← Back to courseVisual Reference

Component Identification Gallery — Aerospace CCA Repair

A visual field guide for new circuit card assembly (CCA) repair technicians. Each card shows what the part looks like on the board, its usual reference designator, what it does, how it typically fails, and how to check it with a digital multimeter (DMM). Drawings are schematic stand-ins for photographs — proportions, colors, and markings match what you will see on real hardware. Rule one on aerospace boards: verify polarity marks and pin-1 marks before you ever lift a part, and document everything per IPC-7711/7721.

1. Through-Hole Passives

yellow – violet – red – gold 4 – 7 – ×100 – ±5% = 4.7 kΩ
Carbon-Film Resistor (axial)R
Tan cylindrical body on axial leads with painted color bands — here yellow-violet-red-gold reads 4.7 kΩ ±5%. It limits current or divides voltage. Failure modes: open circuit or drift high after overheating; look for browned or cracked bodies. DMM check: resistance mode, expect the banded value within tolerance (lift one lead if in-circuit paths skew the reading).
yellow – violet – black – brown – brown 4 – 7 – 0 – ×10 – ±1% = 4.70 kΩ
Metal-Film Resistor (axial, 5-band)R
Light-blue body distinguishes precision metal-film from tan carbon-film; five bands give three significant digits (here 4.70 kΩ ±1%). Used where tolerance and low noise matter — common in aerospace analog circuits. Failure modes: open from surge, or out-of-tolerance drift. DMM check: resistance mode against the marked value; a ±1% part reading 5% off is a reject.
5W 0.1ΩJ SQP-5
Power Wirewound ResistorR
White ceramic rectangular block with the value printed on it (here 0.1 Ω 5%, 5 W) — the cement case hides a wound resistance wire. Handles high power, often as a current-sense or load resistor, and runs hot by design. Failure modes: open circuit (the wire fuses) or cracked cement after thermal cycling. DMM check: resistance mode; an open or wildly high low-ohm part is dead — use 4-wire or relative mode for sub-ohm values.
470µF 25V short lead = − long lead = +
Aluminum Electrolytic Capacitor (radial)C
Upright cylinder in a colored sleeve with a light stripe of “−” marks running down the can on the NEGATIVE side; the shorter lead is negative, and the top is scored with a vent cross. Bulk energy storage and power-rail filtering. Failure modes: dried-out electrolyte (high ESR), and the dead giveaway — a bulged or leaking vent top. DMM check: capacitance mode for value, but a normal-reading cap can still have bad ESR — use an ESR meter; in-circuit, a rail shorted to ground points here too.
104 50V
Ceramic Disc CapacitorC
Flat orange disc on two radial leads, non-polarized. The 3-digit code is first two digits × 10third digit in picofarads: “104” = 10 × 10⁴ pF = 100 nF (0.1 µF). Used for decoupling and RF bypass. Failure modes: cracks from mechanical stress causing leakage or intermittent opens. DMM check: capacitance mode out of circuit; in resistance mode it should read open (no leakage) after the charging kick.
0.1µF K 250V MKT
Film (Box) CapacitorC
Yellow or red rectangular plastic box with the value printed in plain text; non-polarized metallized polyester/polypropylene. Used for timing, filtering, and snubbers where stability matters. Failure modes: rare — gradual capacitance loss after overvoltage self-healing events, or cracked case from board flex. DMM check: capacitance mode for value; resistance mode should show open, never a short.
