Bergquist PCB Material Selection Guide: How to Choose the Right Dielectric

Every Bergquist Thermal Clad IMS board succeeds or fails at the dielectric. The copper circuit layer, the aluminum base, the surface finish — those parameters matter, but they are mostly standard. The dielectric is the decision. It controls how fast heat moves from your power components to the metal base, what voltage isolation your board can sustain, what temperature the substrate can operate at continuously, and whether your assembly process is even compatible with the material you specified. Getting it right the first time saves a redesign. Getting it wrong means a board that either fails in the field or never passes agency certification.

This guide walks through Bergquist PCB material selection as an engineering decision process — not a product brochure summary. Each selection parameter is explained from first principles, matched to the official specification data from the Bergquist Thermal Clad Selection Guide (document Q-6019), and mapped to specific real-world applications with worked decision logic. By the end, you will have a systematic method for arriving at the correct dielectric specification for any power electronics design.

Why Bergquist PCB Material Selection Starts with the Dielectric, Not the Product Code

Most material selection conversations start with a product code — “I need HT-04503” or “quote this in MP-06503.” That approach works if someone else already did the selection work and you trust their output. If you are doing the selection yourself, starting with the product code puts the cart before the horse.

The correct starting point is four application parameters, evaluated in a specific order. The Bergquist Thermal Clad Selection Guide frames these as watt density, electrical isolation, and operating temperature environment. A more actionable way to state them for a working engineer is:

1. What is the maximum continuous operating temperature the substrate will experience?
2. What is the working voltage across the dielectric, and what does your safety standard require?
3. What thermal resistance can the design tolerate across the dielectric layer?
4. What is the assembly process — specifically, does it involve AuSn die attach or gold wire bonding?

Answer those four questions and the product selection is largely determined. The rest of this guide provides the data and logic to turn those answers into a specific Bergquist dielectric specification.

Understanding the Key Specification Parameters in Bergquist PCB Materials

Before the selection table, you need to understand what each specification actually measures and why it drives the decision differently for different applications.

Thermal Impedance vs Thermal Conductivity: Which One to Design Against

These two numbers are related but measure different things, and confusing them leads to wrong conclusions. Thermal conductivity (W/m-K) is a material property — it measures heat flow rate per unit thickness per degree of temperature difference, independent of how thick the layer is. Thermal resistance (°C·in²/W) combines thermal conductivity and thickness — it is what directly predicts temperature rise in your design for a given power density and component footprint area.

The Bergquist Selection Guide states this directly: “Thermal impedance is the only measurement that matters in determining the watt density capability of your application because it measures the temperature drop across the stack-up for each watt of heat flow.”

For practical design work, use thermal resistance (°C·in²/W). Multiply by power density (W/in²) and you get the temperature drop across the dielectric in °C. For a 10 W device on a 0.5 in² footprint (20 W/in²): HT-04503 at 0.05 °C·in²/W gives 1.0°C drop across the dielectric. MP-06503 at 0.09 °C·in²/W gives 1.8°C. The difference grows at higher power density and larger component counts.

Glass Transition Temperature and UL RTI: Two Different Operating Temperature Numbers

The Tg (glass transition temperature) is where the polymer transitions from glassy and rigid to elastomeric and compliant. This is not a failure temperature — it is a property change temperature. The Bergquist Selection Guide is explicit: many Thermal Clad products are used in applications above their rated Tg, and the UL RTI (Relative Thermal Index) — validated through 2,000-hour thermal endurance testing — is the correct long-term continuous operating temperature limit to design against.

The key separation in the Bergquist family: LTI, MP, and CML dielectrics have Tg of 90°C but UL RTI of 130°C. HT dielectrics have Tg of 150°C and UL RTI of 140°C. HT-09009 reaches 150°C UL RTI. The practical impact: if your design runs the substrate at 115°C continuously, LTI-04503 (130°C RTI) is certified for that application, despite the 90°C Tg. If you run it at 135°C, you need an HT product (140°C RTI).

Breakdown Voltage vs Proof Test Voltage vs Working Voltage

Three different numbers, three different purposes. The ASTM D149 AC breakdown voltage is a material characterization test — it tells you how much voltage the dielectric withstands in a laboratory before it fails. The typical proof test voltage is the fabrication hipot test voltage applied to every finished board to verify dielectric integrity. The working voltage is what your circuit actually runs at, governed by the applicable product safety standard.

