Joining Taxonomy — Family Index

Tier 3 family index for permanent and semi-permanent joining methods across metals, polymers, ceramics, and composites. Welding is covered in depth in its own Tier 3 note; this note treats brazing, soldering, adhesive bonding, and mechanical joining as peer families and provides selection heuristics across all five.

1. At a glance — the five families

Family	Mechanism	Base material melted?	Filler T threshold	Reversible?
Welding	Fusion or solid-state coalescence of base material	Yes (fusion) / no (solid-state)	n/a (base melts)	No
Brazing	Capillary filler metal wets base, base remains solid	No	Filler T_liq > 450 °C	Limited (heat to disassemble)
Soldering	Capillary filler metal wets base, base remains solid	No	Filler T_liq < 450 °C	Yes (reflow)
Adhesive bonding	Chemical / mechanical adhesion via polymer film	No	n/a	Limited (chemical strip)
Mechanical	Fastener, rivet, clinch, stake, interference fit, snap-fit	No	n/a	Yes (most), No (rivet/clinch)

Welding is covered in its own Tier 3 note: welding-processes. The 450 °C boundary between brazing and soldering is the AWS / ISO 17672 convention (filler liquidus, not solidus).

2. Brazing fundamentals

Brazing relies on capillary action to draw molten filler into a tight joint gap (typically 0.025–0.250 mm, optimum 0.05–0.12 mm for silver fillers). The base metal is heated above the filler liquidus but stays well below its own solidus. Filler wets the base via flux-cleaned oxide-free surfaces and forms a metallurgical bond — atomic diffusion at the interface gives strength approaching the base metal in shear.

Joint geometry rules:

• Lap joints preferred over butt — overlap distance 3–4× the thinner section gives joint strength equal to base material in shear.
• Joint clearance controls capillary pull. Too tight (<0.025 mm): filler cannot enter. Too loose (>0.25 mm): capillary fails, filler slumps, weak joint. AWS recommendation per filler: BAg fillers 0.05–0.13 mm; BCuP 0.025–0.13 mm; BNi 0.025–0.13 mm at brazing temperature (account for thermal expansion mismatch — a stainless-to-copper joint that is 0.10 mm at RT may be 0.05 mm at 700 °C).
• Filler placement — Pre-placed ring, paste (AMS 4777 for BNi), wire, or face-fed by hand. Furnace cycles use pre-placed paste or foil.

Flux chemistry:

• AWS FB3-A, FB3-C, FB3-D, FB3-E — Boron / borate / fluoride-based fluxes for Ag, Cu, brass, steel. White paste, active to ~870 °C.
• AWS FB4-A — Higher-T flux for stainless and tungsten carbide brazing, active to ~1100 °C.
• Nocolok (Solvay/Lucas) — Potassium fluoroaluminate eutectic for aluminum CAB; non-corrosive, no post-clean needed.
• Vacuum / H₂ atmosphere — Eliminates flux entirely; oxide reduced by atmosphere chemistry.

AWS A5.8 / ISO 17672 filler families:

• BAg (silver-bearing) — 45 %, 50 %, 56 % Ag fillers. Easy-Flo 45 (BAg-1, 45 Ag–15 Cu–16 Zn–24 Cd) historically the workhorse for HVAC and plumbing; cadmium-free BAg-7 (56 Ag–22 Cu–17 Zn–5 Sn) now dominant for food/potable. Liquidus 620–740 °C.
• BCuP (copper-phosphorus, self-fluxing on copper) — BCuP-2 through BCuP-7. Phosphorus reduces Cu₂O at the interface, so no flux needed for Cu-to-Cu joints (refrigeration, electrical, plumbing). Brittle on iron/nickel — never use on ferrous.
• BNi (nickel-based) — BNi-1 through BNi-9, Ni–Cr–B–Si–Fe. For stainless steels, Inconel, and high-T service. Almost always vacuum-furnace or H₂-atmosphere brazed; flux-brazing impractical.
• BCu (pure copper) — Furnace brazing of carbon steel in H₂ or dissociated-ammonia atmosphere. Used on automotive transmission planetary carriers and differential gears (no flux, no post-cleaning).
• BAl-Si (aluminum-silicon) — Al brazing of heat-exchanger cores. Solvay/Lucas Nocolok flux (KAlF₄/K₃AlF₆ eutectic) is the standard for CAB (controlled-atmosphere brazing) — N₂ atmosphere, ~600 °C. Used for automotive radiators, condensers, EV battery cold plates, and aerospace heat exchangers.
• BAu (gold-based) — BAu-1 through BAu-6 (Au–Cu, Au–Ni, Au–Pd). Hermetic electronic packages, surgical implants, sensor windows.
• BNi-Cr (nickel-chromium) — Service to 1000 °C in oxidizing atmospheres; turbine seal segments.