10µF 16V + stripe = +
Tantalum Bead Capacitor (through-hole)C
Dipped orange/yellow teardrop bead; the stripe / “+” mark indicates the POSITIVE lead — opposite of aluminum electrolytics. Stable, compact bulk capacitance on power rails. Failure mode: they fail SHORT, often violently (scorched bead, burned board) — especially if installed reversed or undervolted on rating. DMM check: resistance/diode mode across it — a low-ohm or dead-short reading means it has failed; confirm value out of circuit in capacitance mode.
101 = 100 µH
Radial (Drum) InductorL
Upright drum of visible copper wire wound on a ferrite bobbin, two radial leads; value often coded like resistors or 3-digit (“101” = 100 µH). Stores energy in switching supplies and filters. Failure modes: open winding from overcurrent, or shorted turns (hard to see — the part overheats and the supply sags). DMM check: resistance mode should show near-zero to a few ohms of continuity; open = dead. Shorted turns need an LCR meter.
Toroidal InductorL
Enameled copper wire wound around a gray ferrite or powdered-iron ring; the closed ring keeps the magnetic field contained, so these sit in EMI filters and power stages. Failure modes: broken wire at the lead exits, chipped/cracked core after impact, scorched enamel from overcurrent. DMM check: continuity end-to-end (a few ohms max); inspect winding insulation for darkening.
8.000
Quartz Crystal (HC-49 metal can)Y / X
Low, shiny seam-welded metal can on two leads, stamped with its frequency (“8.000” = 8.000 MHz). Sets the clock for processors and comms. Failure modes: dead or off-frequency after mechanical shock or a drop — a classic “board does nothing” cause. DMM check: it reads OPEN in resistance mode even when good, so a DMM only catches a shorted can; real verification is an oscilloscope or frequency counter on the oscillator pins.
ZTT 4.00MG
Ceramic ResonatorY / X
Small dipped blue or tan block, usually three leads (built-in load capacitors, center pin grounded), frequency printed on the face. Cheaper, less accurate clock source than a quartz crystal. Failure modes: cracked element after shock, frequency drift. DMM check: outer-pin-to-center should look like a capacitor (open at DC); confirm operation with a scope on the oscillator.
F 2A 250V spring clips grip the end caps
Glass Cartridge Fuse & ClipsF
Glass tube with nickel-plated end caps held in two spring clips; the fine element wire is visible through the glass, and the rating is stamped on a cap (“F” = fast, “T” = time-delay). It sacrifices itself on overcurrent. Failure mode: open element — a visibly broken wire or silvered/blackened glass means it blew hard; find out why before replacing. DMM check: continuity across it — good fuses read ~0 Ω; also check the clips for corrosion and spring tension.
X30 72V
PTC Resettable Fuse (Polyfuse)F / RT
Yellow radial disc that looks like a fat ceramic capacitor but is marked with a hold-current code (e.g. “X30” = 3 A hold). On overcurrent it heats, goes high-resistance, and recovers after power-down. Failure modes: stuck high-resistance after repeated trips, or charred from sustained fault. DMM check: resistance mode — cold and healthy it reads well under 1 Ω; ohms or more at room temperature means it is degraded.

