Best PCB Materials for High Power LED Lighting: Bergquist Thermal Clad Guide

Selecting the right PCB material for LED lighting is the single engineering decision with the greatest impact on LED junction temperature, lumen output, colour consistency, and service life — more than driver efficiency, more than heatsink design, more than reflector optics. Every 10°C rise in LED junction temperature approximately halves lumen maintenance lifetime. A substrate choice that adds 8°C to junction temperature does not improve over the life of the product. It compounds.

This guide focuses on Bergquist Thermal Clad IMS materials — the most widely specified name-brand PCB material for LED lighting in power applications — and covers which specific dielectric to choose for each LED application type, why the thermal resistance numbers in the datasheet translate directly into LED lifetime, what design decisions around copper weight and surface finish compound the substrate material choice, and where generic aluminium MCPCB falls short of Bergquist performance specifications. The approach is engineering-first: starting from the LED junction temperature requirement and working backward through the thermal stack to the substrate specification.

Why LED Junction Temperature Is the Central Design Variable

LEDs do not fail suddenly in the way that electrolytic capacitors or switching transistors do. They degrade gradually through lumen depreciation. The industry standard metric is L70 — the time to 70% of initial lumen output — which correlates directly and predictably with junction temperature through the Arrhenius relationship.

At 55°C junction temperature, a quality high-brightness LED rated for 50,000 hours to L70 will comfortably meet that figure. At 85°C junction temperature, the same device may reach L70 in 25,000 hours. At 105°C junction temperature, lifetime to L70 may be 10,000–12,000 hours. These are not hypothetical numbers — LED manufacturers publish LM-80 lumen maintenance data across multiple junction temperatures, and the TM-21 extrapolation standard gives engineers the ability to project L70 lifetime from that data.

The PCB substrate determines what junction temperature a given design achieves for a given input power. The thermal path from LED junction to ambient passes through: the LED package thermal resistance (fixed by the LED supplier), the solder joint, the PCB dielectric, the PCB base metal, the TIM, and the heatsink. For packaged LEDs, the dielectric thermal resistance is frequently the largest controllable variable in that chain — the one parameter the board designer directly determines.

FR-4 at 0.25–0.3 W/m-K thermal conductivity creates a through-plane thermal resistance that dominates the total thermal stack for any LED dissipating more than 1–2 W. Standard generic aluminium MCPCB at 1.0–1.5 W/m-K dielectric conductivity is better, but still 1.5× to 2× higher thermal resistance at the dielectric than the best Bergquist products. At a 10 W LED on a 0.5 cm² thermal pad (20 W/cm²), the difference between a 1.0 W/m-K generic dielectric and Bergquist HT-04503 at 2.2 W/m-K is approximately 4°C of additional junction temperature — enough to reduce L70 lifetime by 15–20% in a 50,000-hour design.

The Bergquist Thermal Clad LED Lighting Dielectric Family

The Bergquist Thermal Clad range includes four dielectrics relevant to LED lighting, each targeting a specific application and power density tier. Understanding where each product sits in the performance hierarchy is the starting point for PCB material for LED lighting specification.

HPL-03015: The LED-Specific Flagship

HPL-03015 is the only Bergquist dielectric engineered specifically for LED applications. The “HPL” designation stands for High Power Lighting, and the product was developed to address the needs of COB LED packaging and high-power LED assemblies that demand the lowest achievable thermal resistance in an IMS substrate.

At 1.5 mil (38 µm) dielectric thickness and 3.0 W/m-K dielectric thermal conductivity, HPL-03015 achieves 0.02 °C·in²/W thermal resistance — the best performance in the entire Bergquist Thermal Clad family. The product thermal conductivity of 7.5 W/m-K (combining the dielectric and copper circuit layer contribution) and thermal impedance of 0.30 °C/W (TO-220 test) represent the IMS substrate performance ceiling for standard LED lighting designs.

The 185°C glass transition temperature — the highest Tg in the Bergquist family — is what makes HPL-03015 specifically appropriate for COB die attach processes. At 150°C substrate temperature during AuSn eutectic die attach (280–320°C peak, 325°C/60s UL rated) or thermosonic gold wire bonding (120–150°C substrate during bonding), HPL-03015 remains fully in its glassy state and mechanically rigid. No other standard Thermal Clad product provides that combination of ultra-low thermal resistance and 185°C Tg.