Plate-fin heat exchangers — Aluminum brazed (Linde, Chart Industries, Modine, Mahle Behr) for cryogenic ASUs and EV battery cooling; stainless diffusion-brazed (Heatric printed-circuit heat exchangers, PCHE) for high-pressure offshore and sCO₂ Brayton cycles. PCHE channel sizes 0.5–2 mm machined by photochemical etching, then stacked and diffusion-bonded at ~1000 °C and ~10 MPa for hours — joint approaches base-material strength.

Transient liquid-phase (TLP) bonding / diffusion brazing — Thin filler (BNi with B as melting-point depressant) isothermally solidifies as B diffuses into base. Used for single-crystal turbine blade tip repair (CMSX-4, René N5).

3. Brazing processes

Process	Heat source	Volume	Notes
Torch brazing	Oxy-acetylene, oxy-propane	Low	Manual, Ag/CuP fillers, plumbing and HVAC
Induction brazing	RF coil (10 kHz–450 kHz)	High	Repeatable, localized heat, automotive
Furnace brazing	Belt or batch furnace	High	H₂ reducing atmosphere (carbon steel + BCu), N₂ (CAB Al)
Vacuum brazing	Vacuum furnace 10⁻⁵ torr	Med	BNi, BAu, stainless, Inconel — no flux, no oxide
Resistance brazing	Electrodes pass current	Low	Carbide tooling onto steel shanks
Dip brazing	Molten salt bath	Med	Declining — environmental + safety
Salt-bath brazing	Chloride/fluoride salts	Med	Al heat exchangers (legacy)

Stop-off compounds — Boron nitride (white) or graphite paste applied to areas where filler must not flow (threads, mating bores). Wesgo Nicrobraz Stop-Off.

Brazing defects to inspect for:

• Erosion — Filler dissolves base; usually from over-temperature or excessive dwell with Ni or Ag fillers.
• Voids / lack of fill — Outgassing, incomplete capillary penetration, or too-tight gap.
• Cracking — Differential thermal contraction in dissimilar joints (e.g., ceramic-metal) — designed-in compliance layer required.
• Stress corrosion — Brass with Hg-contaminated environment, or stainless joints residual flux not cleaned.

4. Soldering — filler chemistries

Soldering uses filler with liquidus below 450 °C, primarily on copper, brass, and tinned steel substrates. Two distinct application worlds: electronic (PCB assembly) and plumbing (potable water and HVAC).

Electronic solders:

• Sn-Pb 60/40 — Traditional electronic, T_liq 188 °C. RoHS-banned in consumer electronics since 2006 (EU Directive 2002/95/EC); still permitted in military, aerospace, medical exemptions.
• Sn-Pb 63/37 eutectic — T_liq = T_sol = 183 °C, sharp melt, classic SnPb reflow. Legacy electronics.
• SAC305 (Sn-3.0Ag-0.5Cu) — IPC J-STD-006 / J-STD-020. Modern lead-free SMT reflow at ~217 °C peak. Industry standard since RoHS.
• SAC405 (Sn-4.0Ag-0.5Cu) — Higher Ag, marginally better creep, more expensive.
• SnCu0.7 — Low-cost lead-free wave-solder alloy; higher T (~227 °C).
• SnBiAg (e.g., Sn-57Bi-1Ag) — Low-T solder, T_liq ~ 138 °C, used for temperature-sensitive components and step-soldering of nested assemblies.
• SnIn (Sn-52In) — Ultra-low T 118 °C for heat-sensitive glass/optic bonding.

Plumbing solders (potable water):

• Sn-Cu (Sn-0.7Cu) and Sn-Ag (Sn-3.5Ag, Sn-5Ag) — Lead-free since the US Safe Drinking Water Act amendments (1986, 0.2 % Pb max in solder for potable systems; 2014 Reduction of Lead in Drinking Water Act tightened wetted-surface limit to 0.25 %).
• 50-50 Sn-Pb — Historical, banned for potable.