2. Resistor Color Code Reference

4 7 ×100 ±5% 4-band: 47 × 100 = 4.7 kΩ ±5% 1st digit multiplier tolerance 1 0 0 ×100 ±1% 5-band: 100 × 100 = 10 kΩ ±1% 3 significant digits tolerance Read from the band group nearest one end; the tolerance band sits apart, nearest the other lead. Gold or silver can never be a first band — if you start on gold/silver, you are reading backwards.
Resistor Color Codereference
ColorSwatchDigitMultiplierTolerance
Black0×1
Brown1×10±1%
Red2×100±2%
Orange3×1k
Yellow4×10k
Green5×100k
Blue6×1M
Violet7×10M
Gray8
White9
Gold×0.1±5%
Silver×0.01±10%
Worked example 1 (4-band): yellow–violet–red–gold → digits 4, 7; multiplier ×100; tolerance ±5% → 4.7 kΩ ±5%.
Worked example 2 (5-band): brown–black–black–red–brown → digits 1, 0, 0; multiplier ×100; tolerance ±1% → 10 kΩ ±1%.
Mnemonic (black→white):Better Be Right Or Your Great Big Venture Goes Wrong” — Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, White.

3. Through-Hole Semiconductors

1N4007 band = cathode (K) anode (A)
Rectifier Diode (DO-41)D / CR
Black plastic cylinder on axial leads with a single silver band marking the CATHODE — the band end lines up with the bar of the diode symbol on the schematic. Conducts one way; used in rectifiers and protection. Failure modes: short (most common after surge) or open. DMM check: diode mode — ~0.5–0.7 V drop forward (red probe on anode), open in reverse; near 0 V both ways = shorted.
BZX85C5V1 (5.1 V) band = cathode
Zener Diode (glass body)D / VR
Smaller than a rectifier, with a translucent orange/red glass body that glints under light; dark band = cathode. Operated in reverse breakdown to clamp or reference a voltage (marked, e.g., 5.1 V). Failure modes: short after overstress, or shifted zener voltage. DMM check: diode mode reads like a normal diode forward (~0.7 V); verifying the actual zener voltage requires powering it through a resistor and measuring across it.
long lead = anode flat side + short lead = cathode
LED, 5 mmD / DS
Colored or water-clear dome with a molded rim — feel/look for the flat spot on the rim and the shorter lead: both mark the CATHODE. Indicator or illumination. Failure modes: open (dead), dim from degradation, or cracked dome. DMM check: diode mode usually lights it faintly and reads ~1.8–3.3 V depending on color; open both ways = dead. Never apply current without a series resistor.
2N3904 1  2 3 flat face toward you
Small-Signal Transistor (TO-92)Q
Black half-cylinder — one flat face carrying the part number — with three leads in a row. BJTs and small FETs share this package. Pinout (E-B-C vs C-B-E vs G-D-S) varies by part number — always confirm against the datasheet, never assume. Failure modes: shorted junctions or open bond wires from overstress. DMM check: diode mode — a good BJT shows two diode drops from base to emitter/collector and open elsewhere.
IRF540N tab often LIVE
TO-220 Power PackageQ / U / VR
Black plastic body with a drilled metal tab for heatsinking; holds power MOSFETs, regulators, and triacs. The tab is usually internally tied to the center pin (drain/output) — treat it as electrically live and check for insulating washers when mounted to a chassis. Failure modes: shorted die (often drain-source), cracked solder on the heavy leads, blown open. DMM check: diode mode pin-to-pin against the datasheet; a MOSFET reading short D-S unpowered is dead.
FGH40N60
TO-247 Power PackageQ
The big brother of the TO-220 — taller, wider, with a mounting hole through the plastic and heavier leads; carries high-power IGBTs, MOSFETs, and rectifiers in motor drives and converters. Many variants have no exposed metal tab (fully isolated), but verify before assuming. Failure modes: shorted die after overcurrent, cracked package, fatigued solder joints from thermal cycling. DMM check: diode mode per the datasheet pinout; check gate-to-source for unexpected leakage.
+ ~ ~ KBP206
Bridge RectifierBR / D
Square (or inline) black package with four leads and corner marks: “+”, “~”, “~”, “−” — AC in on the tildes, DC out on +/−; the + corner is often chamfered or has a dot. Converts AC to pulsating DC at the power input. Failure modes: one shorted internal diode (blows fuses, hums) or an open leg (half output). DMM check: diode mode each AC pin to + and − — four healthy ~0.5–0.7 V drops in the conducting direction, no shorts.
NE555P 12 34 56 78 dot = pin 1
DIP IC (dual in-line package)U
Black rectangular body with two rows of leads through the board; the notch (or molded dot) marks the pin-1 end, and pins number COUNTERCLOCKWISE from pin 1 when viewed from the top. Logic, op-amps, timers — often socketed on legacy aerospace cards. Failure modes: ESD damage, corroded socket contacts, bent or fatigued pins. DMM check: verify supply pins aren’t shorted to ground (resistance mode); functional faults need powered testing per the datasheet.