The working voltage limit of 120 VAC / 170 VDC is the one design constraint engineers must account for. HPL-03015 is suitable for LED string voltage applications (12–48 VDC is the typical LED bus), COB die assemblies, and any LED application where the circuit voltage stays within that envelope.

HT-04503: The High-Temperature Standard

HT-04503 is the most widely specified Bergquist dielectric globally. At 3 mil (76 µm) and 2.2 W/m-K dielectric conductivity, it achieves 0.05 °C·in²/W thermal resistance — 2.5× higher than HPL-03015, but still approximately 8× lower than standard FR-4 at the same thickness. The 150°C Tg and 140°C UL RTI make it the correct choice for automotive LED applications (under-hood to 125°C ambient) and high-ambient industrial LED systems. The 325°C/60s solder limit supports AuSn die attach and thermosonic gold wire bonding, just as HPL-03015 does.

HT-04503 is appropriate for LED applications where: the circuit voltage exceeds HPL-03015’s 170 VDC limit (any LED driver with mains-derived bus voltage); the ambient temperature approaches or exceeds 85°C (automotive under-hood, outdoor summer installations); or the assembly process involves AuSn or wire bonding at a substrate temperature that approaches HT-04503’s Tg.

LTI-04503: The General-Purpose Alternative

LTI-04503 shares identical 2.2 W/m-K dielectric conductivity and 0.05 °C·in²/W thermal resistance with HT-04503 at 3 mil thickness. The difference is its 90°C Tg and 130°C UL RTI versus HT-04503’s 150°C Tg and 140°C UL RTI. For LED applications where the substrate temperature stays reliably below 120°C and no AuSn or gold wire bonding is required — commercial interior lighting, LED strip drivers, retail display lighting — LTI-04503 delivers identical thermal performance to HT-04503 at equivalent cost.

MP-06503: High-Voltage LED Driver Isolation

MP-06503 at 3 mil offers lower thermal conductivity (1.3 W/m-K, 0.09 °C·in²/W) but higher dielectric strength (2800 V/mil versus 2000 V/mil for HT-04503) and higher peel strength (9 lb/in versus 6 lb/in). For LED driver boards where the primary-side circuit operates at 230–480 VAC and maximum electrical isolation margin at 3 mil dielectric thickness is required — combined with moderate power density from the switching transistors — MP-06503 is the correct specification.

Bergquist Thermal Clad LED Dielectric Specification Table

All values from official Bergquist/Henkel TDS documents and the Bergquist Thermal Clad Selection Guide (Q-6019).

Product	Dielectric Thickness	Thermal Conductivity	Thermal Resistance	Glass Transition	UL RTI	Solder Limit	Max Working Voltage	Key LED Application
HPL-03015	1.5 mil / 38 µm	3.0 W/m-K	0.02 °C·in²/W	185°C	140°C	325°C / 60s	120 VAC / 170 VDC	COB die attach, high-power LED, backlighting
HT-04503	3 mil / 76 µm	2.2 W/m-K	0.05 °C·in²/W	150°C	140°C	325°C / 60s	480 VAC	Automotive headlamp, outdoor lighting, high-ambient
LTI-04503	3 mil / 76 µm	2.2 W/m-K	0.05 °C·in²/W	90°C	130°C	260°C	480 VAC	Commercial LED, indoor LED drivers, retail
MP-06503	3 mil / 76 µm	1.3 W/m-K	0.09 °C·in²/W	90°C	130°C	300°C	480 VAC	LED driver primary side, high-voltage isolation
HT-07006	6 mil / 152 µm	2.2 W/m-K	0.11 °C·in²/W	150°C	140°C	325°C / 60s	690 VAC	480–690 VAC LED industrial systems

Matching PCB Material to LED Application Type

Not every LED lighting application has the same thermal requirements. The table below maps common LED lighting application categories to the correct Bergquist dielectric, with the engineering rationale for each.