5. Soldering processes

• Hand iron — Weller WX/WD stations, Hakko FX-951/FM-202, Metcal MX-500 (RF-induction tip with Curie-point temperature regulation). Tip temperatures 350–400 °C for SAC305; 320–360 °C for SnPb eutectic. ESD-safe stations mandatory for sensitive ICs.
• Wave soldering — Older PCB through-hole assembly; molten solder wave contacts board underside. Vitronics Soltec, ERSA, Electrovert.
• Reflow soldering — Modern SMT. Convection ovens dominate (BTU Pyramax, Heller 1900-series, Vitronics-Soltec XPM-3, Rehm VisionX); vapor-phase reflow (Galden perfluoropolyether boiling at 200–240 °C — Solvay) for tight thermal profiles and dense boards (Asscon, IBL). Profile zones: preheat → soak → reflow → cooldown. Peak temperature for SAC305 typically 235–245 °C; time-above-liquidus (TAL) 45–90 s.
• Selective soldering — Through-hole on mixed-tech boards. Vitronics-Soltec ZEVA, ERSA VERSAFLOW, Pillarhouse Jade.
• HASL (hot-air solder level) — Board finish: PCB dipped in molten solder, then air-knife planes off excess. Lead-free HASL replaced SnPb-HASL post-RoHS.
• Alternative PCB finishes — ENIG (electroless Ni / immersion Au, IPC-4552), ENEPIG (Ni / Pd / Au, IPC-4556), immersion silver (ImAg), immersion tin (ImSn), OSP (organic solderability preservative). ENIG and ENEPIG dominate fine-pitch BGA assembly.
• Solder paste — Sn-Ag-Cu powder Type 3 (25–45 μm) or Type 4 (20–38 μm) for fine pitch, suspended in rosin-based flux (ROL0, REL0 per IPC J-STD-004). Stencil-printed; viscosity ~800–1200 kcps for screen printing.

6. Adhesives — taxonomy by chemistry

Chemistry	Typical products	Cure	Lap shear (MPa)	Use case
Cyanoacrylate (CA)	Loctite 401/406/495, 3M CA8	Moisture, sec	10–20	Fast bench fixturing, rubber, plastic
Epoxy 2K	Loctite EA-9396, Hysol 1C, 3M DP460/DP420, West System	RT or heat	25–50	Structural metal, composite
Epoxy 1K heat-cure	3M AF555-555M, AF-191 film	120–180 °C	30–45	Aerospace composite, autoclave
Polyurethane	Sika SikaFlex 252/256, 3M 5200, Henkel Teroson PU8590	Moisture	3–8 (tough)	Auto windshield, marine
Silicone	Dow Corning RTV-3140, Sylgard 184, Dowsil 1-2577	Moisture / heat	1–4	High-T, food/medical, electronic potting
Acrylic / methacrylate	Loctite AA Speedbonder, 3M 8101, Plexus MA300	Anaerobic activator	20–35	Works on oily metal, fast cure
Anaerobic	Loctite 242/243/263/271, 504/518/574, 638	Metal + O₂-free	n/a (specific)	Thread-locker, gasket, retainer
Pressure-sensitive (PSA)	3M VHB 4910/5952, Tesa ACX+	Pressure	0.5–3 (peel)	Car trim, glass, signage
Hot-melt	EVA, polyamide, Henkel Technomelt PUR	Cool	5–15	Packaging, bookbinding, auto headliner
UV-cure / LED-cure	Dymax, Loctite UV adhesives	UV 365–405 nm, sec	15–30	LCD assembly, medical, dental
Phenolic	Cytec FM-300	Heat + pressure	20–40	Aerospace structural (legacy)

Notes on key families:

• Cyanoacrylate poor gap-fill above 0.1 mm; embrittles on rubber over time; sensitive to humidity at use. Toughened CA grades (Loctite 480, 3M PR100) blend rubber particles for impact resistance.
• Epoxy 2K toughened grades (DP460 NS, DP420) tolerate peel and impact; structural mainstay.
• Polyurethane Sika 252 is the OEM windshield-bonding adhesive — full strength in hours.
• Silicone RTV cures release acetic acid (corrosive to copper, silver); neutral-cure grades (oxime, alkoxy) for electronics.
• Methacrylates (Plexus MA300) uniquely tolerate light surface oil — chosen for trailer and equipment OEMs.
• Anaerobics cure only when confined between metal surfaces (Fe, Cu, brass — slow on inert metals like stainless or aluminum without primer 7649/7471). 242 medium-strength, 271 high-strength, 638 retainer (gap 0.05–0.5 mm, slip-fit bearings on shafts).
• 3M VHB acrylic foam — 0.4–3 mm thick, viscoelastic energy absorber; bonds glass curtainwall to mullions. Achieves design strength in 72 h at RT; pre-conditioning surface with VHB Surface Cleaner 9460 or activator improves wet-out.
• Hot-melt PUR (reactive) — Henkel Technomelt PUR — applies hot like a standard hot-melt but moisture-cures into a thermoset, giving heat and chemical resistance unavailable from EVA or APAO hot-melts. Used in automotive headliner bonding and furniture edge banding.
• Pre-applied adhesives on fasteners — Loctite Vibra-Tite, nylon-patch and microencapsulated-adhesive bolt patches (3M Scotch-Grip, ND Industries Vibra-Tite) — cure on assembly, no liquid handling on the line.

7. Adhesive surface preparation

Adhesive bond strength depends more on surface preparation than on adhesive chemistry. Standard sequence: degrease → abrade → re-degrease → prime → bond within open-time.

