Stencil Design for Mixed Technology Boards

A mixed technology board carries both surface mount and through hole parts, and the stencil has to serve both in one print or in two. The design of the foil therefore has to accommodate apertures for fine pitch devices alongside large pads for connectors and thermal features.

The difficulty is that the requirements conflict: the fine pitch region wants a thin foil and small volumes, while the through hole region often wants a heavy deposit. The stencil design is where that conflict is resolved.

Single Print Against Two Passes

A single print deposits paste for everything in one operation, which is faster and requires one stencil. The compromise is that the aperture geometry has to satisfy both the fine pitch and the heavy requirement with one thickness.

Two passes allow each region to have its own foil and its own volume, at the cost of a second print and an alignment step. The choice depends on how far apart the two requirements are.

Step Stencils

A step stencil carries two thicknesses, so a region can be thinner or thicker than the rest of the foil. This is the standard solution where a single thickness cannot serve both areas.

The boundary between the regions and the print direction then become critical, as described for step region quality.

Aperture Design for Through Hole Pads

A through hole pad needs enough paste to fill the barrel and form a fillet during reflow, which for a connector pin can be several times the volume of a chip joint. The aperture is usually a slot with an enlarged area on the pad.

The aperture must not extend over the hole if the paste would drain, so the geometry is often split or offset. The intent is to deliver the volume to the pad rather than into the barrel.

Thermal Pads and Large Openings

A large thermal pad aperture is segmented into a grid to allow gas to escape, which also reduces the volume slightly. On a mixed board this segmentation has to coexist with the mask dam requirements of the fine pitch area.

The segmentation should be designed for the reflow behaviour rather than copied from a previous board, since the voiding depends on the paste and the profile.

Relief and Support

A large aperture in a thin foil leaves a weak area, so a step stencil may need relief cuts or a support pattern to maintain tension. The relief is designed by the stencil supplier and should be reviewed.

Where a relief crosses a paste area it changes the print, so the position has to be chosen with the aperture layout in mind.

Print Parameters for Mixed Boards

The squeegee pressure, speed and separation have to suit both regions, which usually means choosing values that are safe for the fine pitch and acceptable for the coarse. The fine pitch is the limiting requirement.

The parameters should be recorded per product, because a setting that works for one mixed board may not transfer to another with a different balance of features.

Cleaning and Inspection

A stencil with both fine and large apertures may need more than one cleaning pass, since the large apertures hold more paste and the small ones hold it tighter. The cleaning schedule should account for the mixed geometry.

Inspection should check the fine apertures for blockage and the large ones for residue, which are two different checks on the same foil.

Documentation

The stencil drawing should show the thickness, any step, the aperture dimensions with the reduction applied, and the segmentation of large pads. It should also state the print parameters that the design assumes.

These details belong with the assembly documentation that the print process references, alongside the requirements in design release.

Process Control and Verification

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

FAQ

Is a step stencil always needed for mixed technology? Only where a single thickness cannot deliver both the fine pitch volume and the heavy deposit within their windows.

Can one print cover both through hole and surface mount? It can, and the aperture design has to deliver the barrel volume without draining the paste into the hole.

Why segment a thermal pad aperture? To give the gas a route out during reflow, which reduces voiding in the joint.

What limits the fine pitch end? The area ratio of the smallest aperture against the foil thickness, which sets the minimum practical aperture size.