L–Thermal Sensor Bonding

Lab Guide & Certification Protocol

author: U.Warring affiliation: Institute of Physics, University of Freiburg version: 0.85 last_updated: 2025-12-02 review_status: Internal laboratory documentation; not externally peer-reviewed license: CC BY-SA 4.0

1. Overview

For attaching mm-scale thermistors to copper or other substrates under 0–200 °C conditions, three adhesive classes cover many recurring scenarios (under ambient conditions):

1. Industrial thermal epoxies — for external components, housings, and non-optical mounts

2. Optics-grade, low-outgassing epoxies — for internal laser/SHG environments

3. High-temperature sensor-grade epoxies — for occasional >150 °C or structural bonding

⚠️ Laser Safety Reminder: All adhesive work inside laser cavities must only begin once the laser is powered down, locked out, and beam paths secured. Wavelength-specific eyewear is mandatory whenever the enclosure is open. See: Bonding of Optical Components in High-Power Laser Beam Paths for cavity-specific protocols.

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1.5 Safety Prerequisites & Protocols

A. Training & Authorization

• ✓ Completed institutional chemical safety training

• ✓ Cavity Work: LSO authorization + laser safety certification required

• ✓ Must review SDS for all adhesives listed in Section 2 (links provided below)

B. Personal Protective Equipment (PPE)

Standard (All Jobs):

• Nitrile gloves (6 mil+)

• Safety glasses (side shields)

• Lab coat

Cavity/Laser Work:

• Laser safety eyewear (OD 7+ @ 1064 nm / OD 5+ @ 532 nm)

Thermal Work:

• Heat-resistant gloves (for bake-out handling >60°C)

C. Ventilation & Surface Prep

Mixing: Fume hood required for Stycast/EPO-TEK (amine hardeners)

Abrasion Hazards:

• Metals: Standard ventilation

• Ceramics/Glass/FR-4: N95/P95 dust mask or respirator required. Perform in fume hood or use wet-abrasion to control hazardous dust (silica/glass fibers)

• AlN (Aluminum Nitride): Hydrolysis risk (ammonia release) if abrading aged AlN in humid air—use fume hood

D. Chemical & Thermal Response

• Skin Contact: Wash with soap + water ≥15 min. NEVER use solvents on skin

• Thermal: Cool-down time ≥30 min before touching with thermal gloves; ≥60 min for bare hands

• Waste:

  • Uncured epoxy → Chemical Waste

  • Solvents → Organic Waste

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2. Adhesive Comparison Table

Adhesive	Thermal Cond. (k)	Service Temp	Strength / Properties	Best Use
MG Chemicals 8329TCS	0.67 W/m·K	–55 to 130 °C	Lap shear ~7.7 MPa (Al)<br>Moderate rigidity	Low-waste, occasional external thermistor bonding
WEICON Easy-Mix HT 250	0.74 W/m·K	–60 to 200 °C	Shore D > 90 (Very High)<br>Structural standard	Structural, batch bonding on external parts
OMEGABOND 200	1.52 W/m·K	Up to 260 °C	High Shear (~18.6 MPa)<br>Requires Heat Cure	High-T external mounting (reserve option)
Stycast 2850FT	1.04 W/m·K	–65 to 130/155 °C	CTE matched to Copper<br>Low Outgassing (ASTM E595)	Internal SHG/Laser Cavity bonding near optics
EPO-TEK H74	1.3 W/m·K	> 200 °C	High Shear<br>Optical Grade	Alternative for vacuum/cavity work

Standards Referenced

• ASTM E595: Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment – https://store.astm.org/e0595-15r21.html

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3. Use-Case Mapping

A. Internal to Laser/SHG Cavity (Critical Zone)

Use: Stycast 2850FT (Cat 9 or 11) or EPO-TEK H74 Why: Low outgassing (ASTM E595 compliant), CTE match protects optics from stress See also: Bonding of Optical Components in High-Power Laser Beam Paths for surface preparation protocols specific to cavity environments

B. External Parts (Structural)

Use: WEICON Easy-Mix HT 250 Why: Strongest mechanical anchor, reliable to 200 °C, good shelf life

C. Occasional / Low-Waste Jobs

Use: MG Chemicals 8329TCS Why: Minimizes waste for single-sensor jobs

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4. Application Protocol

4.1 Bond Line Thickness

Target: 0.05–0.15 mm Reason: Thick epoxy layers act as a "thermal expansion jack," lifting the sensor or cracking the joint during heating.