• Solvent clean — IPA, MEK, acetone — remove oils.
• Abrade — Scotch-Brite, alumina grit-blast, or peel-ply (composites). Creates mechanical interlock and breaks weak boundary layer.
• Chemical etch (aluminum aerospace) — Forest Products Laboratory (FPL) chromate etch, or chromic-acid anodize (CAA), or phosphoric acid anodize (PAA) per Boeing BAC5555 — gives porous oxide for primer mechanical key.
• Primer — BR-127 corrosion-inhibiting epoxy primer (Cytec/Solvay), or 3M AC-130 Sol-Gel (no-tank surface treatment for field repair).
• Cure — Autoclave 120–180 °C, 6 bar typical for AF-555 / AF-191 film adhesives.
• For polyolefin (PP/PE) — Flame, plasma, corona, or chemical etch raises surface energy from ~30 mN/m to >40 mN/m. Without treatment, no adhesive bonds reliably. Atmospheric plasma (Plasmatreat Openair) widely used inline; corona for film web; chemical adhesion promoters (Loctite SF 770, 3M AP-115) for hand application.
• For composite (CFRP/GFRP) — Peel-ply (nylon or polyester woven sheet co-cured on bonding surface, then removed just before bonding) leaves a textured, contamination-free surface. Avoid release-coated peel-plies (silicone- or fluoro-contaminated) — they leave residue that prevents bonding. Mechanical abrasion or plasma after peel-ply removal is best practice.
• Wettability check — Water break test (ASTM F22): water sheets uniformly on a clean high-energy surface; beads up on a contaminated low-energy one. Dyne-pens / dyne-inks (ASTM D2578) quantify surface energy.

Common adhesive failures and mitigations:

• Adhesive failure (clean release at the substrate interface) — surface prep inadequate; abrade more aggressively, change primer, or check for surface contamination.
• Cohesive failure (failure within the adhesive layer) — adhesive at its limit; specify higher-strength chemistry or thicker bondline.
• Substrate failure (e.g., composite fiber tear-out) — joint is stronger than substrate; design is sound but a thicker / different substrate is needed.
• Creep / cold flow — Polymer adhesives creep under sustained load. Anaerobics, epoxies are more creep-resistant than silicones or PSAs.
• Environmental aging — Moisture, UV, thermal cycling degrade interface. Boeing wedge test (ASTM D3762) is the accelerated screening.
• Galvanic isolation — Adhesive layer can also serve as a galvanic isolator between dissimilar metals (Al-CFRP), but only if it remains continuous; mechanical fastener through the bondline can short the isolation.

8. Mechanical joining — no permanent base change

Mechanical joints preserve the base material chemistry (no heat-affected zone, no metallurgical bond). Categories:

• Threaded fasteners — Bolts, screws, studs, nuts. See fasteners-taxonomy for grades, drives, and torque/preload theory.
• Solid rivets — Hot-driven (boiler, ship hull — legacy) or cold-driven (aircraft skin, Al 2117-T4 AN470). Driven head deforms in upset cavity (bucking bar).
• Blind rivets (pop rivets) — Stem-and-sleeve, pulled with hand tool. Avdel, POP/Stanley, Huck.
• Hi-Lok / Hi-Lite — Threaded pin with frangible torque-off collar. Standard aerospace permanent fastener.
• Huck Lockbolt — Swaged collar onto grooved pin; high preload, vibration-immune. Truck chassis, rail car, structural steel.
• Self-piercing rivet (SPR) — Henrob (now Atlas Copco/Stanley Engineered Fastening) semi-tubular rivet pierces top sheet and flares into a die, locking bottom sheet without piercing it. Pioneered in volume on the 2003 Jaguar XJ aluminum body; now standard in BIW for aluminum-aluminum and aluminum-steel joints (Audi A8, Ford F-150, Tesla Model S/X). 2–5 mm total stack.
• Clinching — Tox Pressotechnik, BTM (Tog-L-Loc). Two sheets pressed into a die, forming a button-shaped mechanical interlock — no consumable. Lower joint strength than SPR but no rivet cost and no piercing chips. Common on HVAC ductwork, appliances, auto seat frames.
• Staking — Localized plastic deformation of a stud, boss, or shaft end to retain a mating part. Roller-staking, orbital-staking (BalTec), impact-staking. Common for retaining sheet-metal shafts in housings and for plastic boss retention.
• Crimping — Cold deformation of a connector barrel around a stranded conductor. Electrical: AMP/TE, Molex, JST tooling; gas-tight crimp per IPC/WHMA-A-620. Hydraulic crimp on large-gauge battery cable lugs; pneumatic crimp on aerospace contacts (M22520 / Daniels Manufacturing).
• Toggle latches and Dzus fasteners — Quick-release service panels (aerospace, motorsport). Reusable, no tools.

