TDS), available on the product page, or by looking on the certificate on conformance (COC). The COC can be downloaded by going to https://www.chemtronics.com/coc. Once you have the shelf life, you will need to add it to the manufacture date for a use-by date. The manufacture date can be identified by the batch number. The batch code used on most of our products are manufacture dates in the Julian Date format. The format is YYDDD, where YY = year, DDD = day. For example, 19200 translates to the 200th day of 2019, or July 19, 2019. This webpage explains and provides charts to help interpret our batch numbers: https://www.chemtronics.com/batch-codes.

The follow are factor to consider when trying to avoid breakage: 1) Curing – Partially cured mask will have more of a tendency to break or leave residue. 2) Temperature and duration in extreme heat – Mask is designed for wave soldering, which is about a second of exposure to molten solder temperatures (generally 550°-650°F). If the mask is sent through a reflow oven the duration is much longer, which can lead to more mask brittleness. If the mask is exposed to high-temperature solder (e.g. silver), it can also lead to more brittleness. 3) Application method – When hand applying, we recommend holding the bottle at about a 60° angle, rather than upright at 90°. That tends to apply the material as a thick blanket over the areas to be masked. When the bottle is held at 90°, the mask is generally applied thinner, and is forced through holes and around components. Those thin areas are more likely to become brittle and stay behind, and mask pushed into holes and around components is harder to remove, so more likely to break.