The relationship between them: Bergquist recommends staying below 480 VAC working voltage for all 3 mil dielectrics. Above 480 VAC, move to 5 or 6 mil dielectric. The proof test voltage (1500 VDC for 3 mil products, 2500 VDC for 6 mil products) confirms the dielectric survived fabrication at a substantial margin above working voltage peak. The breakdown voltage (6.0–20.0 kVAC depending on product) is a materials characterization reference, not a working voltage specification.

The Complete Bergquist PCB Dielectric Specification Table

All values from the official Bergquist Thermal Clad Selection Guide (Q-6019).

Product	Thickness	Conductivity	Thermal Resist.	Breakdown	Proof Test	Tg	UL RTI	Peel	Solder Limit
HPL-03015	1.5 mil / 38 µm	3.0 W/m-K	0.02 °C·in²/W	5.0 kVAC	—	185°C	140°C	5 lb/in	325°C / 60s
LTI-04503	3 mil / 75 µm	2.2 W/m-K	0.05 °C·in²/W	6.5 kVAC	1500 VDC	90°C	130/130°C	6 lb/in	260°C
HT-04503	3 mil / 75 µm	2.2 W/m-K	0.05 °C·in²/W	6.0 kVAC	1500 VDC	150°C	140/140°C	6 lb/in	325°C / 60s
MP-06503	3 mil / 75 µm	1.3 W/m-K	0.09 °C·in²/W	8.5 kVAC	1500 VDC	90°C	130/140°C	9 lb/in	260°C
LTI-06005	5 mil / 125 µm	2.2 W/m-K	0.09 °C·in²/W	9.5 kVAC	2000 VDC	90°C	130°C	6 lb/in	260°C
HT-07006	6 mil / 150 µm	2.2 W/m-K	0.11 °C·in²/W	11.0 kVAC	2500 VDC	150°C	140/140°C	6 lb/in	325°C / 60s
LTI-07006	6 mil / 150 µm	2.2 W/m-K	0.11 °C·in²/W	11.0 kVAC	2500 VDC	90°C	130°C	6 lb/in	260°C
HT-09009	9 mil / 225 µm	2.2 W/m-K	0.16 °C·in²/W	20.0 kVAC	—	150°C	150/150°C	6 lb/in	325°C / 60s
CML-11006	6 mil / 150 µm	1.1 W/m-K	0.21 °C·in²/W	10.0 kVAC	2500 VDC	90°C	130/130°C	10 lb/in	260°C

Note: CML-11006 is available in prepreg form only. All others are fully cured laminates.

Step-by-Step Bergquist PCB Material Selection Logic

Step 1: Determine the Maximum Continuous Substrate Operating Temperature

This single question splits the entire product family into two branches and eliminates half the candidates immediately.

Your Max Continuous Substrate Temperature	Eligible Dielectrics
≤ 130°C	LTI-04503, LTI-06005, LTI-07006, MP-06503, CML-11006, HPL-03015
131°C – 140°C (automotive under-hood, high-temp industrial)	HT-04503, HT-07006, HT-09009
141°C – 150°C (harshest automotive, near-exhaust)	HT-09009 only
COB LED, CPV, bare-die attach priority	HPL-03015 (185°C Tg, lowest thermal resistance)

The “substrate temperature” here means the temperature of the dielectric itself during operation — not ambient, not junction temperature, not case temperature. It is the sum of ambient temperature plus the temperature rise in the metal base due to component power dissipation. For a motor drive with 85°C maximum ambient and a 40 W total dissipation on a 50 cm² aluminum base: the base temperature rise above ambient is modest (aluminum’s 150 W/m-K spreads heat efficiently), but the dielectric sits between the circuit copper and that base. In most standard industrial designs at 40–85°C ambient, LTI-04503’s 130°C RTI is adequate. In automotive under-hood where the ambient itself can reach 105–125°C, HT-04503 at 140°C RTI becomes mandatory.