9. Friction joining (non-melt)

Solid-state joining via friction heating below the melting point. Friction-stir welding (FSW) and friction-stir spot welding (FSSW) are covered as welding processes in welding-processes — they belong to welding by convention but use no melt. FSSW pioneered on the Mazda RX-8 (2003) for aluminum roof-to-body joints. Friction-bit joining (FBJ) drives a coated steel bit through aluminum into steel and friction-welds it to the steel — joins dissimilar Al-steel.

Inertia and direct-drive friction welding (Manufacturing Technology Inc., Thompson Friction Welding) — Two parts spun against each other under axial load; flash forged out, atomic bond at interface. Used for aerospace shaft-to-flange (turbocharger compressor wheels), drill pipe tool joints, and bimetallic engine valves (martensitic stem + austenitic head).

Magnetic-pulse welding (MPW) — Bmax, PSTproducts — high-current pulse drives one tube into another at >300 m/s, forming a jetting / cold-weld bond. Used for aluminum-copper electrical busbar joints in EV battery packs (no IMC layer, low resistance).

10. Interference / press-fit / shrink-fit

ISO 286-2 fit classes describe shaft (lowercase) and hole (uppercase) tolerance combinations.

Fit class	Type	Interference (μm, Ø50 mm shaft)	Use
H7/k6	Transition	−15 to +18	Light press, occasional disassembly
H7/n6	Light interference	−2 to +33	Locating fit
H7/p6	Press fit	+5 to +42	Press-on gears, bearings
H7/r6	Medium drive	+20 to +59	Heavy drive
H7/s6	Shrink fit	+35 to +75	Permanent assembly
H7/u6	Heavy shrink	+60 to +106	Heavy machinery

Assembly methods:

• Press fit — Hydraulic press at room temperature. Limit by yield of hub.
• Shrink fit — Heat hub to expand bore (induction, oven 200–300 °C) — common for gears, bearings, locomotive wheels. Or cool shaft in dry ice (−78 °C) or liquid nitrogen (−196 °C).
• Hydraulic dilation — Pressurize oil between hub and shaft via oil port (SKF oil-injection method); reduces friction during press-on.

Roll-pins and coiled pins (Spirol) press into a slightly undersized hole; spring-back grips the hole wall. Driven with a punch. Coiled pins (Spirol coiled) distribute hoop stress more evenly than slotted spring pins and tolerate looser hole tolerance.

Torque-to-yield (TTY) bolts — Stretch fastener into plastic regime for highest preload uniformity (cylinder head bolts, common rail injector hold-downs). Single-use; replace at every disassembly.

Disassembly methods for interference fits:

• Bearing puller (mechanical) — two-jaw or three-jaw, often used with heat on outer race.
• Hydraulic puller (Enerpac, Posi Lock) — multi-tonne for large gears, couplings.
• Induction heater for the hub (Bessey, SKF TIH) — quick localized heating without affecting shaft material.

Design rules for press / shrink fits:

• Lamé equations govern hoop stress in interference fits. For a solid shaft pressed into a hub: σ_hoop_hub = p · (D_hub² + D_shaft²) / (D_hub² − D_shaft²), where p is the contact pressure from interference. Keep hub hoop stress below 50–60 % of yield.
• Stress concentration at hub edges — Round or chamfer hub bore mouth to avoid edge spalling and shaft fretting.
• Press-on force — F = π · μ · D · L · p, where μ ≈ 0.10–0.15 for clean dry steel, 0.05–0.10 lubricated. Heated assembly reduces required force to zero (slip fit, then contracts).
• Torque capacity of interference fit — T = F · D / (2 · μ). Adding a key or pin in addition to interference doubles or triples reliable torque transmission.

11. Snap-fit (plastic)

Plastic snap-fits are the dominant joining method for injection-molded enclosures (consumer electronics, automotive interior, toys). Three geometry families:

• Cantilever beam snap — Most common. Hook on the end of a cantilever beam deflects during assembly, returns to undercut.
• Annular snap — Cylindrical undercut around a circular boss (e.g., pen cap, marker cap).
• Torsional snap — Lever rotated about a torsion bar; user squeezes to release.

Design rule: Maximum strain ε_max = (1.5 · h · y) / L² for a tapered cantilever, where h is beam thickness at root, y is deflection, L is beam length. Stay below the material’s allowable strain (ε_allow ≈ 50 % of ε_yield for one-time assembly; ≈ 30 % for multiple cycles). Reference: Bayer / Covestro Snap-Fit Design Manual (1998, still the canonical design guide); also DSM, BASF, and SABIC have equivalent application notes.

