Printed Circuit Board Embedded Thin Film Resistors

Applications and Implementation in MEMs and RF Devices

By

Bruce Mahler

Vice President

Ohmega Technologies, Inc.

IMS 2017 olulu,niIMS 2017 Honolulu, Hawaii June 7, 2017

Thin Film Embedded Resistor Technology

• 1. Electrodeposited NiP thin film resistive material

• 2. Standard subtractive PWB processing

• 3. Surface or embedded resistors

• 4. Mature technology (40+ years)

• 5. Field Proven, Excellent Long Term Reliability

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Why use Embedded Resistors?

• Density improvement—free up board area or reduce board size with elimination of discrete resistors

• Improved reliability with reduction in solder joints

• Elimination of parasitic inductance in power dividers

• Critical signal measurements to meet fast rise times and reduce signal delay

• Placement of a termination resistor very close to the drive line

• Very small element sizes with subtractive PCB print/etch

• EMI improvement and improved fidelity in conjunction with a planar capacitor as an RC filter in MEMs microphone modules

• Embedded or integral heaters at board level

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Thin Film Resistive Material Manufacturing

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NiP Sheet Resistivity Offerings

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Sheet Resistivity

(Ω/□)

Material Tolerance

(%)Typical Applications

10 3Series termination, impedance matching, planar heaters

25 5Series/parallel termination resistors, power dividers, filters

40 5

50 5

100 5 Pull-up/pull-down resistors

250 10 High ohmic applications

Basic Design Overview

• Sheet resistivity, stated in Ohms per square is dimensionless• A square area of resistive material equals the sheet resistivity of material

• 25 ohms per square (Ω

■) sheet resistance

• Resistor value = sheet resistivity X ratio of element length to width

• 𝑅 = 𝑅𝑠 ×𝐿

𝑊; where

𝐿

𝑊= 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑞𝑢𝑎𝑟𝑒𝑠 𝑁

• For example:

• Sheet resistivity (Rs) = 25 Ω/□

• Length = 0.030” (30 mils)

• Width = 0.015” (15 mil)

• 𝑅 = 25Ω

∎×

30𝑚𝑖𝑙𝑠

15 𝑚𝑖𝑙𝑠∎

• 𝑅 = 50Ω

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Sheet Resistivity (Rs) Adjustments for PTFE Substrates

• Lamination of NiP to PTFE substrates results in a one time shift in nominal sheet resistivity:• 25 ohm per square → 28 ohm per square

• 50 ohm per square → 60 ohm per square

• 100 ohm per square → 140 ohm per square

• For designs requiring a PTFE substrate, the modified sheet resistivity should be used when calculating resistor footprints.

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Ohms Per Square

• Pull-up/down and termination resistors military/aerospace board

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Ohms Per Square

• Sample Circuit 50 ohm per square on Duroid 6202PR

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PCB Processing of NiP Resistive Conductive Material

• Step 1: Apply Photoresist to Laminate

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• Step 2: Print and Develop Composite Image

PCB Processing of NiP Resistive Conductive Material

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PCB Processing of NiP Resistive Conductive Material

• Step 3: Etch Unwanted Copper Using Any Conventional Etchant (1st etch)

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PCB Processing of NiP Resistive Conductive Material

• Step 4: Etch Unwanted Resistive Material with Copper Sulfate Solution (2nd etching process)

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• Step 5: Strip Photoresist

PCB Processing of NiP Resistive Conductive Material

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• Step 6: Apply Photoresist, Print and Develop Conductor Protect Image (2nd print)

PCB Processing of NiP Resistive Conductive Material

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PCB Processing of NiP Resistive Conductive Material

• Step 7: Etch Away Copper Over the Designed Resistor Using a Selective Alkaline Etchant (3rd etch)

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• Step 8: Strip Photoresist

PCB Processing of NiP Resistive Conductive Material

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Two Print Artwork PCB Processing

Artwork layout

• NiP resistor processing consists of two prints:• 1st print – COMPOSITE image of conductors and resistors

• 2nd print – RESISTOR DEFINE image of resistor elements

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Composite (negative film) resistor define commonly used for

voltage or ground plane with most of the copper preserved.

COPPER

PAD

Composite (negative film) conductor protect commonly used

for signal plane

NiP Resistive Material in RF Applications

• Provides greater packaging densities

• Eliminates resistor assembly

• Eliminates solder joints

• Weight savings

• Reduction in parasitic inductances and capacitances

• Excellent stability beyond 70 GHz

• Low profile copper for low insertion loss

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NiP Resistive Material in RF Applications

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Performance Benefit of NiP Planer Resistor

• Reduction in parasitic capacitance and inductance compared to surface mount components.• Reduce metal-to-metal transitions associated with chip

resistors

• Reduce vias on critical nets

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NiP Resistors in Microwave Applications

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NiP Resistors in Microwave Applications

Enlargement of a four-up array 16-way power divider with 50 /sq NiP resistors

0.026” x 0.0145

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NiP Resistor Designs in Space Applications

NiP resistors in microwave application for Globalstar antenna.

Layer stack up.