LED Application	Power per LED / Power Density	Circuit Voltage	Substrate Temp	Recommended Bergquist Product	Rationale
COB LED die attach (bare die)	50–300 W total / high density	24–48 VDC LED string	≤140°C	HPL-03015	Lowest thermal resistance; 185°C Tg for wire bonding and AuSn; 170 VDC adequate for LED bus
Automotive LED headlamp, packaged LED	1–3 W per LED, 10–30 W total	12 VDC (automotive)	Up to 140°C	HT-04503	140°C UL RTI for under-hood ambient; 325°C solder limit for AuSn or wire bond if COB
LED street light, outdoor commercial	3–10 W per LED, 100–400 W total	48 VDC LED string	≤130°C	LTI-04503 or HT-04503	LTI if ambient ≤ 85°C; HT if outdoor summer ambient pushes substrate above 120°C
High-bay industrial LED, 100–300 W	5–15 W per LED, high density	48 VDC LED string	≤130°C	LTI-04503	Best thermal at 130°C RTI class; cost-effective for high-volume industrial luminaire
Retail/office LED panel, 20–60 W	0.5–2 W per LED	24–48 VDC	≤100°C	LTI-04503	Adequate thermal margin; 130°C RTI more than sufficient for conditioned interior
LED backlight for LCD display	0.5–3 W per LED	12–24 VDC	≤100°C	HPL-03015	Named application in HPL TDS; ultra-low thermal resistance minimises backlight non-uniformity from thermal drift
Projector LED illumination	10–50 W per LED	24–48 VDC	≤130°C	HPL-03015	Named application in HPL TDS; projector LED in confined space demands lowest substrate thermal resistance
Concentrator photovoltaic (CPV) cell	Very high density	Low voltage	≤140°C	HPL-03015	Named CPV application; bare solar cell direct attach requires AuSn compatible ultra-low resistance
LED driver primary side (230 VAC input)	5–20 W switching losses	230–400 VDC bus	≤100°C	MP-06503	Higher isolation margin at 3 mil for mains-connected circuits; thermal performance adequate
Horticultural grow light, 200–600 W	3–8 W per LED	48 VDC LED string	≤120°C	LTI-04503	Long-duration operation requires reliable thermal margin; 130°C RTI adequate in temperature-controlled greenhouse

The Junction Temperature Calculation: From Substrate Spec to LED Lifetime

The thermal path from LED junction to ambient heatsink can be modelled as a series of thermal resistances. For a surface-mount LED on a Bergquist Thermal Clad IMS board mounted to a heatsink:

Total RθJA = RθJC (LED package) + Rsolder joint + RPCB dielectric + Rbase metal spreading + RTIM + Rheatsink

For a typical 3 W packaged LED with RθJC of 8°C/W, on an HPL-03015 board versus LTI-04503 board, mounted to the same heatsink (10°C/W, fan-less), with 0.5 cm² thermal pad and a quality phase-change TIM:

Thermal Stack Component	HPL-03015	LTI-04503	Notes
LED RθJC	8°C/W	8°C/W	Fixed by LED package
Solder joint (100 µm, 1 cm²)	~0.1°C/W	~0.1°C/W	Per Bergquist 100 µm spec
PCB dielectric (0.5 cm²)	0.52°C/W	1.30°C/W	HPL 0.13 °C·cm²/W vs LTI 0.32 °C·cm²/W
Aluminium base spreading	~0.5°C/W	~0.5°C/W	Approximate, geometry dependent
Phase-change TIM (0.1 mm, 0.6 W/m-K)	~1.0°C/W	~1.0°C/W	TIM resistance
Heatsink (fan-less natural convection)	10°C/W	10°C/W	Same heatsink both cases
Total RθJA	~20.1°C/W	~20.9°C/W	0.8°C/W difference
Junction temperature at 3 W, 40°C ambient	100.3°C	102.7°C	2.4°C difference

At 3 W per LED, the junction temperature difference between HPL-03015 and LTI-04503 is 2.4°C. At 10 W per LED (a reasonable high-power COB element), the dielectric thermal resistance difference scales proportionally — delivering approximately 8°C lower junction temperature on HPL-03015 versus LTI-04503 for the same heatsink. At that level, the substrate choice matters directly and measurably for L70 lifetime.

PCB Design Details That Compound the Substrate Material Choice

The Bergquist Thermal Clad Selection Guide emphasises that substrate material selection is necessary but not sufficient. Several design-level decisions multiply or undermine the substrate’s thermal performance.