4. SMD Passives

472 47 × 10² = 4.7 kΩ
Chip Resistor (0603 / 0805)R
Black-topped brick with shiny tin end caps and a white code: 3-digit = two digits × 10third (“472” = 4.7 kΩ); 4-digit precision parts use three digits + multiplier (“4701” = 4.70 kΩ); “0” or “000” marks a 0 Ω jumper. Failure modes: open from overload (look for a burnt spot) or cracked from board flex. DMM check: resistance mode right on the end caps; compare with the printed code.
no marking — value not readable
MLCC (Multilayer Ceramic Chip Capacitor)C
Tan/brown body with tin ends and NO marking — you cannot read the value off the part; pull it from the BOM or schematic by reference designator. Decoupling and filtering everywhere on the board. Failure mode: cracks from board flex or depaneling stress → short or maddening intermittent, often invisible without magnification. DMM check: resistance across it — a low reading flags a cracked/shorted cap; value verification needs an LCR meter out of circuit.
100µF 16V dark sector on can top = NEGATIVE side
SMD Aluminum ElectrolyticC
Silver can sitting on a black plastic base with chamfered corners; the dark stripe/sector printed on the can top marks the NEGATIVE terminal (the base chamfer is on the positive side). Bulk filtering where height allows. Failure modes: dried out (high ESR), and on older boards leaked electrolyte corroding traces under the base — sniff for fish smell, look for stained mask. DMM check: capacitance/ESR out of circuit; inspect underneath when in doubt.
107A stripe = POSITIVE
SMD Tantalum CapacitorC
Yellow/orange molded brick; code like “107A” = 10 × 10⁷ pF = 100 µF, 10 V. The stripe marks the POSITIVE terminal — exactly opposite the electrolytic convention. Failure mode: fails SHORT, sometimes with scorching, especially after reverse installation or surge. DMM check: resistance/diode mode across it — low ohms means it has failed short; replace and investigate the rail.
100 100 = 10 µH
Chip InductorL
Looks like a chip resistor but often gray/dark with a faint winding texture; marked with an inductance code (“100” = 10 × 10⁰ = 10 µH) or unmarked on small sizes. Filters and matches RF and power circuits. Failure modes: open winding, cracked ferrite body. DMM check: continuity — should read near 0 Ω to a few ohms; open = replace. Value confirmation needs an LCR meter.
e.g. 600 Ω @ 100 MHz — usually unmarked FB
Ferrite BeadFB / L / E
Dead ringer for a chip resistor or inductor — dark monolithic body, usually unmarked; when marked, the number is its impedance at 100 MHz (e.g. 600 Ω), not resistance. It passes DC but blocks RF noise on supply and signal lines. Failure modes: cracked open from flex or overcurrent — which silently kills the rail it feeds. DMM check: continuity — a good bead reads well under 1 Ω at DC; open = the hidden break in your dead supply line.
F 2A
SMD FuseF
White or tan ceramic block marked with current rating and speed (“F 2A” = fast 2 A); some have a tiny window. Protects downstream circuitry; you cannot see the element on most. Failure mode: open after overcurrent — and an opened SMD fuse looks identical to a good one. DMM check: continuity across it is the ONLY reliable test — ~0 Ω good, open blown. As always, find the fault that blew it before fitting a new one.
020104020603 08051206 drawn to relative scale
Chip Package Sizesreference
Imperial size codes give length × width in hundredths of an inch.
Codemm (L × W)Note
02010.6 × 0.3tweezers-and-microscope territory
04021.0 × 0.5common on dense digital boards
06031.6 × 0.8everyday workhorse
08052.0 × 1.25easy hand rework
12063.2 × 1.6power/legacy designs

