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Solder Mask for Printed Circuit Boards
Posted on 26/06/2015

At PCB Global, we offer a range of Solder Mask colour options such as green, blue, purple, red, yellow, white, black matt and black gloss as well as the option to have no solder mask. These options apply to our FR4 (2-20 layer), Full Spectrum (1-40 layer) and Aluminum Printed Circuit Boards.

Once you have decided on your design of your Printed Circuit Board you need decide on options that will suit your requirements for your board. One of the options is the choice to have a solder mask and/or silkscreen legend. Why not deviate from the standard green printed circuit board and pick a color that will match your corporate logo?

It is important, even for your prototype PCB’s to have a solder mask and silkscreen legend for many reasons. Without the solder mask, your board would be plain copper, or what is known as a bare board with Tin Plating (tin, lead and usually a silver mixture). The Tin plating is placed on the bare board to allow soldering to take place and also slows down the rate of oxidation in the copper.

Today the Solder Mask application is LPI (Liquid Photo Image), which is applied to the production panel in a silkscreen printing process one side at a time. It is then dried in an oven and then the solder mask film is aligned to the pads and bared in an UV exposing machine then placed in a conveyorised developer to remove all the areas (SMT pads and holes) that will be required to have the surface finish applied later like HASL, Immersion Gold, OSP, etc.

The solder mask is a coating that protects the circuit from electrical shorts and corrosion as well as providing electrical insulation, allowing higher voltage traces to be placed at a closer proximity to each other. The most important function of the solder mask is to keep the solder on the pads, which will reduce the formation of bridges or unintended connections. In some cases, such as wave soldering, solder masks are a critical element of this process whereas in other cases, it enables a more efficient and more accurate hand solder job as the solder covers the entire board except for the pads. The colour of the solder mask is not vital to the function of the board, rather providing a distinctive colour to your design and enabling better visibility to your copper connections. There is an option to have a partial solder mask that will cover a specified part of the pad, rather then exposing the entire pad.  This would be ideal if there is a pad underneath a chip, leaving the pad exposed and vulnerable to solder bridges.

Exposed vias are usually covered with the solder mask and silkscreen as this will provide protection from corrosion and electrical shorts while also providing a larger surface area. If a board does not contain a solder mask, it is not an issue, it will mean that care must be taken when soldering as the solder mask reduces cost of mistakes and time taken to complete the solder and components.

Physical and Electrical Properties of the LPI Solder Mask Include:

 

Adhesion

100/100 (JIS D-202)

Pencil Hardness

7H (JIS D-202)

HASL Resistance

260℃/10 seconds ×3 times

Solder Heat Resistance

260℃/10 seconds ×3 times

Acid Resistance (before the solder)

≧30 minutes (10% H2SO4(aq),20℃)

Alkali Resistance (before the solder)

≧30 minutes (10% NaOH(aq),20℃)

Solvent Resistance

≧30 minutes (Trichloroethane,20℃)

Pressure Cooker Test

121℃2atm, 5Hr

Boiling Test

100℃, 5Hr

Dielectric Strength

1000V DC/mil

Volume Resistance

1×1015Ω.cm

Surface Resistance

5×1014Ω

Insulation Resistance

5×1013Ω

Moisture and Insulation Resistance

5×1010Ω (In moisture)

1×1012Ω (Out of chamber)

Dielectric Loss Factor (tanδ)

0.025 (1MHz)

Dielectric Constant (ε)

3.4 (1MHz)

Flammability

UL 94V-0

 

 

 

Next comes the option for the Silkscreen layer which is also called a ‘legend’ as this is a non-conductive ink used to place traces on a board as a means of instructions. It is utilised as a pathway to allow multiple traces and pathways, as this will not be conductive in the copper layers alone as the copper traces would interrupt the circuit. This also makes better use of the space on the board and provides a better contrast for a faster assembly. For prototype PCB’s the silkscreen is usually restricted to the top part of the board but in some cases it can be placed on both sides of the board if required.

PCB Global was created to minimize time taken in waiting for quotes and orders to be processed. We utilise these processes by adding you, the designer and engineer, to quote and order PCB’s to your specifications 24/7, 365 days a year, allowing you to test or create your product for your needs at a faster rate.