Step 2: Establish the Voltage Isolation Requirement

After eliminating products on temperature grounds, apply the voltage filter. The key rule from the Bergquist Selection Guide: for applications above 480 VAC expected working voltage, use dielectric thickness greater than 3 mil (75 µm).

Working Voltage (AC or DC Peak)	Recommended Dielectric Thickness	Products to Consider
Low voltage systems (LED, 12/24/48 VDC)	3 mil	All 3 mil products
Standard industrial ≤ 480 VAC	3 mil	HT-04503, LTI-04503, MP-06503
Need high isolation at 3 mil (reinforced insulation per IEC 62368 or similar)	3 mil with highest breakdown	MP-06503 (8.5 kVAC)
480 VAC – 690 VAC	5 or 6 mil	LTI-06005, HT-07006, LTI-07006, CML-11006
Above 690 VAC, traction, high-voltage industrial	9 mil	HT-09009 (20.0 kVAC)

One subtlety worth knowing: within the 3 mil products, MP-06503 has a notably higher 8.5 kVAC breakdown compared to HT-04503 (6.0 kVAC) and LTI-04503 (6.5 kVAC). For designs at standard industrial voltage that still need maximum 3 mil isolation margin — certain medical equipment classes, reinforced insulation requirements, or product safety standards requiring higher working voltage test margins — MP-06503 is the correct call even though its thermal conductivity is lower.

Step 3: Select for Thermal Performance Within Your Eligible Products

After Steps 1 and 2, you have a short list. Within that list, select the dielectric that meets your thermal resistance requirement.

Worked Example: 48 VDC Industrial Motor Drive, 85°C Max Ambient

Step 1: Max substrate temperature estimated at 120°C (85°C ambient + ~35°C rise). Eligible: LTI, MP, CML. Step 2: Working voltage 48 VDC — far below 480 VAC threshold. All 3 mil products eligible. Step 3: Power density at the IGBT thermal pad: 30 W on 1.5 cm² = 20 W/in² = 3.1 W/cm². Allowable temperature rise across dielectric: 5°C budget. Required thermal resistance: 5°C ÷ 20 W/in² = 0.25 °C·in²/W. LTI-04503 at 0.05 °C·in²/W easily meets this. MP-06503 at 0.09 °C·in²/W also meets it. Step 4: Assembly is standard SAC305 reflow. No AuSn, no wire bonding. Selection: LTI-04503 — maximum thermal performance at the correct operating temperature class. MP-06503 is thermal overkill in the negative direction (lower conductivity than needed) and costs the same or more.

Worked Example: Automotive LED Headlamp Module, 125°C Under-Hood Ambient

Step 1: Substrate temperature potentially reaches 135–140°C at full load. Eligible: HT-04503, HT-07006, HT-09009. Step 2: Working voltage is 12 VDC LED system. All HT products pass voltage. Prefer minimum thermal resistance — use 3 mil (HT-04503). Step 3: COB LED with direct die attach — AuSn solder process at 290°C. Step 4 drives this decision. Step 4: AuSn process requires 325°C/60s solder rating. HT-04503 is rated 325°C/60s. LTI would fail at this process temperature. Selection: HT-04503 — HT’s 150°C Tg and 325°C solder rating are both required.

Worked Example: 400 VAC Industrial SSR, 70°C Max Ambient

Step 1: Substrate temperature estimated at 100°C. Eligible: LTI, MP, CML, HPL (all 130°C RTI products qualify). Step 2: 400 VAC working voltage. 3 mil is acceptable (below 480 VAC threshold). MP-06503 at 8.5 kVAC offers better margin over 400 VAC peak (566 VDC) than HT-04503 at 6.0 kVAC, especially if the safety standard requires double insulation margin. Step 3: Power density is moderate — a thyristor at 15 W on 2 cm². Both LTI and MP meet thermal budget. Step 4: Standard SAC305 reflow. No special process constraints. Selection: MP-06503 — the combination of 8.5 kVAC breakdown at 3 mil and adequate thermal performance makes it the correct specification for a high-voltage SSR where the insulation margin matters more than squeezing the last fraction of a degree from thermal performance.