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Globalstar Antenna – NiP Resistor Inner Layer

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NiP Resistor Designs in Military/Aerospace Applications

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Active Phased Array Antenna

NiP Resistive Material in R-Cards and Absorbers

• Can be laminated to a variety of substrate materials with different permittivities

• Create repetitive, planar patterns using standard photolithography techniques (subtractive print/etch)

• Cost reduction

• Weight savings (reduced thicknesses)

• Increased bandwidth and improved performance covering wider angles of incidence

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NiP Resistors in RF Electromagnetic Absorbers and R-cards

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NiP Resistors in RF Electromagnetic Absorbers and R-cards

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NiP Resistive Conductive Material in Sensors

• Increased PCB density (small element sizes)

• Reduced PCB thickness (eliminate SMT-R)

• Reduced assembly (resistor built into PCB)

• Improved Reliability (elimination of solder joints)

• Improved electrical performance (reduced EMI)

• Cost savings (replacement of discrete SMT-R)

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NiP Resistors in MEMs Microphone Sensor

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NiP Resistors in MEMs Microphone Sensor

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3.5”

2.5”

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Consumer Electronics

MEMs Microphones

NiP Resistors in MEMs Microphone Sensor

NiP Resistor Reliability

• Researchers at Alcatel Bell tested NiP resistors for broadband (45 MHz-5 GHz) telecom applications to compare the reliability of NiP resistors to 0805 discrete thick film chip resistors rated at 125mW. The NiP resistors were as good as, or better than, the chip resistor in all performed tests.

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Type of TestMeasured ΔR

(Alcatel Tested)Ohmega Specifications

Thick Film Chip (0805)

Humidity TestTemp: 40°CRH: 93%

After 21 days:• 0.22% for 25 Ohm/sq.• 0.07% for 100 Ohm/sq.• 0.10% for 250 Ohm/sq.After 56 days:• 0.74% for 25 Ohm/sq.• 0.14% for 100 Ohm/sq.• 0.22% for 250 Ohm/sq.

After 10 days:• 0.5% for 25 Ohm/sq.• 1.0% for 100 Ohm/sq.

After 56 days:≤ ± 1.5%

Thermal CyclingHigh: 125°CLow: -25°C

After 100 Cycles• - 0.03 % for 25 Ohm/sq.• 0.03 % for 100 Ohm/sq.• - 0.08 % for 250 Ohm/sq.

After 25 Cyles• - 0.5% for 25 Ohm/sq.• 1% for 100 Ohm/sq.

≤ ± 25%

Unbiased Aging125°C

After 100 Hrs.• 0.10% for 25 Ohm/sq.• 0.08% for 100 Ohm/sq.• - 0.13% for 250 Ohm/sq.

Not specified Not specified

Solder Float260°C20 seconds

• - 0.02% for 25 Ohm/sq.• 0.01% for 100 Ohm/sq.• - 0.01% for 250 Ohm/sq.

• 0.5% for 25 Ohm/sq.• 1% for 100 Ohm/sq.

≤ ± 25%

NiP Resistor Reliability

• Dassault Electronique (Thales) did a 2 year study of NiP resistors for an active phased array antenna (X-band). The resistors were constructed on Rogers RT Duroid® 6002 substrate and fusion bonded inside a multilayer package. The NiP resistors were selected over printed polymer ink and chip resistors.

Method Result

Etching Tolerance (%) 5

Minimum Resistor Width (um) 200

Tolerance After Fusion Bonding (%) 7

Influence of NiP Layer on Microwave Properties

Negligible

Thermal Cycle (% ΔR)• -55°C to 125°C, 500 cycles

• Microstrip 2%• Stripline 3%

Thermal Coefficient of Resistance (% ΔR)• -55°C to 125°C

• Microstrip +/- 2%• Stripline +/- 3%

Power (mW) 300

2-Port Power Divider Environmental Stress Test (% ΔR)• Thermal Cycle: -55°C to 125°C, 500 cycles• High Temperature Storage: 125°C, 500 hours• Static Humidity: 40°C, 95% RH, 40 days• Salt Spray: 48 hours

Negligible

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NiP Resistor Reliability

• NiP for lead-free assembly. Highly Accelerated Thermal Shock Test (HATS)• -40°C to 145°C

• 1000 cycles

• 10.85 minutes per cycle

Resistor Network15 Coupons per Resistor network

% ΔR

1 0.20

2 0.15

3 0.20

4 0.17

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NiP Resistor Reliability

• Stability over long term continuous operation

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NiP Resistor Reliability

• Application shows a heater used to bring the X-Ray Spectrometer (XRS) biasing and pre-amplification electronics to -50 degrees Celsius in the Mars Beagle 2 Lander.

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Summary

• NiP thin film resistive material used extensively in RF and MEMs designs

• Improved package densities

• Improved electrical performance

• Improved reliability

• Standard subtractive PCB processing

• Growing applications in IC packaging, IOT sensor technologies, 5G and DDR4 memory devices

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OHMEGA TECHNOLOGIES, INC.

4031 ELENDA STREET

CULVER CITY, CA 90232-3799

PHONE: (310)559-4400

FAX: (310)837-5268

WEB SITE: http://www.ohmega.com

Copyright 2017 Ohmega Technologies, Inc.