A stencil aperture review should compare the opening geometry, foil thickness and intended paste deposit for each critical component. A percentage reduction in the paste layer is not sufficient evidence that the stencil will print consistently. Calculate aperture area ratio, estimate theoretical volume and verify actual deposits with the selected paste and printing process before releasing production tooling.
This review method is intended for PCB designers, assembly engineers and buyers resolving a stencil engineering question. It concentrates on decisions that can be checked before a first build. Numerical examples below are hypothetical geometry comparisons, not recommended apertures for an unspecified package.
Review the package, copper, mask and paste as separate objects
Begin with the exact manufacturer part number and package drawing revision. Compare the component terminals with the released copper land pattern, solder-mask openings and paste artwork. Record dimensions in one unit system. A library footprint that has the right pin count can still carry a paste pattern copied from a different package variant.
Ask who owns stencil adjustments. The PCB designer may supply a starting paste layer while the assembler proposes changes for its process. Retain the original artwork and approve the modified tooling file as a separate revision. An email saying “reduce paste” is incomplete unless it identifies which apertures change, by how much, and against which reference geometry.
TI’s QFN and SON PCB attachment guide, SLUA271C treats perimeter terminals and the exposed pad separately. It also connects stencil geometry to printing and assembly conditions. Use the package-specific recommendation where available; a general application note does not establish a universal rule for every QFN on a mixed-component board.
Calculate aperture area ratio before changing thickness
For a straight-walled rectangular opening with length L, width W and foil thickness T, the opening area is L × W and the wall area is 2 × (L + W) × T. Therefore, area ratio is (L × W) / [2 × (L + W) × T]. All dimensions must use the same units. The result is dimensionless.
For a circular opening of diameter D, the corresponding ratio is D / (4T). These simple expressions describe ideal geometry. Rounded corners, taper and the manufactured aperture contour can change the actual areas. Request the stencil supplier’s calculation for unusual shapes instead of replacing them with an unqualified bounding rectangle.
Indium’s powder choice and stencil design application note discusses 0.66 as an area-ratio guideline and links smaller openings with powder selection and stencil characteristics. Treat a screening guideline as a reason to investigate process capability. It does not guarantee a particular transfer efficiency, nor does falling below it prove that every process must fail.
Worked comparison: the same opening with two foil thicknesses
Assume an ideal rectangular aperture measuring 0.25 mm × 0.60 mm. Compare 0.10 mm and 0.125 mm foil thicknesses. For a third case, reduce both opening dimensions by 10% while keeping the 0.10 mm foil. The values below are calculated geometry only; no paste-release measurement is assumed.
| Case | Opening, mm | Thickness, mm | Area ratio | Aperture volume, mm³ |
|---|---|---|---|---|
| A: starting geometry | 0.25 × 0.60 | 0.100 | 0.882 | 0.01500 |
| B: thicker foil | 0.25 × 0.60 | 0.125 | 0.706 | 0.01875 |
| C: both dimensions reduced 10% | 0.225 × 0.54 | 0.100 | 0.794 | 0.01215 |
Increasing thickness raises theoretical volume by 25% between A and B, while lowering area ratio. Reducing both planar dimensions by 10% reduces opening area and theoretical volume by 19%, not 10%. That distinction is important when an engineering change specifies a reduction without saying whether it applies to width, length or total area.
For case A, the aspect ratio based on the shorter opening dimension is W/T = 2.5. Aspect ratio and area ratio answer different geometric questions. Record both where the supplier’s design rules require them, and avoid comparing a measured width-to-thickness ratio against an area-ratio limit.
Separate theoretical volume from the deposit you actually print
The theoretical aperture volume is opening area multiplied by foil thickness. Transfer efficiency compares the measured printed paste volume with that aperture volume. Indium’s discussion of stencil print metrics emphasizes both transferred volume and consistency. A mean value alone can hide deposits that vary enough to affect individual joints.
As an arithmetic example, an assumed measured deposit of 0.0135 mm³ from case A gives 0.0135 / 0.0150 × 100 = 90% transfer efficiency. This is not a prediction that case A will achieve 90%. If case B transferred only 65%, its deposit would be 0.01219 mm³, less than that hypothetical case A result despite the thicker foil.
The example explains why requesting more stencil thickness may not solve insufficient paste. Identify whether the problem is aperture capacity, incomplete release, printing setup or measurement configuration. Also distinguish wet paste volume from final solder volume: the paste contains flux as well as metal, so the printed deposit is not a direct measurement of the finished joint.
Use component geometry to define the question
Build a short critical-component list rather than reviewing every resistor identically. Include the smallest openings, closely spaced terminals, large exposed pads and components needing substantially different paste volumes. For each family, state the suspected failure mechanism and the evidence that would confirm it.
For an exposed-pad package, document the aperture segmentation, total opening area and the position of vias relative to the print. “Coverage” must name its denominator: total stencil opening area divided by copper-pad area is a different quantity from post-reflow solder coverage or X-ray void percentage. Do not use one percentage as a substitute for another acceptance measurement.
For a pair of small passive-component pads, compare the two deposits as a pair as well as individually. For a connector beside fine-pitch components, check whether the common foil thickness can support both needs. A local thickness change may be worth discussing, but it also needs supplier input on transition placement and printability. It should not be inserted into the tooling drawing as an unsupported shortcut.
Plan a trial that can explain a bad result
Freeze the starting conditions before comparing stencil variants: PCB revision, paste product and lot, conditioning procedure, foil revision, board support, printing settings and inspection program. Record deviations. If several conditions change simultaneously, an improved result does not identify which change caused the improvement.
Sample normal printing and defined restart conditions. Keep the pause duration and cleaning sequence in the record. Review deposits by aperture family and board location, not only as a single board-wide average. A location-dependent issue calls for a different investigation from one that follows the smallest geometry everywhere on the panel.
| Record | Purpose |
|---|---|
| Board, package and tooling revisions | Connect results to the actual released geometry |
| Paste identification and handling conditions | Distinguish material or handling changes from tooling changes |
| Critical aperture dimensions and foil thickness | Reproduce ratio and volume calculations |
| Deposit volume, position and spread | Assess consistency by pad family and panel location |
| Print sequence and pause conditions | Identify restart or repeated-print effects |
| Post-reflow inspection findings | Connect print evidence with the assembled joint |
Close the review with an explicit release decision
Define acceptance criteria before interpreting the trial. Limits should come from the product requirements, applicable component guidance and demonstrated process capability. A supplier’s default inspection program may be a useful starting point, but its percentage bands must be checked against the intended deposit and joint.
If an aperture fails the review, assign an owner and a next action. Examples include correcting a mismatched footprint, requesting an alternative foil proposal, checking a print-inspection reference volume or repeating a trial with controlled paste handling. Keep rejected variants with their findings so the same unhelpful change is not reintroduced on a repeat order.
A complete release record identifies the approved stencil drawing, critical dimensions, material pairing, inspection approach and any unresolved exceptions. It should also state which future changes require another review. A new package source or paste product can matter even when the PCB part number stays the same.
Include stencil decisions in the assembly package
When preparing a PCB assembly build, provide the paste layers together with the component list and assembly drawing. Use the assembly file checklist to keep revisions aligned. Flag fine-pitch devices, exposed pads and any customer-controlled stencil features before tooling is ordered.
For a manufacturing review and quotation, include the board files, BOM and known print concerns. Ask for proposed stencil changes to be returned with dimensions and rationale. That gives engineering and purchasing a concrete decision to approve, and leaves a usable record for the next production run.