Bergquist HPL-03015 vs HT-04503: Choosing Between High Power & High Temp Dielectrics

HPL-03015 vs HT-04503 comes up constantly when engineers are specifying Bergquist Thermal Clad IMS for LED and power electronics designs. Both products share the same HT family solder rating (325°C/60 seconds UL 796), both carry UL 94 V-0 flammability, and both are compatible with AuSn eutectic die attach — a combination that immediately separates them from every LTI, MP, and CML product in the Thermal Clad lineup. At that point, a lot of engineers assume the two products are interchangeable and default to HT-04503 because it is more widely available and more commonly specified by fabricators.

That assumption causes real problems. HPL-03015 has a hard working voltage ceiling of 120 VAC / 170 VDC continuous. HT-04503 operates up to 480 VAC. On the flip side, HPL-03015 achieves 0.02 °C·in²/W thermal resistance — 2.5× lower than HT-04503’s 0.05 °C·in²/W. Those two facts together define a clear engineering decision: HPL-03015 vs HT-04503 is a choice between maximum thermal performance within a voltage-limited design envelope, or proven high-voltage capability with strong (but not best-in-class) thermal performance. This guide works through that decision with the official specification data from both Henkel/Bergquist TDS documents, application-specific worked logic, and the key design constraints that make one product the right call in each scenario.

What Separates HPL from HT: Polymer Chemistry and Dielectric Thickness

The products diverge at the dielectric formulation level, not just at the thickness level. HT-04503 uses the standard HT ceramic-polymer dielectric at 3 mil (76 µm) thickness with 2.2 W/m-K thermal conductivity. HPL-03015 uses a different polymer chemistry specifically engineered for maximum thermal conductivity — 3.0 W/m-K dielectric thermal conductivity (ASTM D5470) — at half the thickness: 1.5 mil (38 µm). The thinner dielectric at higher conductivity combines to deliver a 0.02 °C·in²/W thermal resistance that is 60% lower than HT-04503.

The higher-conductivity polymer in HPL-03015 also produces a higher glass transition temperature: 185°C versus HT-04503’s 150°C. That 35°C Tg advantage is what makes HPL-03015 the correct material for direct bare-die mounting with thermosonic gold wire bonding when the substrate temperature during bonding approaches or exceeds 150°C — at that temperature, HT-04503 is at its Tg limit while HPL-03015 still has 35°C of headroom before property transition. Both products maintain full glassy-state stiffness through AuSn solder attach processes at 280–320°C, which is what the 325°C/60s UL solder limit certifies.

The trade-off the HPL formulation makes for that thermal performance is reduced dielectric thickness, which directly limits electrical breakdown strength. HPL-03015 has a 5.0 kVAC ASTM D149 breakdown voltage and a stated continuous operating limit of 120 VAC / 170 VDC. That ceiling is not a fabrication limitation or a test anomaly — it is the physics of a 38 µm dielectric, and no surface finish, soldermask choice, or board design can change it.

HPL-03015 vs HT-04503: Full Specification Comparison Table

All values from the official Bergquist/Henkel Technical Data Sheet documents.

Parameter	HPL-03015	HT-04503	Notes
Dielectric Thickness	1.5 mil (38 µm)	3 mil (76 µm)	HPL is exactly 2× thinner
Dielectric Thermal Conductivity	3.0 W/m-K	2.2 W/m-K	HPL is 36% higher conductivity
Product Thermal Conductivity	7.5 W/m-K	4.1 W/m-K	HPL combined system performance
Thermal Resistance (ASTM D5470)	0.02 °C·in²/W	0.05 °C·in²/W	HPL is 2.5× lower
Thermal Impedance (TO-220 test)	0.30 °C/W	0.45 °C/W	HPL is 33% lower
Glass Transition Temperature	185°C	150°C	HPL is 35°C higher
UL Max Operating Temperature	140°C	140°C	Identical
UL Solder Limit	325°C / 60s	325°C / 60s	Identical
AC Breakdown Voltage (ASTM D149)	5.0 kVAC	8.5 kVAC (TDS)	HT-04503 is substantially higher
Continuous Working Voltage	120 VAC / 170 VDC	Up to 480 VAC	Critical design constraint
Dielectric Constant	6.6	7	Slightly different; same family
Dissipation Factor (1 kHz / 1 MHz)	0.003 / 0.005	0.0033 / 0.0148	HPL slightly lower at 1 MHz
Capacitance	925 pF/in² (140 pF/cm²)	540 pF/in² (85 pF/cm²)	HPL is ~1.7× higher
Volume Resistivity	10¹⁴ Ω·m	10¹⁴ Ω·m	Identical
Peel Strength	5 lb/in (0.9 N/mm)	6 lb/in (1.1 N/mm)	HT-04503 slightly higher
CTE below Tg	35 µm/m°C	25 µm/m°C	HPL is 40% higher CTE
CTE above Tg	85 µm/m°C	95 µm/m°C	HT-04503 slightly higher above Tg
Storage Modulus @ 25°C	18 GPa	16 GPa	HPL marginally stiffer
Storage Modulus @ 150°C	12 GPa	7 GPa	HPL significantly stiffer at 150°C
UL Flammability	V-0	V-0	Identical
CTI	0 / 600	0 / 600	Identical
AuSn Die Attach Compatible	✓	✓	Both rated 325°C/60s
RoHS / Lead-Free	✓	✓	Identical