Designing for repeated assembly cycles — A snap-fit rated for 1 cycle may fail at cycle 50 due to creep, stress relaxation, or fatigue crack at the root. For consumer products with hundreds of opening cycles (e.g., battery covers), keep maximum strain below 30 % of yield strain and add a generous root fillet (R ≥ 0.5 × beam thickness).

12. Heat-stake and ultrasonic-stake; ultrasonic welding

• Heat-stake — Plastic boss extends through a sheet-metal or PCB hole, then a heated tool (200–300 °C) melts and forms the boss into a rivet head retaining the part. Used heavily for PCB-to-housing and nameplate retention.
• Ultrasonic-stake — Same geometry, but boss melted by ultrasonic vibration (20–40 kHz). Faster, lower thermal stress.
• Ultrasonic welding (thermoplastic) — Branson (Emerson), Telsonic, Herrmann Ultrasonics, Dukane, Sonics & Materials. Two thermoplastic parts vibrated at 20/30/40 kHz under axial pressure; energy directors (small triangular ridges on the joint face) concentrate friction heat; weld in 0.2–2 s. Standard for automotive instrument-cluster housings, medical-device housings, and HVAC components. Compatible plastics: amorphous (PC, ABS, PS, PMMA — best) and some semi-crystalline (PA, PET, POM — harder, need higher amplitude). Dissimilar plastics generally do not ultrasonically weld; transmission of energy depends on stiff modulus path from horn to joint.
• Hot-plate welding — Two thermoplastic parts pressed against a heated platen, then mated. Slow (10–30 s) but tolerant of large parts and dissimilar/semi-crystalline plastics. Automotive intake manifolds (PA-GF), bumpers, water bottles.
• Spin welding — Cylindrical thermoplastic joints, one part spun against the other under axial load. Cooling lines, fuel tanks (HDPE), aerosol containers.
• Vibration (linear friction) welding — Linear oscillation at 100–240 Hz, larger amplitude than ultrasonic, joins large parts (intake manifolds, instrument-panel substrates). Branson, Bielomatik.
• Laser plastic welding — Transmission laser through a clear top part absorbed by a carbon-black-loaded bottom part; precise, no flash, no vibration. LPKF, Leister, Branson. Used for medical disposables and clean automotive sensors.

12a. Industry-specific case studies

• Apple iPhone enclosure (2010–present) — Mix of pressure-sensitive adhesive (display to chassis), screws (T2, T5 pentalobe), and clips. Service requires heat to soften the perimeter adhesive (iOpener pads ~50 °C) and suction. Modern models add screw retention on key components for FFR (right-to-repair) compliance.
• Boeing 787 fuselage — CFRP barrel sections joined circumferentially with titanium Hi-Lok fasteners through co-cured CFRP layups. Skin-to-stringer bonding uses Cytec FM-300 / 3M AF-555 film adhesive co-cured in autoclave.
• Ford F-150 (2015–present) — Aluminum body. ~2000 SPR joints + ~120 m of structural adhesive (Henkel Terokal 5089) per vehicle. SPR pioneered for production in Jaguar XJ; F-150 brought it to high-volume mass-market trucks.
• Tesla Model Y Giga Casting (2020–present) — Large Al die-castings reduce joint count; remaining joints are SPR + adhesive for body, and TIG/laser-welded busbars and battery interconnects.
• EV battery pack — Cell-to-busbar: laser welding (Trumpet, IPG), ultrasonic welding (Branson, Sonics), wire-bonding (Hesse Mechatronics). Module assembly: structural adhesive (3M / Sika) + mechanical fasteners. Cooling plates: vacuum-brazed aluminum.
• Aerospace gas turbine — Single-crystal blade root attached by fir-tree mechanical fit; integral disk-blade BLISK by linear friction welding; nozzle guide vane segments by vacuum brazing (BNi-2, BNi-7).
• Refrigeration tube assembly — Copper-to-copper joints by BCuP-2 / BCuP-5 brazing (no flux). Copper-to-brass or copper-to-steel by BAg with flux.
• Smartwatch / wearable — Plastic housing ultrasonically welded; rear glass to titanium ring by structural acrylic adhesive (Henkel Loctite 3525, 3M LSE adhesive) or hot-melt PUR.