5. SMD Semiconductor & IC Packages

A7W 123
SOT-23Q / D / U
Tiny 3-lead package — two gull-wing leads on one side, one on the other; holds small transistors, dual diodes, or regulators. The body carries a short marking code (“A7W”), not a part number — decode it via the manufacturer’s marking tables. Failure modes: ESD damage, cracked solder on the single lead. DMM check: diode mode pin-to-pin once you know what is inside; the same code can mean different parts from different vendors.
AMS1117 tab = heatsink pad
SOT-223VR / Q / U
Flat package with three small leads on one side and one wide tab lead opposite that doubles as the heat spreader — commonly LDO regulators and medium-power transistors. The tab is usually tied to the center pin/output, so treat it as live. Failure modes: thermal shutdown/degradation from inadequate copper, shorted pass element. DMM check: with power off, no pin-to-pin shorts; powered, verify regulator output voltage at the tab.
SS14 band = cathode
SOD-123 / SMA DiodeD / CR
Flat two-terminal black brick with a white/silver band at the CATHODE end, same convention as through-hole diodes. SMA/SMB/SMC are the heavier-current siblings. Rectification, clamping, and TVS transient protection. Failure modes: short after surge (very common for TVS parts doing their job), or open. DMM check: diode mode — ~0.2–0.4 V for Schottky, ~0.6 V silicon, open reverse; 0 V both ways = shorted.
LM358 12 34 5678 dot = pin 1; numbers run counterclockwise
SOIC-8U
Small-outline IC with gull-wing leads on two sides at 1.27 mm pitch; the molded dot (or beveled edge) marks pin 1, numbering counterclockwise from above. Op-amps, EEPROMs, drivers. Failure modes: lifted or cracked gull-wing joints from thermal cycling, ESD. DMM check: continuity from each lead to its board pad catches cracked joints (probe the lead, not the solder); check V+ to GND for shorts.
PCA9555 0.65 mm (or finer) lead pitch
TSSOPU
Thin shrink small-outline package — like a SOIC but thinner with much finer 0.65/0.5 mm pitch leads; pin-1 dot and counterclockwise numbering as usual. Failure modes: solder bridges and bent leads during rework, cracked joints. DMM check: continuity lead-to-pad under magnification; after any rework, buzz adjacent pins against each other for bridges before power-up.
STM32F1 pin-1 dot
QFP (Quad Flat Pack)U
Square body with gull-wing leads on all four sides (32–208+ pins); the corner dot or chamfer marks pin 1, numbering counterclockwise. Microcontrollers and FPGAs. Failure modes: bent/coplanarity-damaged leads, bridges, cracked corner joints from board flex. DMM check: lead-to-pad continuity on suspect corners; buzz neighboring leads for bridges after rework. Inspect leads at an angle with magnification.
thermal pad bottom view — pads, no leads
QFN (Quad Flat No-lead)U
Square package with no protruding leads — flat pads under the edges plus a large exposed thermal pad in the center; pin 1 marked by a corner chamfer or dot. Joints hide under the body, making visual inspection nearly impossible — side-view fillets are tiny when present at all. Failure modes: voided/unsoldered thermal pad (overheating), open perimeter joints. DMM check: pad-to-net continuity from the tiny exposed side faces; suspect joints often need X-ray to confirm.
A1 corner bottom view — solder balls
BGA (Ball Grid Array)U
Solder-ball array entirely under the package; a corner triangle/dot marks ball A1 (rows lettered, columns numbered). You CANNOT probe or visually inspect the joints — this is X-ray territory, and reflow/reball is specialized rework. Classic failure: cracked balls from thermal cycling and vibration → intermittents that come and go with temperature or flex. DMM checks are limited to vias/test points that fan out from the part.
B55NF06 tab = drain/collector center lead cut short
DPAK / D²PAK (TO-252 / TO-263)Q / VR
Surface-mount power package: black body, two (or more) leads on one side, and a broad metal tab soldered to a large copper pour — the tab carries the drain/collector and the heat; the center lead is usually clipped off. Failure modes: shorted die, cracked tab joint causing thermal runaway. DMM check: diode mode per datasheet; verify tab-to-pour continuity — a part that runs scorching hot may have a voided tab joint.
16.000 4 corner pads: 2 crystal, 2 ground/case
SMD CrystalY / X
Low rectangular ceramic body with a seam-welded metal lid stamped with the frequency; typically four corner pads (two active, two case-ground). Clock source for MCUs and serial comms. Failure modes: fractured blank after shock/vibration — board runs dead or the UART babbles at the wrong baud. DMM check: open at DC even when good, so a DMM proves little; verify with a scope on the OSC pins or swap-test.
mark = cathode
SMD LEDD / DS
White ceramic/plastic body with a yellow phosphor window (white LEDs) or clear epoxy; the cathode is marked by a corner notch, green/black bar, or “T” symbol on the underside — conventions vary by maker, so confirm with diode mode. Failure modes: open from thermal stress, cracked from collision during handling. DMM check: diode mode lights most indicator LEDs faintly; reversed installation after rework is the classic self-inflicted fault.