The Working Voltage Ceiling: HPL-03015’s Critical Design Constraint

The single most important number in the HPL-03015 vs HT-04503 comparison is not thermal resistance. It is the 120 VAC / 170 VDC working voltage limit on HPL-03015. This is documented directly in the older HPL TDS (Bergquist company version) and referenced in the Thermal Clad Selection Guide. It is the direct consequence of a 38 µm dielectric — at that thickness, even with a 3.0 W/m-K ceramic-polymer formulation, the available electrical insulation margin above working voltage is limited.

For designs where working voltage falls within these limits, this constraint is irrelevant. LED driver output circuits operating at 12–48 VDC LED string voltage, COB die boards with 24 VDC LED bus, concentrator photovoltaic (CPV) cell assemblies at low voltage — none of these come close to 170 VDC. HPL-03015 is fully appropriate and delivers its 2.5× thermal advantage without compromise.

The constraint becomes critical the moment the circuit design involves even modest AC or DC bus voltages above 170 VDC. A 48 V to 400 VDC boost converter output stage — HT-04503 or higher. A 230 VAC LED driver primary-side board — HT-04503. Any design with a DC bus derived from rectified European mains (230 VAC × 1.414 = 325 VDC peak) — HT-04503. The HPL working voltage limit in those cases is not a conservative interpretation open to engineering judgment — it is a published material specification that cannot be overridden by design choices.

HPL-03015 vs HT-04503: The Thermal Advantage in Practice

For applications that do stay within HPL-03015’s voltage class, the 2.5× thermal resistance advantage translates into meaningful design benefits that HT-04503 cannot match.

Junction Temperature Reduction at High LED Power Density

For a COB LED module with 20 W dissipated across a 1 cm² thermal pad (20 W/cm² = 129 W/in²), the temperature rise across the dielectric alone is 2.58°C for HPL-03015 versus 6.45°C for HT-04503 — a difference of 3.87°C. That difference feeds directly into LED junction temperature. For high-brightness white LEDs, every 7–10°C reduction in junction temperature approximately doubles lumen maintenance life to L70 (the standard LED lifetime metric). A 3.87°C improvement from the dielectric alone at 20 W/cm² does not double lifetime, but it contributes meaningfully to the total thermal stack improvement that enables hitting L70 lifetime targets without overbuilding the heatsink.

At the highest power densities used in automotive headlamp COB assemblies (50 W or more on a 2–3 cm² die area), the HPL-03015 thermal advantage over HT-04503 approaches 10°C in dielectric temperature rise — at which point the junction temperature impact on LED lifetime or maximum allowable drive current becomes significant enough to affect the specification of the entire luminaire.

Wire Bonding at High Substrate Temperature

HPL-03015’s 185°C Tg provides a specific advantage for direct die assemblies that require wire bonding at elevated substrate temperature. The standard thermosonic gold wire bonding process heats the substrate to 120–150°C for bond formation. HT-04503 at Tg 150°C is precisely at its glass transition temperature during this process — mechanically transitioning from glassy to elastomeric — which reduces the acoustic coupling efficiency and can affect bond pull strength. HPL-03015 at Tg 185°C remains solidly in its glassy state through the entire wire bond process, providing superior substrate modulus and more consistent bond quality for high-power LED COB and semiconductor die assemblies.

Lower Solder Joint Fatigue Risk from the Thinner Dielectric

A less-discussed benefit of HPL-03015’s 38 µm dielectric thickness is that the thinner layer places the circuit copper more intimately in contact with the aluminum base thermally. This means more rapid and uniform heat spreading into the base metal during and after solder reflow — which actually improves solder joint quality during assembly. The faster heat removal during cooling reduces the time solder spends in a partially solidified state, improving grain structure. For high-cycle-count thermal fatigue applications, this marginally improves solder joint reliability compared to HT-04503 when working voltage permits HPL-03015.