Copper Weight Selection for LED Boards

Heavier copper circuit layers reduce the thermal resistance between the LED thermal pad copper and the dielectric layer below it by improving lateral spreading before heat concentrates into the dielectric. For single-LED footprints, the improvement is marginal. For multi-LED boards with shared thermal copper regions, 2 oz copper versus 1 oz copper can reduce effective board thermal resistance by 10–20% by spreading heat over a larger dielectric area.

The Bergquist Selection Guide documents copper weight selection data with a specific note for LED applications: heavier copper on Thermal Clad allows circuit traces to carry higher currents because the substrate’s thermal management dissipates I²R losses in the copper itself more effectively than FR-4. This means heavier copper on Bergquist Thermal Clad is doubly beneficial — it spreads heat and handles higher current simultaneously.

For standard packaged LED boards (1–3 W per LED), 1 oz copper is adequate. For COB LED boards and high-density arrays (5 W or more per LED), 2 oz copper is recommended. For bare die attach and high-power modules, 3–4 oz copper under the die attach pads reduces the thermal concentration effect at the LED thermal footprint.

White Soldermask: The Lumen Output Multiplier

White soldermask on LED boards is not just cosmetic. The reflectivity of the soldermask surface between and around LED packages contributes to total luminous flux output by recycling photons emitted sideways from LED packages back into the beam. Studies measuring LED module lumen output on identical boards with different soldermask colours consistently show 5–10% higher total luminous flux from white soldermask versus black or green soldermask.

Bergquist Thermal Clad is compatible with all standard soldermask systems. For LED applications, white soldermask should be specified on the fabrication drawing as a design requirement, not left as a fabricator default. On HPL-03015 boards particularly — where thermal management and optical efficiency are both maximised — white soldermask completes the system optimisation.

The 100 µm Solder Thickness Rule for Thermal Clad LED Boards

The Bergquist Selection Guide states this directly: “No other decision will affect the reliability of the solder joint as much as the thickness of the solder to be used. A minimum of 0.004 inch (100 µm) is recommended (after reflow).” This requirement exists because Thermal Clad’s metal base conducts heat away from the solder joint faster than FR-4 — which means the joint cools more rapidly during reflow, increasing the cyclic thermal stress on the joint over the product lifetime. A 100 µm solder joint has more compliance than a thin 25–50 µm joint, absorbing the CTE mismatch stress through plastic deformation rather than crack propagation.

For LED applications with 10,000+ power cycles over product lifetime (commercial lighting cycling on and off), the 100 µm solder thickness is not a suggestion — it is a reliability design requirement for Thermal Clad.

Surface Finish Selection for LED Assembly

Surface Finish	Properties	LED Application Recommendation
ENIG (Electroless Nickel Immersion Gold)	Flat surface, excellent solderability, stable shelf life	Preferred for LED SMT assembly — flat surface improves solder paste printing accuracy and paste release under small LED thermal pads
HASL Lead-Free	Slight surface topography, good solderability	Acceptable for 1206 and larger LED packages; less suitable for fine-pitch COB footprints
OSP (Organic Solderability Preservative)	Flat, very thin, short shelf life	Suitable for high-volume production with controlled assembly timing
ENEPIG	ENIG + palladium layer, enables aluminium wire bonding	Recommended for COB wire bonding applications
Immersion Silver	Flat, high reflectivity	Useful where additional surface reflectivity between LEDs contributes to optical efficiency

For COB bare die attachment, ENIG or ENEPIG is the correct surface finish choice. The flat ENIG surface minimises solder void formation under the die attach pad, which is critical for thermal performance — a 10% void in the AuSn die attach joint can increase die-to-board thermal resistance by 15–25%.

Bergquist vs Generic Aluminium MCPCB for LED Lighting

The LED PCB supply chain includes a large number of Chinese and Asian manufacturers offering generic aluminium MCPCB at lower cost than Bergquist Thermal Clad. Engineers evaluating cost reduction through substrate substitution need to understand specifically what changes when Bergquist is replaced with generic material.