13. Selection heuristics by use case

Use case	Recommended joining method
Permanent metal joint, sheet metal (≤3 mm)	Spot welding (RSW) or clinching (Tox) or SPR
Permanent dissimilar Al-steel BIW	SPR + structural adhesive (Henkel Terokal 5089, Dow Betamate)
Permanent metal joint, high-strength structural	GTAW, EBW, laser welding
High-strength + heat-sensitive substrate	2K epoxy (Loctite EA-9396, 3M DP460) or methacrylate
Field-serviceable	Bolted joint, optionally with Heli-Coil or Time-Sert insert
Vibration + thread-loosening	Nord-Lock wedge-lock washer + Loctite 243 thread-locker
Sealing fluid + repairable	Bolted joint with gasket (spiral-wound, PTFE) or O-ring face seal
Hermetic seal (electronic, vacuum)	Vacuum brazing (BAu, BNi) or EBW or laser weld
Plumbing copper tube	BCuP-2 (no flux) or lead-free Sn-Cu solder
HVAC stainless tube	BAg-7 with brazing flux (corrosive — must post-clean)
Auto windshield to body	Polyurethane (Sika 252)
Auto BIW mixed Al + steel	SPR + structural adhesive (Henkel Terokal 5089)
Aerospace composite skin to honeycomb	3M AF-191 / AF-555 film adhesive, autoclave 121–177 °C
Plastic housing + boss	Ultrasonic weld or heat-stake
Electronics SMT assembly	SAC305 reflow per IPC J-STD-020
Press-fit bearing on shaft	H7/s6 interference, optional Loctite 638 retainer
Trailer / equipment with surface oil	Methacrylate (Plexus MA300)
Glass-to-metal hermetic	Glass-to-Kovar matched seal, or solder seal

Hybrid joining (weld-bond, rivet-bond) is increasingly common in BIW. Adhesive provides stiffness and durability (NVH, fatigue), spot welds or SPR provide handling strength before cure and peel resistance. Henkel Terokal 5089 and Dow Betamate 1496V are the volume crash-durable epoxies used by VW, BMW, GM, and Ford on aluminum-intensive vehicles.

14. Inspection and NDT

• Adhesive bonds — Tap test (sonic) for disbonds, ultrasonic C-scan for void mapping, infrared thermography (flash thermography), pull-tab coupons cured alongside the part. ASTM D1002 (single-lap shear), D3163 (rigid plastic lap), D5868 (composite lap shear), D3433 (cleavage), D3762 (wedge crack — Boeing wedge test for durability).
• Brazed joints — Radiography (X-ray) for filler distribution and voids, dye penetrant (PT) for surface cracks, helium leak test for hermetic.
• Mechanical joints — Torque audit (in-process and final), bolt preload via ultrasonic stretch measurement (Strainsert, Norbar), push-out / pull-out test for interference fits and rivets.
• Welded joints — See welding-processes for radiography, UT, PT, MT.
• Soldered joints (electronic) — AOI (automated optical inspection, Koh Young, Omron, MIRTEC), X-ray inspection for BGA / QFN solder balls (Nordson Dage, ScienScope), in-circuit test (ICT, Teradyne), flying-probe test. IPC-A-610 is the canonical workmanship standard (Class 1 consumer, Class 2 industrial, Class 3 high-reliability/aerospace).
• Soldered joints (plumbing) — Pressure test, visual inspection for full filler distribution around the bell of the joint.

Bond strength characterization is destructive: lap shear coupons (ASTM D1002, D3163, D5868), peel (ASTM D1876 T-peel, D903 180° peel), wedge crack durability (ASTM D3762 — measures crack growth in a wet/humid environment, the most predictive aging test for aerospace bonds), and fracture mechanics (Mode I G_Ic per DCB ASTM D5528, Mode II per ENF).

14a. Lifecycle, repair, and recyclability

Joining choice influences end-of-life and serviceability:

• Welded structures — Cuttable (plasma, oxyfuel, abrasive saw) but not non-destructively separable. Recyclable as base material if single-alloy.
• Brazed joints — Reheatable above filler liquidus to disassemble. Filler contaminates base scrap (Ag, Cu, Ni in steel stream — low concentration usually acceptable).
• Soldered joints — Reflowable; SMT rework stations (PDR, Air-Vac, Metcal APR) reflow individual components without disturbing neighbors. Hand desoldering with braid or pump.
• Adhesive joints — Disassemble by heat (epoxies soften above Tg, ~120–180 °C), chemical strip, or controlled fracture (Henkel debondable adhesives release on induction heating). Sustainability concern: bonded multi-material assemblies (Al-CFRP) resist separation for recycling.
• Mechanical joints — Mostly serviceable. Bolted joints fully reversible; SPR and rivets must be drilled out; clinch joints can be punched out but leave damaged sheet.

For circular-economy design (right-to-repair, EU ESPR — Ecodesign for Sustainable Products Regulation), preference is shifting toward reversible mechanical and thermally debondable adhesives, particularly in consumer electronics and EV battery packs where module-level repair matters.