Application-by-Application Selection Guide

Application	Recommended Product	Decision Rationale
COB LED lighting, 12–48 VDC LED bus	HPL-03015	Maximum thermal performance, well within voltage class, AuSn compatible
Automotive LED headlamp, low-voltage COB die	HPL-03015	185°C Tg for wire bonding, 140°C UL RTI for automotive, best thermal performance
Concentrator photovoltaic (CPV) cell mount	HPL-03015	Ultra-low thermal resistance critical, low working voltage
Automotive headlamp with isolated AC driver circuit	HT-04503	Driver circuit voltage exceeds HPL’s 120 VAC limit
Industrial LED driver, 230 VAC input stage	HT-04503	Mains voltage far exceeds HPL’s 170 VDC limit
DC-DC power converter, 48 V bus	HT-04503	48 VDC within either product; but converter output may exceed 170 VDC
Power LED on 24 VDC system	HPL-03015 (preferred)	Best thermal; 24 VDC comfortably within 170 VDC limit
SSR control board, 120 VAC switching	HT-04503	120 VAC is at HPL’s exact stated limit — HT-04503 provides proper margin
LED backlighting, monitor/TV, 12 VDC	HPL-03015	Specifically called out in HPL TDS; thermal performance maximized
Automotive under-hood power module, 12 V	HT-04503 preferred	Under-hood harsh environment; HT-04503’s higher peel strength and lower CTE more suitable
High-power COB projector, low-voltage	HPL-03015	Projector LED backlighting is a named HPL-03015 TDS application

HPL-03015 vs HT-04503: The CTE Difference and Its Solder Joint Impact

One difference that does not appear in a first-pass comparison but matters for reliability in thermally cycled designs: CTE below Tg. HPL-03015 has 35 µm/m°C CTE in the XY plane below Tg, while HT-04503 has 25 µm/m°C. Both are higher than alumina ceramic packages (6–8 µm/m°C) and silicon (2.6 µm/m°C), which means both products impose solder joint fatigue considerations for bare-die assemblies with CTE-mismatched components. However, HPL-03015’s 40% higher XY CTE creates proportionally larger solder joint strain during thermal cycling for the same temperature excursion.

For fixed-installation LED lighting that cycles from cold start to operating temperature but does not cycle rapidly, this is generally not a concern — the joint count is low, cycle count over product lifetime is modest (fewer than 10,000 power-on cycles over 20 years is typical), and the Bergquist-recommended 100 µm minimum solder thickness accommodates the CTE mismatch stress. For automotive applications with aggressive thermal cycling (-40°C to 125°C, thousands of cycles over vehicle lifetime), HPL-03015’s higher CTE warrants more careful solder joint fatigue modeling than HT-04503. This is an additional reason why HT-04503 dominates in automotive power modules with demanding thermal cycle specifications.

Comparison Summary: When to Choose HPL-03015 vs HT-04503

Criterion	HPL-03015 Wins	HT-04503 Wins
Lowest thermal resistance	✓ (0.02 °C·in²/W)	—
Working voltage above 170 VDC	—	✓ (up to 480 VAC)
High-Tg for wire bonding at 150°C substrate	✓ (185°C Tg)	— (at Tg limit)
Low-voltage COB LED, CPV, backlighting	✓	—
Industrial/commercial power electronics	—	✓
Automotive under-hood (voltage ≤170 VDC)	✓ (if thermal limited)	✓ (if cycling or CTE critical)
Thinner board profile	✓ (38 µm dielectric)	—
Higher peel strength	—	✓ (6 vs 5 lb/in)
Lower dielectric CTE (less joint fatigue)	—	✓ (25 vs 35 µm/m°C)
Higher breakdown voltage	—	✓ (8.5 vs 5.0 kVAC)

For Bergquist PCB designs where the working voltage allows, HPL-03015 is the correct material whenever maximum thermal extraction from the dielectric is the design objective. For everything that involves voltages above 170 VDC — which covers the majority of industrial, commercial, and mains-connected power electronics — HT-04503 is the appropriate specification.

Useful Resources for HPL-03015 vs HT-04503 Decisions

Resource	Content	Link
HPL-03015 Official TDS (Bergquist/mclpcb)	Full standalone datasheet: 3.0 W/m-K, 0.02 °C·in²/W, 185°C Tg, 5.0 kVAC breakdown	PDF
HT-04503 Official TDS (Bergquist/mclpcb)	Full standalone datasheet: 2.2 W/m-K, 0.05 °C·in²/W, 150°C Tg, 8.5 kVAC breakdown	PDF
Bergquist Thermal Clad Selection Guide (Digikey)	Master spec table with voltage guidance and thermal impedance graphs across all dielectrics	PDF
Bergquist Thermal Clad Selection Guide (tjk.com.au)	Current version Q-6019 with updated family specs	PDF
HPL-03015 TDS (reliance EMS)	Alternate hosted version with full electrical table including operating voltage limits	PDF
Henkel Electronics Portal	Technical support, SDS, current documentation for all Thermal Clad products	henkel.com/electronics
IPC-2221B	PCB design standard for creepage, clearance, and electrical isolation requirements	ipc.org

FAQs: HPL-03015 vs HT-04503

Q1: Why does HPL-03015 have a 140°C UL max operating temperature when its Tg is 185°C? Shouldn’t the higher Tg allow a higher operating temperature rating?

