5.2 ICT & Fixture Design

Bed-of-nails rules, probe types, keepouts, and the care/feeding that keeps fixtures fast and trustworthy.

In-circuit test (ICT) works only as well as the fixture that delivers it, and that fixture is essentially a precision handshake between the tester and your PCB. The bed-of-nails must contact every targeted pad cleanly, hold the board flat without stress, and survive hundreds of thousands of cycles with minimal upkeep. Achieving that means respecting a realistic probe grid, choosing the right tip style for each surface, and designing the PCB with accessible, mask-free test pads, clear keepouts, and solid mechanical support. Electrical performance hinges on guarded paths for sensitive measurements, Kelvin pairs for low-ohm checks, and safe power-up sequencing. A disciplined maintenance routine—cleaning probes, replacing foams, inspecting seals—keeps results stable, while fixture self-tests prevent chasing phantom board faults. When PCB DFT, fixture mechanics, and upkeep all align, ICT becomes a fast, dependable structural gate that catches assembly faults before functional testing ever starts.

5.2.1 ICT in plain words (what the fixture must do)

An ICT fixture is a repeatable handshake between the tester and your PCB. It must:

Touch the right nets (pads/vias/test points) reliably,
Hold the board flat without cracking anything,
Power up safely (when needed) and measure cleanly, and
Survive hundreds of thousands of cycles with only light TLC.

Design the board for test first (DFT), then design the fixture so operators can’t not succeed.

5.2.2 Bed-of-nails basics (grids, counts, forces)

Parameter	Practical starting point	Why it matters
Probe grid	100 mil (2.54 mm) default; 75 mil for dense boards; 50 mil only if you truly need it	Tighter grids raise cost/fragility; 100 mil keeps fixtures robust
Test pad Ø	1.0–1.2 mm on 100 mil grid · 0.7–0.9 mm on 75 mil	Gives crowns/cups a clean hit without skating
Min pad pitch	Pad center ≥ probe grid	Avoids bent pins and pad nicks
Probe force per pin	3–8 oz typical	Too little → contact noise; too much → board bow
Total force	Keep board deflection ≤ 0.5–1.0 mm	Use plenty of backup pins and a stiff pressure plate
Actuation	Vacuum for inline volume; clamshell/pneumatic for benchtop	Vacuum is fast/consistent; clamshell is flexible for odd shapes

Support, support, support: lay out a backup pin field under large copper pours, BGAs, and the thinnest laminate regions so the board doesn’t oil-can when vacuum pulls down.

Tip style	Best for	Notes
Crown (4/5-point)	Solder-masked pads, ENIG/OSP test points	General purpose; bites through light films
Spear/Conical	Via holes, small vias-as-pads	Don’t overdrive—can “drill”
Flat/Cup	Component leads, round posts, battery tabs	Gentle on plated posts; stable contact area
Chisel/Serrated	Oxidized or rough HASL	Aggressive; increases wear—use sparingly
Kelvin pair	Low-ohm or precision measurements	Two pins for force, two for sense on the same net
Long-travel / double-plunger	Stack-up tolerances, thick boards	Absorbs Z-variation; slower and costlier

Keep a probe legend in the fixture docs (part numbers, spring force, tip style, installed locations).

5.2.4 DFT keepouts & test-point rules (do this on the PCB)

Reserve a grid: mark a 100 mil access grid early; avoid placing tall parts in the nail field you’ll need later.
Test point spec: round ENIG pads preferred; open mask (no tent). Size per Section 11.2.2.
Spacing & relief: keep ≥ 1.5 mm solder mask clearance around each test pad so tips don’t skate on gloss.
Edge clearance: keep tall parts ≥ 5 mm from edges where vacuum seals ride; add tooling holes (3.0/3.2 mm) and global fiducials.
Component keepouts under pressure posts: mark no-crush zones for electrolytics, crystals, wirewounds—give the fixture plate somewhere safe to push.
High-current rails: add Kelvin pads (two close pads) for 4-wire resistance/IR drop checks.
JTAG/BSCAN header/pads: plan chain order and a small header or robust pads (see 11.1 and 3.4).

Golden rule: one net = one reachable point (more for critical nets). Don’t rely on vias buried under parts unless you spec spear tips and access height.

