Printing Peelable Solder Mask

The use of temporary solder mask during mixed technology electronics assembly is a well established, if time consuming and costly, practice.

These single part elastomeric materials are generally spot applied by hand, but can be dispensed, or even printed using conventional screen techniques.

Hand applying peelable solder mask is both inconsistent and time consuming, as is the removal of the individual spot deposits after flow soldering.

Dispensing is slow and only copes with the smallest hole tenting requirements. With screen printing, the difficulty lies in achieving sufficient deposit thickness through a 50 - 60% open area screen mesh to tent holes and protect solderable surfaces. Large area deposits and contiguous deposits are achievable but many have had to resorted to multiple print / cure cycles to address the thickness problem. The screen itself lacks durability and regular cleaning can cause delamination of the emulsion.

Unlike with solder paste printing, the use of conventional stencils does not provide an answer. Although able to print deposits of 1.0 mm and more, conventional stencils are incapable of printing contiguous deposits so essential when producing a web of material to assist temporary solder mask stripping after the soldering stage. These stencils cannot effectively print large area deposits, as squeegees tend to scoop out or scavenge material from the middle of the printed area. Printing round ?island sites? is also impossible with conventional stencils.

Now, with the introduction of the MicroMesh stencil, contiguous deposits and large areas, even with island sites, can be printed? achieving sufficient deposit thickness to tent holes of 6mm and above with a single print pass.

MicroMesh, a differentially etched stencil system that is saving users much time and cost. It is a patented, stainless steel stencil system that combines all the benefits of stencils, in terms of durability, precision and optimised aperture fill, with some of the benefits of screens, namely the ability to print large deposits and continuous tracks.

The top side or squeegee side of the stencil emulates a woven screen by having perforations in the stainless steel to form a mesh. This mesh prevents any squeegee scavenging of the material, even on large area deposits. With an open area of up to 90% achievable, the mesh does not restrict the flow of the polymer but merely prevents the squeegee scooping material away from the centre of the deposit.

The underside or board side is formed into a cavity, that matches the precise dimensions of the required print area.

For large print areas, this cavity is prevented from collapsing by the inclusion of minute columns on the underside to support the mesh and within the space of the cavity. These columns do not significantly reduce the volume of the cavity but maintain its integrity during the print cycle. They prevent its collapse? thus guaranteeing deposit volume for optimum printing results.

Deposit thickness is a function of the mesh parameters in a conventional screen. It can be varied, within limits, by changing the mesh but you can only have one mesh size on one screen so deposit thickness is virtually fixed. As well as changing the material thickness of the MicroMesh stencil to affect the deposit thickness, any number of mesh parameters can be incorporated into one MicroMesh stencil so the deposit thickness can even be varied within a single squeegee pass. (Different areas of the PCB can receive different thicknesses of solder mask deposit.)

Print register, definition and repeatability are all high, as one would expect from a steel stencil.

The ability to print continuous tracks helps in removing the mask after wave soldering. By printing a web that joins the individual spot deposits, all the mask can be removed in a single operation, saving operators much time and effort as well as reducing the overall cost of the process.

MicroMesh stencils are individually manufactured; the design for each being derived from customers? original Gerber data. A simple sketch indicating areas to be protected and holes to be tented should accompany the Gerber data and the Company will design the mesh pattern accordingly.

MicroMesh has been shown to save time and cost in the application of temporary solder mask but is also suitable for printing a wide range of other materials where a substantial material deposit is required over a large or contiguous area.