We've gathered together answers to some of the most common questions regarding stencils - but please don’t hesitate to contact us to discuss any of these or other stencil related issues.

What thickness stencil do I need ?

Stencil thickness should be determined by the finest pitch device or smallest aperture size on the image. Boards without fine pitch devices are fine on 6 thou (0.15mm), or if you need large volumes of paste then 8 thou (0.2mm) might be an option.

However if the image contains devices of 20 thou pitch or below, then a 5 thou (0.125mm) stencil is best. Images with very small μBGA’s or similar device may need 4 thou (0.1mm).

What aperture modifications should be applied ?

For most conventional devices (e.g. resistors, capacitors and non fine-pitch IC’s) an overall reduction of 5 to 10% from pad size is best. Fine pitch IC’s should normally be reduced in width to give a equal aperture to gap ratio (i.e. 10 thou aperture on a 20 thou pitch device).

Specialist devices such as BGA’s, flip-chips and 0201 chips need to be looked at on an individual basis - we are happy to recommend on these aspects of stencil design.

We can also advise on more elaborate modifications to help with particular printing problems such as under chip solder balls.

Are my existing stencils suitable for lead free pastes ?

All stencils supplied by Sparks are produced on hardened high nickel content stainless steel ensuring compatibility with lead free paste.

What data files are needed to produce a stencil ?

Ideally your data should include paste layers which are unmodified (if your data is modified it’s important to notify us to avoid a second reduction being applied). Where possible including tracking, resist and silk screen layers can help in the pre-production of your stencil.

Ideally data should be supplied in gerber (RS247X) format but we can also accept DXF and HPGL files.