Solder mask functions and properties

Solder mask functions

Solder mask (also known as ‘solder resist’) is used on the vast majority (97.7%) of boards (IPC 1996 survey), and has many functions. Our first list, expanded from Wallig’s contribution to Coombs Printed Circuits Handbook, contained seven items:

To reduce solder bridging and electrical shorts
To reduce the volume of solder pickup to obtain cost and weight savings
To reduce solder pot contamination (primarily by dissolved copper and gold)
To protect circuitry from handling damage, dirt and fingerprints
To provide an environmental barrier (especially against moisture) and an electromigration barrier against dendritic growth
For high frequency applications, to fill the space between conductor lines and pads with material of known dielectric characteristics
To act as an insulation or dielectric barrier between electrical components and conductor tracks or via interconnections when components are mounted directly on top of the conductor layer.

The most crucial ideas relate to the environmental protection and the control of solder. How many of these did you identify as you went through the activity?

The items we missed out first time around were:

The use of solder resist during subsequent processing, to act as a plating resist with finishes such as nickel-gold, palladium, silver and tin
The important part that solder mask plays in determining the fillet shape in applications such as those BGA designs where the sides of the well in the solder mask actually come into contact with the solder ball. You’ll get a reminder later that the surface of a printed circuit board has three dimensions, and not just two
A final point, made by one of the reviewers, is that solder mask does not always come in green, as was implied in the activity. Sometimes there are cosmetic reasons for preferring an alternative colour; other users differentiate prototype boards from production circuits by using a different solder mask colour, for example production boards using green solder mask, but prototype boards being in red. To go with different colours of solder mask, there may be a requirement to have the component identification print in different colours, although the common white and yellow choices work well on both green and red solder mask.

Solder mask properties

In the words of a British Standard, solder mask is a ‘permanent polymeric coating’, and it has to be permanent despite the harsh nature of the application and environment.

Figure 1: A ‘permanent polymeric coating’ that cracked up!

One list of the properties required of a solder mask that we used to publish was that a solder mask must:

not soften during soldering
not be affected by any fluxes or cleaning fluids used
have good insulation and humidity resistance over life
be non-flammable
have good scratch resistance
not allow solder pick up during solder levelling or soldering¹ operations.

¹ This is crucial during wave soldering, especially in a nitrogen atmosphere, where certain kinds of solder mask have been implicated in the formation of small solder balls which partially embed themselves in the mask and create a reliability hazard

How many of these did you identify as you went through the activity? And did your list have any more? [Apart, that is, from sordid commercial considerations such as low price and ready availability!]

Based on a memory of how solder masks are applied, you might have tied this into the polymer and flow information, and listed some qualities associated with the intended application method, such as:

having the correct rheology for the process being used – printing would need a mask which shear thins; dispensing a resin that flows easily
having a sufficient pot life
being photosensitive and thus able to be imaged with ultraviolet light.

But these aren’t all the qualities a solder resist needs. When you think of the requirements of the fabrication process, you will realise that:

Solder mask has to have good adhesion both to the base laminate and to the surface metallisation on the foil
If it is planned to apply any coating on top of the solder resist, then the two materials have to be compatible: they must stick to each other and the mask must not inhibit the curing of the next layer or modify its properties. This applies to legend applied to selected parts of the board just as much as to a conformal coat that covers the whole surface
During its flow stage, solder mask has to have the right surface energy to be able to wet the surface of the board, without leaving voids or any areas unwetted
Solder mask must not produce or leave any contaminants which adversely affect the solderability of the board.

One thing no solder resist has is the ability to remain firmly attached and unwrinkled if it is applied to a solder surface which is subsequently reflowed.

Figure 2: Resist crinkle due to remelting of underlying solder

Particularly if thick, a solder coating will remelt when the assembly is soldered, causing solder mask to wrinkle over the track areas. Not only is the visual result unsatisfactory, but there is also a reliability aspect, that the mask can flake off and be a potential hazard at contact areas. For these reasons, the SMOBC (solder mask over bare copper) sequence is generally preferred.

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