How do you use a PCB stencil?
Support the PCB so it cannot move, align every stencil aperture over its pad, secure the stencil as a repeatable hinge, condition the solder paste, make one controlled squeegee stroke, and lift the stencil vertically. Inspect the deposits before placing components; smear, missing paste or bridging should be corrected before reflow.
Choose framed or frameless stencil setup
| Setup | Best fit | Main control |
|---|---|---|
| Frameless prototype stencil | Small batches and bench printing | Board nest and hinge |
| Framed stencil | Repeated manual or machine printing | Tension and printer setup |
| Step stencil | Mixed paste-volume needs | Qualified aperture/thickness design |
| Rework stencil | Localized paste application | Access and registration |
Confirm stencil revision and aperture side
Match the stencil ID and revision to the PCB and paste-layer data. Confirm top/bottom side and orientation using fiducials, asymmetrical apertures or a documented reference. A mirrored stencil can appear close until fine-pitch apertures fail to align.
Build a flat, repeatable board nest
Place the PCB on a clean flat surface and surround it with boards or shims of the same thickness, or use a dedicated nest. The board must not rock or slide. Tape the stencil along one edge to create a hinge only after all apertures align.
Prepare solder paste correctly
Use the specified paste within its storage, thawing, working-life and handling limits. Mix or condition it only as the supplier instructs. Record lot, time out of storage and start time. Paste that is dry, separated, contaminated or beyond its working life should not be rescued by adding unapproved material.
Align apertures to pads under magnification
Check fine-pitch devices, small passives and thermal pads, not just large corner pads. Aperture edges should register consistently across the board. If one region aligns and another does not, stop and verify board/stencil revision, scaling, flatness and orientation.
Make a controlled squeegee stroke
- Lower the clean stencil onto the supported board.
- Place a paste bead ahead of the squeegee.
- Hold a consistent angle and pressure.
- Move across the apertures in one steady stroke.
- Confirm apertures were filled without repeated random scraping.
Exact angle, speed and pressure depend on paste, stencil, aperture and equipment. Establish a repeatable setting rather than copying one universal number.
Lift the stencil without smearing deposits
Hold the PCB stationary and raise the stencil evenly, preferably using the hinge or controlled separation mechanism. Sideways motion can drag paste off pads. Do not touch deposits while deciding whether they look acceptable.
Inspect solder paste deposits
| Finding | Likely cause | Action |
|---|---|---|
| Centered, complete deposits | Stable alignment and release | Proceed to placement |
| Smear or bridging | Board movement, underside paste or lateral lift | Clean board/stencil and reprint |
| Missing/partial deposits | Blocked aperture or poor fill | Clean, review stroke and reprint |
| Peaked/torn deposits | Poor release or paste condition | Review separation and paste |
| Consistent excess paste | Aperture/thickness/process mismatch | Engineering review |
Know when to clean and reprint
Clean the stencil underside when paste transfers outside apertures, alignment marks become obscured or deposits begin to smear. Follow the stencil and paste cleaning instructions. If a print is unacceptable, remove all paste from the PCB and restart; spot-correcting many deposits produces uncontrolled volume.
Place components before paste degrades
Move the printed board through placement and reflow within the paste supplier’s allowed exposure time. Avoid vibration, dust and accidental contact. Verify component polarity, package and centroid data before placement begins.
Record a first-print approval
- PCB and stencil revision
- Paste manufacturer, alloy, lot and working time
- Fixture/nest and print side
- Squeegee, stroke direction and repeatable settings
- Photos or inspection result for fine-pitch and thermal-pad deposits
- Observed defect and correction
- Approved first board identifier
Connect stencil results to the reflow outcome
Bridges, opens, solder balls, tombstoning and void behavior can originate from paste print, placement, pad design, component termination or reflow. Preserve the print record so failure analysis does not blame the oven by default.
PCB stencil FAQ
Do I need a stencil for one PCB?
Not always, but a stencil gives more repeatable volume for fine-pitch and multi-pad assemblies than dispensing each deposit by hand.
How thick should a PCB stencil be?
Thickness depends on aperture dimensions, paste release, component mix and required volume. It should be selected with the stencil/assembly provider, not by one universal value.
Can I use a credit card as a squeegee?
It may work for a prototype, but edge straightness, stiffness and cleanliness affect consistency. A suitable squeegee is easier to control and document.
Why does paste stick in apertures?
Paste condition, aperture aspect/area ratio, wall quality, contamination and separation method can all reduce release.
Should I make several strokes?
Use the validated process. Repeated uncontrolled strokes can change volume and force paste under the stencil; correct setup before adding strokes.
Can a stencil be reused?
Yes when it is undamaged, correctly cleaned, stored and still matches the board revision. Inspect apertures and tension before reuse.
How do I clean solder paste from a PCB?
Use the paste supplier’s approved method and remove all residue before reprinting. Do not spread contaminated paste into vias or connectors.
What causes solder bridges after reflow?
Excess or smeared paste, placement offset, aperture design, pad geometry and reflow behavior are possible causes. Review print evidence first.
What files are needed to order a stencil?
Provide the correct paste-layer Gerber or approved stencil data, PCB outline, side, revision, panel data and any aperture modifications or thickness requirements.
Request stencil and assembly review
Send PCB Gerbers, paste layers, BOM, centroid, assembly drawing and critical-package notes through the QFPCB contact page. For production support, describe stencil, placement, reflow and inspection requirements with the PCB assembly request.