Parameter	Generic Aluminium MCPCB	Bergquist Thermal Clad HT-04503	Bergquist Thermal Clad HPL-03015
Dielectric thermal conductivity	1.0–2.0 W/m-K (claimed)	2.2 W/m-K (ASTM D5470 tested)	3.0 W/m-K (ASTM D5470 tested)
Test method verification	Often unverified or non-standard method	ASTM D5470 steady-state guarded hot plate	ASTM D5470
UL recognition	Often none or non-Bergquist UL file	Yes — Bergquist UL file, specific product	Yes
Qualification reliability testing	Not typically disclosed	Q-6019: 2000-hr thermal bias aging, 500-cycle temp cycling	Q-6019 program
Production consistency	Batch-to-batch variation often undocumented	Monthly production audits, Certificate of Conformance with lot traceability	Same
Solder limit	Usually 260°C stated, process not tested	325°C/60s (HT) or 260°C (LTI) UL 796 certified	325°C/60s UL 796 certified

For Bergquist PCB designs in safety-certified products — luminaires requiring UL 8750 or CE marking, automotive headlamp modules, medical lighting — the Bergquist UL component recognition and lot-traceable Certificate of Conformance are non-negotiable procurement requirements. For commercial LED applications without safety agency certification requirements, the generic MCPCB substitution decision should be made only with third-party ASTM D5470 thermal conductivity verification of the specific material being considered — not on the basis of claimed specifications.

LED Lighting Application Decision Summary

The decision logic for selecting Bergquist PCB material for LED lighting consolidates to four questions:

1. What is the maximum substrate temperature? Above 130°C, only HT products qualify. Below 130°C, LTI or HPL products are suitable.

2. What is the circuit working voltage? Above 170 VDC, HPL-03015 is disqualified — use HT-04503 or LTI-04503. Above 480 VAC, use HT-07006.

3. Is COB die attach (AuSn) or gold wire bonding required? Yes → HPL-03015 or HT products (325°C solder limit, appropriate Tg). No → all products eligible.

4. What is the power density at the LED thermal pad? Above 15–20 W/cm², specify HPL-03015 for lowest possible junction temperature. Below 10 W/cm², LTI-04503 or HT-04503 provide adequate performance.

Useful Resources for PCB Material for LED Lighting

Resource	Content	Link
Bergquist Thermal Clad Selection Guide (Digikey)	Complete dielectric selection guide with LED application examples, thermal impedance graphs, and voltage guidance	PDF
Bergquist Thermal Clad Selection Guide (tjk.com.au)	Current Q-6019 version with full dielectric spec tables	PDF
HPL-03015 Official TDS (Bergquist/mclpcb)	Full datasheet: 3.0 W/m-K, 0.02 °C·in²/W, 185°C Tg, 325°C solder limit	PDF
HT-04503 Official TDS (Bergquist/mclpcb)	Full datasheet: 2.2 W/m-K, 0.05 °C·in²/W, 150°C Tg, 325°C solder limit	PDF
HT-07006 Official TDS (Henkel/mclpcb)	Full datasheet for the 6 mil HT product: 0.11 °C·in²/W, 11.0 kVAC	PDF
IES LM-80	ANSI/IES standard for LED lumen maintenance measurement — basis for L70 lifetime data	ies.org
IES TM-21	Standard for projecting long-term LED lumen maintenance from LM-80 data	ies.org
Henkel Electronics Portal	Current Thermal Clad documentation and technical support	henkel.com/electronics

FAQs: PCB Material for LED Lighting

Q1: My LED lighting design uses 3 W LEDs and the supplier’s LED datasheet specifies a maximum junction temperature of 135°C. What substrate thermal resistance do I need to hit a safe operating margin?

Working backward from a 105°C target junction temperature (30°C margin below the maximum) with a 45°C maximum ambient: the available temperature rise is 60°C. Subtracting the LED package thermal resistance (assume RθJC of 10°C/W per supplier datasheet, adding 30°C at 3 W), the heatsink, TIM, and base metal thermal resistance together, you need the dielectric thermal resistance to contribute as little as possible to the remaining budget. At 3 W on a 0.5 cm² LED thermal pad (6 W/cm²), HPL-03015 at 0.02 °C·in²/W (0.13 °C·cm²/W) adds 0.78°C dielectric temperature rise. LTI-04503 at 0.05 °C·in²/W adds 1.94°C. HT-04503 at 3 mil adds the same 1.94°C. For a 3 W LED with 60°C total temperature budget and 10°C/W LED package resistance, the dielectric choice makes a measurable but not dominant difference — the heatsink is the primary variable. As power per LED increases to 5–10 W, the dielectric contribution scales proportionally and the substrate material becomes progressively more critical to the thermal budget.

Q2: Can I use HPL-03015 for an LED driver board that includes both the LED string and the 230 VAC input switching circuit on the same substrate?