6. Electromechanical & Board Hardware

2×5, 2.54 mm pitch, gold plated
Pin HeaderJ / P
Rows of square gold- or tin-plated pins in a black plastic carrier, usually 2.54 mm or 2.00 mm pitch; mates with ribbon-cable sockets or jumper shunts. Failure modes: bent/broken pins, worn plating causing intermittent contact, cracked solder where cables strain the header. DMM check: continuity pin-to-pad while gently wiggling — a flickering beep means a cracked joint.
trapezoid shell prevents reversed mating
D-Subminiature (DB-9)J / P
Trapezoidal metal shell with two staggered rows of pins (5 over 4 on a DB-9) and jackscrew holes; the D shape keys the connection. Serial data, instrumentation, legacy avionics I/O. Failure modes: bent/recessed pins, broken solder-cup wires inside the hood, loose jackscrews letting it vibrate free. DMM check: continuity each pin through to its wire or board pad; wiggle-test the harness while watching for dropouts.
master key at 12 o’clock; knurled coupling ring
Circular Military Connector (MIL-DTL-38999 style)J / P
Round plated shell with a knurled or bayonet coupling ring, keyed insert so it only mates in one clocking, and gold crimp contacts identified by letter. The aerospace workhorse. Coupling rings are torqued to spec and often safety-lockwired — never spin one off without authorization and document re-torque. Failure modes: recessed/pushed-back contacts, corrosion, damaged keyways. DMM check: pin-to-pin continuity per the wiring diagram using the correct-size probe tip (oversize probes splay female contacts).
key slot gold fingers mate with socket below
Card-Edge ConnectorJ / XA
Gold-plated “fingers” etched directly on the board edge, sliding into a slotted socket; a keying slot prevents reversed insertion. Backplane and module interconnect on rack-mounted avionics. Failure modes: worn or contaminated gold (intermittents — never eraser-scrub plated fingers harder than approved), cracked socket contacts. DMM check: continuity finger-to-circuit, and from socket contact through to the backplane while flexing gently.
G5V-2 12VDC 2A 30VDC
Relay (PCB cube)K
Sealed plastic cube with the coil voltage and contact rating printed on top; inside, a coil pulls an armature to switch isolated contacts — you can often hear the click. Failure modes: burned/pitted contacts (closes but doesn’t conduct), open coil, sticking armature. DMM check: coil resistance against datasheet (typically tens of ohms to a few kΩ); energize off-board at rated voltage and verify contact continuity — listen for the click, then measure near-0 Ω across closed contacts.
ON 1234
DIP SwitchS / SW
Red or blue DIP-outline body with tiny white sliders, “ON” and position numbers molded in. Sets configuration: addresses, modes, termination. Failure modes: oxidized contacts after years untouched (exercise them), broken actuators. DMM check: continuity across each position in both states; photograph the original settings before touching anything — wrong DIP settings masquerade as dead boards.
legs are paired — two per side internally common
Tactile PushbuttonS / SW
Small square switch with a silver metal top, round black plunger, and four legs; gives a tactile click when pressed. Resets, menu buttons, test inputs. Note the four legs are two internally-joined pairs — get the orientation right or the switch reads permanently closed. Failure modes: worn dome (no click, intermittent), flux ingress. DMM check: continuity across opposite-corner legs — open released, ~0 Ω pressed, no scratchiness.
center pin usually +, but verify the marking
Barrel Jack (DC power)J
Black plastic block with a round opening and center pin; three solder legs (center, sleeve, and a switch contact that opens when a plug inserts). Center positive is common — but always confirm against the polarity symbol. Failure modes: cracked solder joints from cable strain (the #1 “works if I hold the plug” fault), worn sleeve contact. DMM check: continuity from each leg while wiggling a mating plug; check the internal switch contact opens on insertion.
Screw Terminal BlockTB / J
Green (or blue) plastic block with screw heads on top and wire-entry cages on the side; clamps stripped field wiring to the board. Failure modes: loose screws from vibration (heat, arcing, intermittents), over-torqued screws cracking the clamp, corroded wire under the cage. DMM check: continuity from the wire itself (not the screw head) through to the board pad; a gentle tug on every wire is part of the inspection.
TO-220 clamped beneath
HeatsinkHS / MK
Black-anodized or bare aluminum extrusion with fins, clipped, screwed, or bonded over a power device, often with thermal pad or grease between. Failure modes: it doesn’t fail — its interface does: dried grease, missing insulator washer (short to chassis!), loose hardware after vibration. DMM check: resistance from heatsink to the device tab and to chassis ground — confirm it matches the design intent (isolated vs grounded) before power-up.
hex body, threaded both ends
Mounting StandoffMP / H
Brass or stainless hex pillar threaded male/female at the ends, spacing the CCA off the chassis and often providing the chassis-ground path through a plated mounting hole. Failure modes: loosening under vibration (aerospace hardware gets torque-striped or thread-locked), crushed boards from over-torque, broken ground path. DMM check: resistance from the board’s ground plane to chassis through the standoff — should be near 0 Ω where grounding is intended.
shunt bridges pins 1–2; pin 3 open
Jumper / ShuntJP
Small black (sometimes colored) plastic cap containing a metal clip that bridges two adjacent header pins — a removable configuration link for boot modes, write protect, voltage selection. Failure modes: lost shunts, oxidized clip, placed on the wrong pair. DMM check: continuity across the two bridged pins (~0 Ω); record shunt positions before removal — an undocumented jumper change can chase you for days.

