A round-and-diamond pins arrangement locates a PCB through two holes while allowing controlled variation in the distance between them. The round pin establishes the primary in-plane location. The diamond pin provides the secondary directional constraint needed to control rotation without unnecessarily duplicating the first pin’s locating function.
How Round and Diamond Pins Locate a PCB
A round-and-diamond pins divides the in-plane locating task between two features.
The round pin works as the primary fixed datum. It restricts movement along both X and Y axes for the PCB tooling hole, locking the board’s base position.
The diamond pins acts as the secondary anti-rotation stop. Its machined flat profile releases constraint along the centerline that connects the two pins. This intentional clearance along that line lets the PCB expand or contract with temperature changes without jamming against both pins at the same time.
The term diamond pins refers to the relieved locating geometry—not a diamond cutting material.
| Fixture element | Main function | Design consideration |
|---|---|---|
| Round locating pin | Establish the primary in-plane position | Fit, hole quality, and location relative to the required datum |
| Diamond alignment pins | Control in-plane rotation through the second hole | Orientation, locating width, and clearance in the relieved direction |
| Support surfaces | Establish seating height and support the board | Contact locations, flatness, and component clearance |
| Clamps or retainers | Keep the board seated during the intended operation | Force direction, deformation, and interaction with locators |
Build the Hole-and-Pin Tolerance Stack
Separate the mounting fit from the locating fit. The pin-to-fixture interface secures the pin; the pin-to-PCB interface determines loading clearance and positional freedom. Specify and inspect these interfaces independently.
For the diamond pins, evaluate its effective locating width perpendicular to the pin-center line and its relieved profile along that line. Include orientation tolerance and use the actual pin geometry rather than treating it as a smaller round pin.
The complete analysis should cover:
- Finished PCB hole sizes, positions, and condition.
- Hole locations relative to the functional PCB features.
- Installed pin sizes, positions, and orientation.
- Seating height, engagement length, and support conditions.
- Thermal dimensional changes where the fixture enters a heated process.
Check both limiting outcomes: the tightest condition must allow the intended loading and seating, while the loosest condition must still meet the positioning requirement. Do not assume that increasing clearance solves both.
Separate Location, Support, Clamping, and Vision
Treat these as connected systems with different jobs. A locating pin establishes a mechanical reference; it does not by itself establish board flatness or final machine alignment.
| System | Main job | What to verify |
|---|---|---|
| Round and diamond locators | Establish in-plane mechanical position | Fit, orientation, accuracy, and repeatability |
| Support tooling | Resist deflection and establish working height | Support locations, height, flatness, and clearances |
| Clamps or vacuum | Retain the board on its references | Movement, deformation, and reliable release |
| Vision alignment, where used | Establish the machine’s relationship to board features | Recognition, correction range, and process result |
Review the board underside in every production configuration. Support and clamp locations must respect components, solder joints, test points, and other keepouts.
Account for Temperature and Fixture Materials
Evaluate temperature according to the fixture’s actual route. A room-temperature printing fixture and a carrier traveling through reflow have different exposure conditions.
For a preliminary uniform, in-plane expansion estimate:
Relative spacing change ≈ L × (αPCB × ΔTPCB − αfixture × ΔTfixture).
L is the initial locating baseline; α values are the applicable directional linear expansion coefficients; ΔT values are temperature changes from the common reference condition. Use consistent units. The relationship assumes free expansion, approximately constant coefficients over the range, and sufficiently uniform temperatures.
Validate the Fixture and Diagnose Binding
Inspect assembled pin locations, orientation, engagement, and support heights. Then test representative boards across the approved variation range. Record insertion and removal behavior without using the clamps to overcome interference.
Measure accuracy against the required datum or process target. Measure repeatability through repeated unloading and reloading under a defined procedure. Separate these evaluations: a fixture can return to the same wrong position consistently.
| Symptom | Possible contributors | Useful check |
|---|---|---|
| Board stops before seating | Burrs, taper interference, spacing mismatch, contamination | Inspect the contact point and compare actual geometry |
| Board rotates after loading | Excess clearance, incorrect diamond orientation, loose mounting | Measure transverse play and inspect retention |
| Board bows when clamped | Uneven support, interference, unsuitable clamp load | Compare free and clamped board condition |
| Binding appears after heating | Differential expansion, blocked relief, thermal distortion | Review hot-state geometry and release conditions |
FAQ
Can an SMT fixture use two round pins?
Yes, when the complete fit and tolerance analysis supports the arrangement. A round-and-diamond pins is a common way to avoid redundant constraint when hole spacing varies.
Does a diamond locating pin require a diamond-shaped PCB hole?
No. The usual round-and-diamond pins arrangement uses a relieved diamond pins in a round locating hole. The relief is provided by the pin geometry. Confirm the selected product’s drawing and intended application.
Are diamond pins, press pin, Carlson pin and routing pin the same?
No. Locating pin is the general term.
Diamond pins describes a relieved pin geometry for anti-binding. Press pin refers to press-fit installation style. Carlson pin is an industry name for locating pins dedicated to PCB screen printing jigs. Routing pin only means registration pins used on PCB router fixtures. One pin can carry multiple labels, for example a press-fit diamond locating pin used on a routing jig.
Can the same pin design be used for room-temperature fixtures and reflow carriers?
Thermal carriers require additional evaluation of materials, expansion, restraint, and post-cycle loading behavior. The appropriate design must be established for the actual process.
French 
English 
German 
Spanish 
Vietnamese 
Russian