5.2.5 Fixture mechanics (vacuum vs clamshell)

Vacuum (inline volume)

Pros: fast, consistent force, great for conveyors; easy operator training.
Cons: needs a gasket seal and a reasonably rectangular outline; sensitive to board bow and leaks.

Clamshell / pneumatic (flexible)

Pros: loves odd shapes; top plates can “kiss” tall parts with soft foam standoffs; easy debug access.
Cons: more moving parts; alignment and parallelism matter; cycle time slightly slower.

Always include

Datum pins + tooling holes → repeatable X/Y/θ.
Pressure plate with replaceable standoffs/foams → touches only designated topside keepouts.
Pushback/strippers → keep the board from sticking to nails on release.
Interlocks → no power unless fixture is closed and vacuum/air is good.

5.2.6 Electrical design (clean measurements, safe power)

Guarding & shielding: for high-impedance or leakage tests, route guarded traces near sense nets; use shielded cables to the matrix.
Kelvin for low ohms: use 4-wire on shunts, fuses, MOSFET Rds(on), and battery paths; place pads adjacent to reduce loop area.
Power-on strategy: do power-off shorts/open checks first, then bring rails up through current-limited supplies with fast cut-off.
Loads & relays: use solid-state where you can; keep relay coils away from tiny analog nodes; snub the inductive stuff.
ESD discipline: ground the fixture frame; add wrist-strap posts; don’t turn your bed-of-nails into a static cannon.

5.2.7 Program & coverage (make the hardware earn its keep)

Netlist compare: import CAD and run learned vs expected to catch mis-mapped nails.
Structural first: opens/shorts/value/orientation → fast, nearly full coverage.
Power-on next: rail presence, inrush/steady current, oscillator alive, reset behavior.
BSCAN hooks: kick boundary-scan interconnect/pin toggles through the same fixture; program flash/MCU here to save FCT time.
Guard-banding: crisp limits on Class-A risks (polarity, shorts); looser on passives where AOI already watches cosmetics.

5.2.8 Maintenance & reliability (fixtures like small rituals)

Probe life: track hit counts; many pins last 100k–500k cycles depending on tip and surface. Replace by refdes group before they go noisy.
Cleaning cadence: dry brush + vacuum daily; IPA swab for flux film weekly; never soak springs.
Gaskets & seals: inspect monthly; leaks turn passes into flicker-fails.
Pressure plate foams: replace when shiny/packed; they stop protecting parts when they stop springing back.
Continuity self-test: a “fixture loopback” coupon proves harness/probe health before blaming boards.
Spare kits: keep a probe kit (10% spares of every tip/force), gaskets, foams, pushers, springs.

5.2.9 Common pain → fast fixes

Symptom	Likely cause	First fix
Random opens on same net	Worn/dirty tip; board bow	Replace a small zone of pins; add backup pins; clean plate
High false shorts	Flux film; too-aggressive tips	Weekly IPA swab; switch crowns → cups on those pads
Fail on first board after lunch	Humidity/cleaning drift; vacuum leak	Run self-test coupon after idle; check gasket
Cracked parts post-ICT	Pressure plate hitting no-go area	Update top stencil map; add foam islands; expand keepouts
Low-ohm measurements noisy	2-wire setup; long loops	Move to Kelvin; shorten harness; shield/guard

5.2.10 Release checklists (one for PCB, one for fixture)

PCB (DFT)

Test pads per net (size per grid) with open mask, ENIG or good OSP
Tooling holes + fiducials present; tall-part keepouts defined
Kelvin pads on high-current/precision rails; JTAG/BSCAN pads/header placed
Edge clearance for vacuum seals; pressure-post keepouts marked

Fixture

Probe map matches CAD; backup pins under weak areas
Tip styles/forces documented; spare kit stocked
Gasket seals, pushers/strippers, interlocks verified
Power-off → power-on sequence safe (current-limited)
Self-test coupon passes; maintenance cadence posted

Bottom line: give ICT clean, reachable pads and room to push; choose probe tips that fit the surface; hold the board flat and safe; and keep the fixture on a light, regular maintenance diet. Do that and ICT becomes your fastest, cheapest quality gate—quiet, repeatable, and fearless about catching the stuff you never want FCT to find.