This is a commonly misread specification. The UL maximum operating temperature is determined by the UL 746E Relative Thermal Index (RTI) testing process — 2,000 hours of thermal endurance at the rated temperature, with electrical properties measured at intervals to confirm the material has not degraded beyond defined limits. That testing established 140°C as the RTI for HPL-03015’s polymer chemistry under the UL test conditions, independent of the Tg. HT-04503 also has 140°C UL RTI despite its lower 150°C Tg. The Tg tells you when the polymer transitions from glassy to elastomeric — not how long it survives continuous thermal exposure. HPL-03015’s 185°C Tg is important for assembly processes (it stays stiff during wire bonding at 150°C), but for long-term continuous operating temperature the relevant number is the 140°C UL RTI, which is the same for both products.

Q2: Can I use HPL-03015 for an automotive LED headlamp that uses a 48 VDC LED driver output?

Yes, and in most cases HPL-03015 is the preferred material for that application. Automotive LED headlamp COB assemblies frequently operate LED strings at 30–48 VDC forward voltage — well within HPL-03015’s 170 VDC limit. The HPL dielectric’s 0.02 °C·in²/W thermal resistance (versus HT-04503’s 0.05 °C·in²/W) directly reduces LED junction temperature for a given drive current, which is the primary performance target in headlamp design. The 185°C Tg is advantageous for any thermosonic gold wire bonding used in COB die assembly. The only scenarios where you would switch from HPL-03015 to HT-04503 in an automotive headlamp are: the driver circuit board shares the same substrate and operates at voltages above 170 VDC; the assembly requires CTE-matched substrate for aggressive thermal cycling reliability; or the fabrication process requires the higher 6 lb/in peel strength of HT-04503 for a mechanically demanding forming or heat-rail application.

Q3: Both products say they support AuSn eutectic die attach. Are there any process differences I need to account for?

The AuSn compatibility is identical between HPL-03015 and HT-04503 in terms of solder process temperatures — both are UL 796 solder rated to 325°C/60 seconds, which comfortably brackets the AuSn eutectic reflow profile (peak 295–320°C). The process differences are about substrate behavior, not solder temperature. HPL-03015’s higher storage modulus at 150°C (12 GPa versus HT-04503’s 7 GPa) means it is a stiffer substrate at die attach temperatures — which can be an advantage for uniform die seating and void-free attach, particularly for large-area die where substrate deflection during the high-temperature process affects bond layer uniformity. The thinner 38 µm dielectric also means HPL-03015 heats up and cools down more quickly than HT-04503 during a die attach cycle, which requires tighter control of ramp rates in the attach furnace profile to avoid thermal overshoot. This is not a problem, but it is a parameter that process engineers need to verify when switching from HT-04503 to HPL-03015 for an existing die attach process.

Q4: Is HPL-03015 widely available from fabricators, or is it harder to source than HT-04503?

HT-04503 is significantly more widely stocked and processed by IMS fabricators globally than HPL-03015. HT-04503 is the Bergquist workhorse product — it covers the broadest range of power electronics applications and virtually every qualified IMS fabricator stocks it. HPL-03015 is a specialty product targeting the premium LED and COB die attach segment. Its 38 µm dielectric requires more precise processing than the 76 µm HT-04503 dielectric — specifically, the thinner layer is more sensitive to surface contamination, handling damage, and press lamination parameters. Fabricators experienced with HPL-03015 typically charge a small premium over HT-04503 boards of the same layer count and copper weight, reflecting the tighter process control required. For prototyping, it is worth confirming your fabricator has specific HPL-03015 experience and can provide boards fabricated from verified Bergquist/Henkel material with a Certificate of Conformance before committing to production volumes.

Q5: My design runs at 24 VDC but sees occasional voltage spikes up to 60–80 VDC from inductive switching transients. Is HPL-03015 safe in that case?

The 170 VDC continuous rating of HPL-03015 refers to steady-state working voltage. Transient overvoltages of the magnitude you describe (60–80 VDC spikes on a 24 VDC rail) are orders of magnitude below the 5.0 kVAC (approximately 7,071 VDC peak equivalent) breakdown voltage of HPL-03015’s dielectric. They present no risk to the dielectric layer. The continuous voltage limit is about sustained dielectric stress and leakage current at elevated temperature — not peak impulse withstand voltage. Your 24 VDC system with 60–80 VDC switching transients is well within HPL-03015’s design envelope, and the material’s thermal advantage over HT-04503 applies fully. For designs with sustained DC bus voltages above 170 VDC — such as a 400 VDC rectified mains bus — the continuous voltage specification does matter and HT-04503 is required regardless of transient behavior.