7. Board-Level Features

barrel crack cross-section: copper barrel joins top and bottom copper
Via
A small plated hole whose copper barrel carries a signal between layers; on the surface it’s just a tiny tented or open dot. Cracked via barrels are a classic intermittent on thermally cycled aerospace boards — the joint opens when hot or flexed and closes again on the bench. DMM check: continuity across the via (top pad to bottom pad or along the net) while heating/cooling or gently flexing the area; an intermittent beep convicts the barrel.
solder wets both sides — fillets top and bottom
Plated Through-Hole (PTH)
A component hole with a copper-plated barrel; solder wicks the full barrel and fillets on both sides, anchoring leaded parts mechanically and electrically. Failure modes: lifted pads/barrels from bad rework (too much heat or pull), voided barrels. DMM check: continuity from the component lead to the connecting trace on each layer; inspect both-side fillets — a top-side fillet missing on a hand-soldered joint suggests poor barrel fill.
R12 exposed pad
Solder Mask & Silkscreen
The green (or other color) lacquer is solder mask — it insulates copper and confines solder to the exposed pads; the white printed legend (“R12”, outlines) is silkscreen identifying designators and orientation. Failure modes: scratched mask exposing copper (corrosion/short risk — repair per IPC-7721), mask lifting after overheating. DMM check: where mask is scratched over a trace, verify the trace still has continuity and isn’t shorted to neighbors.
bare copper dot inside a mask-free ring
Fiducial MarkFID
A bare round copper dot (typically 1 mm) inside a solder-mask keepout ring, placed at board corners and near fine-pitch parts. It is the optical alignment target for pick-and-place and AOI machines — not an electrical feature. Failure modes: none electrically, but never solder over, scratch, or probe-gouge one; a damaged fiducial can make the board unbuildable on automated rework equipment. No DMM check applies.
TP3
Test PadTP
A round or square exposed plated pad labeled “TPx” in silkscreen, deliberately placed so you can probe a net without touching component leads. Use these first — probing tiny SMD pins risks slips and bridges. Failure modes: none of their own; oxidation can give a flaky reading (light scratch with the probe tip fixes it). DMM check: this IS the DMM check — map TP numbers to nets via the schematic/test procedure and measure there.
glossy film follows component contours
Conformal Coating
A thin transparent film (acrylic, urethane, silicone, or parylene) flowed over the whole assembly — look for glossy sheen, meniscus edges at part bodies, and UV fluorescence under blacklight. Protects against humidity, contamination, and arcing. You must remove it before rework and restore it afterward per IPC-7711/7721, using the approved method for the coating type. DMM note: coating is an insulator — probes read open until you reach bare metal at a test pad or breakthrough point.
ESD-Sensitive Symbolcaution
The yellow triangle with a reaching hand crossed by a slash marks ESD-sensitive assemblies and packaging. Treat every CCA as ESD-sensitive: wrist strap on, dissipative mat, grounded iron, parts in shielded bags. Failure mode caused: latent ESD damage — the part works today and fails in flight; you will never see it on a DMM. Verification: of your workstation, not the part — check wrist-strap and mat resistance daily per your facility’s EOS/ESD program.
MSL 3 168 h floor life after bag open desiccant
Moisture Sensitivity Level (MSL)handling
Plastic-bodied ICs absorb moisture; reflow heat can then crack them from the inside (“popcorning”). The MSL label on the moisture-barrier bag sets the floor life after opening — MSL 1 is unlimited, MSL 3 gives 168 hours, higher numbers less. Out-of-time parts must be baked per J-STD-033 before soldering. Failure mode caused: internal delamination and cracked dies, invisible externally. No DMM check — control it by tracking exposure time and bake logs.

8. Polarity Quick-Reference

Polarity Marks at a Glancepin this up
ComponentPolarity markGotcha
Aluminum electrolytic capacitorStripe on the sleeve/can = NEGATIVE; short lead = negativeReversed installation vents or bursts the can under power.
Tantalum capacitorStripe / “+” = POSITIVETHE CLASSIC TRAP: the tantalum stripe means POSITIVE — the exact opposite of an electrolytic. Reverse one and it fails short, often with fire. Check twice.
DiodeBand = cathodeBand end matches the bar on the schematic symbol; backwards diode = no rectification or instant short path.
LEDFlat side & short lead = cathodeSMD LED cathode marks vary by manufacturer — confirm with DMM diode mode before soldering.
DIP / SOIC ICNotch or molded dot = pin 1; pins count counterclockwise from aboveAn IC rotated 180° often powers up reversed and dies instantly; match the notch to the silkscreen outline.
BGACorner triangle / dot = ball A1You cannot see the balls after placement — verify A1 against silkscreen before reflow, because rework is the only undo.