4.2 Surface Preparation

1. Abrade: Light Scotch-Brite or fine sandpaper (320–600 grit) ⚠️ Wear mask for Ceramic/FR-4 substrates (silica dust hazard)

2. Clean: IPA or Acetone in Fume Hood See: Cleaning Components After Manufacturing for detailed solvent protocols

3. Dry: Ensure full evaporation before applying glue (≥5 min air dry)

4.3 Bake-Out (Optics-Grade Only)

Temp/Time: 80–100 °C for 4–12 hours Atmosphere: Vacuum (preferred) or flowing dry N₂ Goal: Drive off volatiles before the part enters the laser cavity

Verification: Visual inspection under magnification (20×) for micro-cracks or haze after cooldown

4.4 Strain Relief

Crucial: Always anchor the cable jacket separately from the sensor head using a cable clamp or secondary glue dot. Do not rely on the thermistor bead to hold the cable weight.

4.5 Substrate Compatibility

Substrate Type	Adhesion Quality	Special Notes
Metals (Cu, Al, SS)	Excellent	Bond Aluminium within 1h of abrasion (oxide reformation)
Ceramics (Al₂O₃, AlN)	Good (Stycast matched)	Safety: Dust mask mandatory during abrasion
Glass	Good	Low CTE mismatch with Stycast
Plastics (FR-4, G-10)	Good	Safety: Mask required for fiberglass dust
Difficult (PTFE, PP)	Poor	Require primers (avoid if possible)

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5. Dry-Run Training Protocol (Mandatory)

Every user must complete this before their first live cavity installation.

Dry Run A (Mixing): Practice weighing Stycast components (0.01g precision) and mixing without introducing bubbles

Dry Run B (Geometry): Bond a dummy slug. Verify 0.05–0.15 mm thickness and minimal fillet

Dry Run C (Bake-Out): Simulate oven loading and thermal safety (gloves/cooldown)

Dry Run D (Inspection): Use a loupe (10–20× magnification) to check cured dummy for micro-cracks or haze

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6. Recommended Lab Stock

Item	Purpose	Supplier Link
MG 8329TCS	Drawer essential (occasional use)
WEICON HT 250	Structural standard
Stycast 2850FT + Cat 9/11	Cavity standard
Precision Scale (0.01g)	Catalyst weighing
Heat-Resistant Gloves	Bake-out handling
Scotch-Brite Pads (7447)	Surface prep
Loupe (10–20×)	Bond inspection

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APPENDIX A: Certification Checklist

Trainee: __________ | Supervisor: __________ | Date: _____

PHASE A: Safety & PPE

• [ ] PPE: Glasses, Gloves, Lab Coat donned?

• [ ] Hazards: Dust mask used for Ceramic/FR-4? Fume hood used for solvents?

• [ ] Laser: (If applicable) Lockout and Eyewear protocol demonstrated?

• [ ] SDS Review: All four adhesive SDS reviewed? (Circle: Yes / No)

PHASE B: The Mix

• [ ] Stycast: Bulk container stirred? Ratio calculated by WEIGHT?

• [ ] WEICON: Nozzle purged? Color uniform?

PHASE C: Application & Process

• [ ] Prep: Surface abraded (320–600 grit) & solvent cleaned?

• [ ] Thickness: Visual check 0.05–0.15 mm?

• [ ] Thermal: Bake-out temp set correctly? Thermal gloves used?

• [ ] Inspection: Loupe check for cracks/haze?

SIGN-OFF

• [ ] Result: Dummy bond is mechanically sound, clean, and safe?

✓ PERMISSION GRANTED FOR LIVE CAVITY WORK

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APPENDIX B: Stycast 2850FT Data Card

LAB STANDARD: STYCAST 2850FT (Blue/Black)

Internal Optics / High Vacuum Only

Parameter	Catalyst 9 (General)	Catalyst 11 (High Spec)
Mix Ratio (Weight)	100 : 3.5	100 : 5.5
Cure	16h @ 25°C	Heat Only: 2h @ 100°C
Max Service Temp	130°C	155°C
ASTM E595 (TML/CVCM)	Pass (<1.0% / <0.1%)	Pass (<1.0% / <0.1%)
CTE (α)	~25 ppm/K	~25 ppm/K
Safety	Fume Hood Recommended	Fume Hood REQUIRED

⚠️ WARNING: Stir bulk resin before weighing. Filler settles!

Source: Stycast 2850FT TDS

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Safety Data Sheets (SDS)

All users must review the following before handling:

1. Stycast 2850FT (Base + Catalyst 9/11): https://api.henkeldx.com/v2/mysds/SafetyDataSheetSet?Appid=%27YPSSW_SDSUA_EXT%27&Matnr=%271188119%27&Laiso=%27EN%27&Rvlid=%27US%27&Dmskey=%27%27&X-Api-Key=Auy4a2t5e00VU2Ju93GSKHeybHFVjYLX

2. EPO-TEK H74: https://www.epotek.com/docs/en/SDS/H74%20SDS.pdf

3. WEICON Easy-Mix HT 250: https://media.weicon.de/fmds/433287/dld:inline and https://media.weicon.de/fmds/433288/dld:inline

4. MG Chemicals 8329TCS: https://www.mgchemicals.com/downloads/msds/01%20English%20Can-USA%20SDS/sds-8329tcs-part-a.pdf

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