15. Cross-references

• joining-welding — parent Tier 2 note for joining technologies
• welding-processes — welding family in depth
• fasteners-taxonomy — threaded fasteners detail
• seals-taxonomy — gaskets, O-rings, mechanical seals
• polymers-taxonomy — adhesive base chemistries
• composites-taxonomy — composite-bonded joints, film adhesives

15a. Standards and codes — quick map

Domain	Standard	Scope
Brazing filler	AWS A5.8 / ISO 17672	Filler classification and composition
Brazing procedure	AWS B2.2 / ISO 13585	Procedure and operator qualification
Brazing flux	AWS A5.31 / ISO 9454	Flux classification
Soldering electronic	IPC J-STD-001 / J-STD-006 / J-STD-020	Process, alloy, moisture/reflow
Soldering electronic workmanship	IPC-A-610	Acceptability of electronic assemblies
PCB design	IPC-2221 / IPC-2222	Generic and rigid board design
Adhesive structural	ASTM D1002 / D3163 / D5868	Lap shear test methods
Adhesive durability	ASTM D3762	Boeing wedge test
Mechanical fastener	ISO 898-1 / SAE J429 / ASTM A574	Bolt grades
Interference fit	ISO 286-2 / ANSI B4.1	Hole / shaft tolerance classes
Aerospace bonded structure	MIL-HDBK-17 / CMH-17	Composite handbook (joining sections)
RoHS lead-free	EU Directive 2011/65/EU	Restriction of Pb, Hg, Cd, Cr⁶⁺, PBB, PBDE
Lead-free plumbing	US SDWA / NSF/ANSI 372	<0.25 % Pb on wetted surface

16. Citations

• AWS, Brazing Handbook, 5th ed., 2007.
• AWS A5.8 / ISO 17672 — Filler metals for brazing.
• ASTM D1002 — Single-lap shear of adhesively bonded metal.
• ASTM D3163 — Lap shear of rigid plastic.
• ASTM D5868 — Lap shear of fiber-reinforced plastic.
• ASTM D3433 — Cleavage of adhesive in bonded metal joints.
• ASTM D3762 — Wedge crack durability (Boeing wedge test).
• IPC J-STD-006 — Requirements for electronic-grade solder alloy.
• IPC J-STD-020 — Moisture / reflow sensitivity classification.
• ISO 286-2 — Limits and fits.
• 3M Adhesive Selection Guide (current edition).
• Henkel Loctite Anaerobic Adhesive Selection Guide.
• Bayer / Covestro, Snap-Fit Design Manual, 1998.
• E. M. Petrie, Handbook of Adhesives and Sealants, 2nd ed., McGraw-Hill, 2007.
• Boeing BAC5555 — Phosphoric acid anodize for adhesive bonding.
• US Safe Drinking Water Act (1986 amendments; 2014 Reduction of Lead in Drinking Water Act).
• AWS D17.1 — Fusion welding for aerospace applications.
• ISO 18278 — Resistance welding — Weldability.
• Atlas Copco / Henrob — Self-Pierce Riveting Technical Guide.
• ISO 14373 — Resistance spot welding of low-carbon steel.
• IPC/WHMA-A-620 — Requirements and acceptance for cable and wire harness assemblies (crimp specifically).
• Solvay technical notes on Nocolok flux for CAB aluminum brazing.
• Branson / Emerson — Ultrasonic Plastics Assembly Design Manual.
• SAE AS5202 — Threaded fastener torque guidelines for aerospace.

17. Glossary

• Capillary action — Liquid metal drawn into a narrow gap by surface tension; the fundamental mechanism of brazing and soldering.
• Eutectic — Mixture of two metals with a single sharp melting point lower than either pure constituent (e.g., Sn-Pb 63/37 at 183 °C).
• Filler metal liquidus / solidus — Upper and lower temperature bounds of the two-phase mushy zone.
• Wetting — Spreading of molten filler across a clean surface; quantified by contact angle (<90° wets; <30° excellent for brazing).
• Intermetallic compound (IMC) — Brittle phase that forms at the solder/copper interface (Cu₆Sn₅, Cu₃Sn). Thin IMC is necessary for a bond; thick IMC weakens it.
• HAZ (heat-affected zone) — Region of base material altered by joining heat (welding, brazing). Adhesive joints have no HAZ — a structural advantage for heat-sensitive materials.
• Cohesive vs adhesive failure — Within the bondline (cohesive) versus at the interface (adhesive); cohesive failure indicates that surface prep is good and adhesive is at its limit.
• Preload — Axial tension induced in a bolt at assembly; controls clamp force, fatigue life, and joint stiffness.
• TLP (transient liquid phase) — Brazing variant where filler isothermally solidifies as melting-point depressant diffuses into base; joint approaches base-material composition.
• Energy director — Triangular ridge molded into a thermoplastic part to concentrate ultrasonic-weld energy at the joint line.
• CAB (controlled-atmosphere brazing) — Aluminum brazing in N₂ atmosphere with Nocolok flux, the standard for high-volume Al heat exchangers.
• Galvanic isolation — Use of a non-conductive adhesive or gasket to prevent galvanic corrosion between dissimilar metals in contact with a common electrolyte.