SIPAD Soldermask Requirements

The design and application of the soldermask coating on a SIPAD board is critical. Without the proper soldermask application, it may be impossible to process certain printed circuits with SIPAD ssd.

There are 3 soldermask rules:

Isolate each smd pad within the footprint. Soldermask dams between pads.
Isolate all holes from smd pads including via holes as well as component holes.
Apply soldermask thick enough to form a physical well to hold more solder paste. Soldermask should be flat/planar to the surface of the boards

Tooling Hole Requirements

Soldermask Design, Isolating smd pads

It is very important that the soldermask completely isolate every surface mount land from other smd pads as well as through holes and vias. There should be a soldermask dam between pads even at .4mm (.0157) pitch components. These dams should be well registered and strong.

• Soldermask design does not provide dams between smd pads

• Broken soldermask dams can be worse than missing dams

Soldermask Design, Isolating holes from smd pads

• Holes open to surface mount pads can rob solder from smd.
• BGA's require solder mask touch up before processing.

Soldermask Thickness

Typical liquid photo image (LPI) soldermask thickness ranges from .001 - .0015. (This figure is based on a 1 ounce copper clad material plated to 2 ounce).

The target mask thickness for a SIPAD boards should be .0015 - .002. Because there are numerous application techniques for LPI mask it is up to each individual facility to determine their limits when it comes to imaging and mask thickness. Ideally the mask should be built up so that it forms a well around the smd lands, the mask being higher than the base copper+ plating+ HASL (or other final surface finish).

Some SIPAD applications do not involve fine pitch components and heavier copper requirements. When 2 ounce copper clad laminate is specified, the soldermask should be applied even thicker to help create a well around the smd pads. Normally designs that require 2 ounce base copper will not use fine pitch components so the soldermask features should be easier to handle with the thicker requirement. Please notify SIPAD Systems when quoting the bare boards.

Potential Processing Problems

• Damaged pads, shop debris, and soldermask debris can lead to rejects.

Surface Finish Issues - Solderabilty

The SIPAD ssd process depends on a good solderable surface finish from the printed circuit board vendor. Because the SIPAD board is reflowed without components it is very easy to see if the surface finish is wetting to the solder paste. If the surface finish is contaminated or not properly applied the SIPAD ssd finish will not be successful. Potential surface finish issues are listed below.

• Poor HASL Finish.

• Solder paste will not wet properly to HASL finish.

• Flattened SIPAD ssd's will not wet to contaminated HASL finish.

Excess Solder in Via Holes, Nodules

Excess solder in via holes can create an issue during SIPAD processing. Solder in via holes can flow out during flattening creating a solder deposit that could cause quality issues after final assembly. Nodules can also cause printing issues, preventing the stencil from forming a gasket during the printing process.

• Solder in via holes can flow out to cause quality issues.

• Nodules will cause printing problems.

Packing and Shipping

• Warp and twist creates scrap.
• Boards should be packed properly for transport to SIPAD Systems as well as the end user.
• Eraser debris on boards from manufacturing