Stencil Aperture: Design Rules and Process Limits
A stencil aperture is the opening through which the solder paste is printed onto a pad. It is a few tenths of a millimetre across, and it decides the volume of solder that will form the joint. The pad is the electrical feature and the aperture is the process feature, and the two do not have to be the same size or the same shape.
What the Aperture Controls
The aperture controls the paste volume that lands on the pad, and that volume is the product of the aperture area and the stencil thickness. The whole design is therefore a volume calculation.
The aperture controls the volume of paste that lands on the pad, and the volume is the product of the aperture area and the stencil thickness. The whole design is a volume calculation.
Too little paste gives a fillet that is concave and a joint that may not be reliable, while too much gives a bulbous joint and a risk of bridging to the neighbour.
The aperture is therefore a tuning parameter. The pad comes from the component datasheet and the aperture comes from the assembly process, and they should not be assumed to be identical.
The relationship between the pad and the aperture is described in the pad standards material, and the aperture is often written as a percentage of the pad area.
Area Ratio and Transfer Efficiency
The area ratio is the area of the aperture divided by the area of its walls. It is the number that predicts how much of the paste will actually transfer from the stencil to the pad.
A high area ratio gives a clean release, while a low one leaves paste behind on the wall of the aperture. The usual rule is to keep the ratio above about 0.66.
The ratio falls as the aperture gets smaller and as the stencil gets thicker, which is why a fine pitch part forces a thinner stencil. A thick stencil with a small aperture is a combination that does not release.
The calculation is quick and it should be done before the stencil is ordered, because the stencil is a physical object that cannot be adjusted afterwards.
Aperture Shape and Reduction
A rectangular aperture is the default, and it is used where the pad is rectangular and the pitch is comfortable. The corners are usually rounded to help the paste release.
A home plate or a rounded shape is used to reduce the volume at the ends of a pad, which is where a bridge usually starts. The shape change is small and the effect on the yield is large.
A reduced aperture is used to lower the volume on a pad that would otherwise receive too much. The reduction is usually between five and twenty per cent of the linear dimension.
An enlarged aperture is used on a pad that needs more solder, such as a thermal tab or a connector pin. The enlargement is limited by the space to the neighbouring pad.
Stencil Thickness and Volume
The stencil thickness sets the height of the paste deposit, and the volume scales with it. A thicker stencil gives more volume for the same aperture area.
The thickness is chosen for the whole board, so a component that needs more volume has to get it from a larger aperture rather than from a thicker stencil. That is the origin of most of the aperture design work.
A common thickness is 0.12 mm for a board with a mixed component range, and a thinner or a thicker foil is used where the range is narrow. A stepped stencil, which is thinner in one region, is used where the range cannot be accommodated.
A stepped stencil solves the problem by thinning one region of the foil, but the step is a mechanical feature that collects paste and has to be cleaned more often. Where the volume range is very wide, the step is still the practical answer.
The stencil material matters as well, since a laser cut stainless foil gives a cleaner wall than a chemically etched one and releases the paste better.
Bridging and Slumping
Bridging happens when two adjacent deposits join together, either during printing or during reflow. It is the most common defect and the easiest to design against.
The gap between two apertures must be large enough for the stencil web to survive and for the paste to stay separate. A web that is too thin breaks during printing and leaves a smear of paste across the pads.
Slumping is the flow of the paste after the stencil is lifted. It depends on the viscosity, the particle size and the area of the deposit, and a large aperture slumps more than a small one.
The bridging risk is also affected by the pad geometry, since a pad that is wider than the lead allows the solder to spread sideways. The aperture and the pad have to be designed together.
Special Cases
A thermal tab under a power device needs a large volume of paste, and a single large aperture slumps and traps flux. The usual solution is a grid of smaller openings with gaps between them.
A fine pitch part needs a small volume and a thin stencil, and the aperture is reduced relative to the pad. The area ratio is the limiting factor and it should be calculated for the smallest aperture on the board.
A pad that is used as a test point should not receive paste, and the aperture is simply omitted. That is easy to miss when the aperture layer is generated automatically from the pad layer, and it results in a solder blob on the test point.
A connector with a through hole pin needs paste for the pin as well as for the surface pad, and the volume is often larger than the pad alone would suggest.
A paste deposit that is inspected optically is easier to check when the aperture has a defined shape, so the AOI process benefits from a consistent design.
Practical Rules
Calculate the area ratio for the smallest aperture on the board, and reduce the stencil thickness if the ratio is below about 0.66.
Keep the aperture separate from the pad dimension, and apply a reduction or an enlargement where the volume demands it.
Round the corners and consider a home plate shape on a pad that is prone to bridging, since the change costs nothing.
Record the aperture design with the stencil order and the release checklist, so that the same design is reused on the next revision.
Process Control and Verification
On a design of this kind, area ratio is the item that decides how the rest of the board is arranged. 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.
FAQ
What is a good area ratio? Above about 0.66, which is the usual threshold for reliable paste release. A lower value leaves paste on the aperture wall.
Why is a thermal tab aperture divided into a grid? Because a single large opening slumps and traps flux, which forms voids. A grid of smaller openings releases better and lets the volatiles escape.
Should the aperture match the pad? Not always. The pad is an electrical feature and the aperture is a process feature, and a reduction is often used to control the volume.