Step 4: Verify Assembly Process Compatibility

Two specific assembly processes restrict product choice regardless of what Steps 1–3 indicate:

Thermosonic gold wire bonding: Requires HT dielectric. The Bergquist Selection Guide explicitly states this: for direct die applications using thermosonic gold wire bonding, “it is important to use HT dielectric because of its high Tg (thus higher modulus) at wire bond temperatures.” Wire bonding occurs at 120–150°C substrate temperature — at those temperatures, LTI and MP dielectrics (Tg 90°C) are in their elastomeric state and cannot support the ultrasonic energy needed for a reliable bond. HT dielectric at Tg 150°C remains in its glassy state through the wire bond process.

AuSn eutectic die attach (280–320°C): Requires HT or HPL dielectric. LTI, MP, and CML products are solder-rated to 260°C — AuSn processes exceed this limit. HT dielectrics are UL solder-rated to 325°C/60 seconds, and HPL-03015 also supports AuSn attach.

Assembly Process	LTI Products	HT Products	HPL-03015	CML-11006
SAC305 lead-free reflow (peak 260°C)	✓	✓	✓	✓
AuSn eutectic die attach (280–320°C)	✗	✓ (325°C rated)	✓ (325°C rated)	✗
Thermosonic gold wire bonding	✗	✓ (Tg 150°C)	Limited (Tg 185°C)	✗
Aluminum wire bonding	✓ with ENIG/ENEPIG	✓ with ENIG/ENEPIG	✓	✓

Bergquist PCB Material Selection by Application Category

The selection logic above maps to predictable product choices for common application families. This table is an engineering shortcut — always verify against your specific operating conditions.

Application	Typical Material Selection	Rationale
High-brightness LED COB, CPV solar cells	HPL-03015	Lowest thermal resistance (0.02 °C·in²/W), Tg 185°C, AuSn compatible
LED street lighting, commercial LED drivers	LTI-04503 or HT-04503	LTI if ambient ≤85°C; HT if under-hood or wire bond required
DC-DC power converters, telecom rectifiers	LTI-04503	High thermal performance at standard operating temperatures
Standard industrial SSR (≤480 VAC, ≤85°C)	LTI-04503 or MP-06503	LTI for thermal priority; MP for higher voltage isolation at 3 mil
High-voltage SSR, 600 VAC industrial	HT-07006 or LTI-07006	6 mil for 11.0 kVAC breakdown; HT if >130°C substrate
Motor drives (HVAC, industrial, <130°C)	LTI-04503	Standard thermal performance, adequate operating temperature
Automotive under-hood electronics	HT-04503	140°C UL RTI mandatory for under-hood environment
Automotive LED headlamps with COB die	HT-04503	AuSn or wire bond compatible, 140°C RTI
High-voltage industrial (690+ VAC, traction)	HT-09009	20.0 kVAC breakdown, 150°C RTI
Two-circuit-layer power/control integration	CML-11006 (prepreg)	Only prepreg in Thermal Clad family; enables multi-layer IMS
Audio amplifier heat-rail, formed structures	LTI-04503 or HT-04503	LTI for standard ambient; HT where forming temp or environment demands it

Selecting the Bergquist Base Metal: Aluminum vs Copper

Bergquist PCB material selection does not end at the dielectric. The base metal selection — aluminum versus copper — affects thermal spreading, solder joint fatigue life, board flatness, and cost.

When to Choose Aluminum Base

Aluminum (5052 or 6061 alloy, ~150 W/m-K) is the default for the vast majority of Thermal Clad applications. It is lighter, lower cost, and available in the full range of standard thicknesses (0.8 mm to 3.2 mm for Thermal Clad). The higher CTE of aluminum (~25 ppm/°C) versus ceramic components (~4–8 ppm/°C) means solder joint fatigue is a consideration for large passive components on thermally cycled designs — mitigated by the 100 µm minimum solder thickness Bergquist specifies.

When to Choose Copper Base

Copper base (~400 W/m-K) is specified when: localized power density is high enough that aluminum’s 150 W/m-K spreading capacity is the bottleneck; the CTE of copper (~17 ppm/°C) provides better solder joint fatigue life for ceramic components than aluminum’s 25 ppm/°C; or an electrical connection from the circuit layer to the base plate is needed (copper is more compatible with via-to-base electrical connections than aluminum). The Bergquist Selection Guide notes a useful cost reference: 1.0 mm copper base costs approximately the same as 3.2 mm aluminum base — copper is not always more expensive if design allows a thinner base.