No — and this is a common design mistake. HPL-03015 has a maximum continuous working voltage of 120 VAC / 170 VDC. A 230 VAC mains input circuit on the same board violates this limit. The correct approach is either to use HT-04503 for the entire board (which handles up to 480 VAC and also supports the LED string voltage), or to use a hybrid board architecture: the LED string power stage on HPL-03015 IMS for maximum LED thermal performance, the 230 VAC input and control circuitry on a separate FR-4 PCB, with appropriate galvanic isolation between the two boards. The hybrid architecture is more complex but allows each substrate to perform optimally in its role. For single-board designs that must include mains voltage circuitry and LED strings together, HT-04503 is the correct specification.

Q3: We have been using generic aluminium MCPCB at 1.5 W/m-K for our 50 W LED street light. Would switching to Bergquist HT-04503 at 2.2 W/m-K make a noticeable difference to LED lifetime?

The dielectric conductivity improvement from 1.5 W/m-K to 2.2 W/m-K translates to a 32% reduction in dielectric thermal resistance at the same 3 mil thickness. For a 50 W street light with 10 LEDs at 5 W each on 0.5 cm² thermal pads (10 W/cm²), the dielectric temperature rise per LED changes from approximately 5.8°C (at 1.5 W/m-K) to 3.9°C (at 2.2 W/m-K) — a 1.9°C reduction. Whether this translates to measurably longer field lifetime depends on where that 1.9°C sits in the total thermal budget. For an outdoor installation in a hot climate where junction temperatures are already running close to 95–100°C, a 1.9°C reduction per LED is meaningful — it moves the design from the steep slope of the Arrhenius curve toward a safer operating point. For a well-designed luminaire with ample heatsink area running LEDs at 65–70°C junction temperature in a temperate climate, the improvement is proportionally smaller. The other compelling reason to specify Bergquist over generic material is lot-traceable thermal conductivity verification — many “1.5 W/m-K” generic materials tested by independent labs come in at 0.8–1.2 W/m-K actual, meaning the thermal model was wrong from the start.

Q4: For a COB LED board using bare die with AuSn attach and wire bonding, what is the complete Bergquist substrate specification I should put on my fabrication drawing?

The complete specification for an HPL-03015 COB LED board: material Bergquist HPL-03015 per Selection Guide Q-6019, ASTM D5470 thermal conductivity minimum 3.0 W/m-K, glass transition minimum 185°C (ASTM E1356), UL 94 V-0, UL solder limit 325°C/60s (UL 796), dielectric thickness 1.5 mil ±10%, copper weight as specified (2 oz recommended for COB die pad), base metal 1.5 mm or 2.0 mm aluminium alloy 5052 or 6061 (specify), surface finish ENEPIG (for aluminium wire bonding) or ENIG (for gold wire bonding), white soldermask (specify coverage with mask clearance from die attach pads), solder mask compatible with 325°C assembly process. Certificate of Conformance required with Bergquist/Henkel lot number and ASTM D5470 thermal conductivity test result. Hipot test 1500 VDC minimum (specify controlled ramp rate per Bergquist guidance). Board must be fully discharged before removal from hipot test fixture.

Q5: At what LED power level does the substrate material choice become genuinely critical for an L50,000 lifetime target?

The crossover point depends on ambient temperature, heatsink quality, and the LED manufacturer’s L70 temperature sensitivity data. As a practical guideline: for LEDs with LM-80 data showing 30% lumen depreciation between 55°C and 85°C junction temperature, the substrate material becomes a critical lifetime driver at per-LED power dissipation above approximately 3 W in a fanless design with a modest heatsink (10–15°C/W). Below 1 W per LED, the dielectric thermal resistance is a small fraction of total RθJA and the substrate material has minimal impact on lifetime. Between 1–3 W per LED, the substrate material choice affects junction temperature by 2–5°C depending on thermal pad area — meaningful but not dominant. Above 5 W per LED, the substrate choice can shift junction temperature by 8–15°C, which directly determines whether a 50,000-hour L70 target is achievable or not with a given heatsink design. The most efficient design approach is to specify the best substrate material first (HPL-03015 or HT-04503 depending on voltage constraints), then size the heatsink to achieve the target junction temperature, rather than over-building the heatsink to compensate for a lower-performance substrate.