The 10% Flatness Rule for Copper Weight on Aluminum Base

A practical fabrication rule from the Bergquist Selection Guide: to maintain circuit flatness on aluminum base, keep copper circuit layer thickness at 10% or less of the aluminum base thickness. Exceeding this ratio allows the copper’s internal stress to dominate and bow the board. Example: 1.6 mm aluminum base — copper circuit weight should stay at or below 160 µm, which is approximately 4.5 oz. For heavy copper designs (6 oz or more), use thicker aluminum base or switch to copper base.

Common Bergquist PCB Material Selection Mistakes and How to Avoid Them

Specifying HT-04503 when LTI-04503 is sufficient. HT-04503 is the most-quoted Bergquist product, so engineers default to it. But if the design runs below 130°C continuously and doesn’t require AuSn or gold wire bonding, LTI-04503 delivers identical thermal performance (same 0.05 °C·in²/W at 3 mil) with a slightly higher 6.5 kVAC breakdown voltage. Specifying HT when LTI is adequate may cost more depending on supply chain, and if the fabricator substitutes a generic “HT equivalent,” you may not know.

Using a 3 mil product above 480 VAC. Bergquist is direct about this: “For applications with an expected voltage over 480 Volts AC, Bergquist recommends a dielectric thickness greater than 0.003 inch.” Specifying HT-04503 for a 600 VAC SSR design is not the correct path. LTI-07006 or HT-07006 at 6 mil is required.

Ignoring the assembly process until the BOM is locked. If your design requires COB bare die with AuSn die attach and you specified LTI-04503 (260°C solder limit), you have a problem that requires either a material redesign or a process change. Verify assembly process requirements in Step 4 of the selection, not after prototype boards are ordered.

Accepting “equivalent” IMS material without data. A generic 1.0–1.5 W/m-K aluminum MCPCB from a commodity supplier is not equivalent to Bergquist HT-04503 at 2.2 W/m-K. The thermal resistance difference is a factor of 1.5× to 2.2× at the same dielectric thickness — and your thermal model assumed the Bergquist number. Require ASTM D5470 thermal conductivity data and UL recognition documentation for any substitute material.

Not specifying soldermask. The Bergquist Selection Guide states that soldermask use is mandatory on Thermal Clad boards. Soldermask protects the dielectric layer from environmental degradation that affects electrical isolation over time.

Useful Resources for Bergquist PCB Material Selection

Resource	What It Contains	Link
Bergquist Thermal Clad Selection Guide (tjk.com.au)	Official complete spec table for all dielectrics, selection logic, base metal guidance, assembly recommendations — the primary engineering reference	PDF
Bergquist Thermal Clad Selection Guide (Digikey)	Alternate hosted version of Q-6019 with identical content	PDF
HT-04503 TDS (Henkel/mclpcb)	Individual TDS for the flagship 3 mil HT dielectric including full electrical, thermal, mechanical, and chemical data	PDF
HT-07006 TDS (Henkel/mclpcb)	Individual TDS for the 6 mil HT dielectric — the 11.0 kVAC, 140°C RTI high-voltage product	PDF
HPL-03015 TDS (Henkel/mclpcb)	Individual TDS for the 1.5 mil ultra-thin LED dielectric	PDF
MP-06503 TDS (Henkel/mclpcb)	Individual TDS for the 8.5 kVAC multi-purpose dielectric	PDF
Henkel Electronics Portal	Current product documentation, SDS, technical enquiries	henkel.com/electronics
IPC-2221B	PCB design standard — creepage/clearance, trace width, current rating rules	ipc.org
ASTM D5470	Standard thermal conductivity test method — basis for all Thermal Clad thermal resistance specs	astm.org

FAQs: Bergquist PCB Material Selection

Q1: I have a 230 VAC product. Does the Bergquist Selection Guide’s “480 VAC” threshold mean my 3 mil product is fine?

Yes, but with an important caveat about how “480 VAC” works. Peak AC voltage at 480 VAC RMS is 679 VDC. The Bergquist 3 mil dielectric proof test at 1500 VDC provides more than 2× margin over that peak, which is why 3 mil is acceptable up to 480 VAC. At 230 VAC (RMS peak 325 VDC), the margin is even more comfortable. The actual limit for your design depends on which product safety standard governs your product — IEC 62368-1, IEC 60335, IEC 61010, or another — and what isolation class (basic, supplementary, reinforced, double) your circuit requires. The 480 VAC threshold in the Bergquist guide is a conservative general-purpose guidance value. Your safety standard may require a higher or lower proof test voltage independent of the Bergquist material specification.

Q2: Can I use LTI-04503 in an automotive application, or does automotive always require HT?

It depends entirely on where in the vehicle and what the maximum substrate temperature is. Automotive body electronics — cabin lighting, seat control modules, window motor drivers, infotainment power stages — operate in a protected environment where ambient temperatures stay within 40–85°C and substrate temperatures stay well within LTI-04503’s 130°C UL RTI. LTI-04503 is appropriate there. Automotive under-hood applications — engine bay electronics, transmission control modules, near-exhaust sensors — can see ambient temperatures of 105–125°C. Adding the thermal rise from component power dissipation puts the substrate temperature above 130°C, making HT-04503 (140°C RTI) necessary. The question is not “automotive?” but “what is the maximum substrate temperature in this specific mounting location?” If you can keep the substrate below 130°C in your thermal analysis, LTI-04503 is certifiable. If not, specify HT-04503.

Q3: My design needs multi-layer routing — power stage on one side, gate driver on the other. Is Bergquist Thermal Clad the right approach?

CML-11006 is the Bergquist product that enables this, and yes, it is the right approach when moderate power density allows the 0.21 °C·in²/W interlayer thermal resistance to be managed with a well-designed thermal via array. The Bergquist Selection Guide documents this construction: two copper circuit layers bonded with CML-11006 prepreg interlayer, laminated to an aluminum or copper base using a standard HT-04503 or LTI-04503 dielectric on the base side. The gate driver and control circuitry occupy the bottom circuit layer; the power FETs and high-current bus work occupy the top circuit layer. Thermal vias drilled through the CML-11006 interlayer and copper-plated connect the top thermal pad copper to the bottom copper and onward to the base. Via density under power device pads is the primary thermal design variable. If your power density is high enough that thermal vias cannot adequately compensate for 0.21 °C·in²/W, CML-11006 is not the right approach and a single-layer HT-04503 board with a conventional two-PCB assembly (power board + control board) may be more practical.

Q4: How do I specify Bergquist material to a fabricator to prevent substitution with generic IMS?

The specification on the fabrication drawing or purchase order should include: material designation “Bergquist Thermal Clad

per Bergquist document Q-6019″; Certificate of Conformance required with Henkel/Bergquist lot number; thermal conductivity minimum [2.2 W/m-K or as applicable] per ASTM D5470; AC breakdown voltage minimum [per product spec] per ASTM D149; UL 94 V-0 and UL RTI [temperature class] per UL 746E. Requiring lot traceability to Henkel is the key clause — a fabricator using genuine Bergquist material can provide this, while one using a generic substitute cannot. Additionally, specify the hipot test voltage explicitly (1500 VDC for 3 mil products, 2500 VDC for 6 mil), the copper weight, base metal alloy and thickness, surface finish, and that soldermask use is mandatory per Bergquist’s own specification.

Q5: Is Bergquist Thermal Clad suitable for both prototype and high-volume production, and does the material change between quantities?

Bergquist Thermal Clad is used from prototype quantities through high-volume automotive and industrial production. The material itself does not change between quantities — it is manufactured at Bergquist’s Prescott, Wisconsin facility under ISO 9001:2000 with the monthly production audit program described in the Selection Guide. What does change between prototype and production volumes is practical supply chain access: prototype quantities are typically sourced from Digikey, Mouser, or directly through a Bergquist-authorized distributor in standard panel sizes (18″ × 24″ or 20″ × 24″). Production volumes may access better pricing through direct Henkel/Bergquist purchasing programs or through qualified fabricators who stock the material. The fabrication process for Thermal Clad is essentially the same as standard aluminum MCPCB production for fabricators experienced with IMS material, so qualifying a fabricator for prototype should translate directly to production without material-related process changes.