RF Module Substrate with Low Dk, Low Df, and Low CTE,
Ideal for Multilayering and Miniaturization

Wireless communications are advancing from 5G to 5G-Advanced and Beyond 5G (6G) in pursuit of higher speeds and larger capacity. To keep pace with these growing needs, smartphones and other mobile devices contain a burgeoning number of radio frequency (RF) communication chips, including amplifiers and filters, to handle multiple radio frequency bands and radio waves such as Bluetooth, Wi-Fi, and near-field communication (NFC). Accordingly, RF chips are increasingly modularized, with chips mounted in close proximity on a single substrate, while demand rises for highly integrated RF chips with chip-to-chip signals transmitted much faster in wider bandwidths. Space saving through modularization also helps equip smartphones with multiple functions, such as high-performance cameras and various types of sensors.

RF module substrates require low dielectric properties (low dielectric constant (Dk) and low dielectric loss tangent (Df)) with low transmission loss at high frequencies as well as low coefficients of thermal expansion (CTE) to ensure reliability.

Bismaleimide triazine resins are commonly used to produce these substrates. However, smaller RF modules are essential to create more space in response to expanding frequency bands and smartphones’ multifunctional capabilities. Multilayering with thinner substrates is an effective way to meet these demands, and substrates must also provide a lower Dk than conventional models.

Resonac recommends its RF module substrate MCL-HS200 (Type D) (hereinafter, “HS200 (D)”) that features both low dielectric properties and low CTE, and is suitable for multilayering. This product provides a lower Dk than conventional models, and allows for thinner substrates, multilayered substrates, and smaller modules.

Trends of modularization

DUP = Duplexer,PA = Power Amplifier,LNA = Low Noise Amplifier,Div. M = Diversity Module,FEMiD = Front End Module integrated Duplexer,PAMiD = PA module integrated Duplexer

Innovative solution

Achieve low dielectric properties, low CTE, and high Tg

Substrates for RF modules must provide low dielectric properties, low CTE, and high glass transition temperatures (Tg). Dk and Df need to be lowered to reduce the dielectric loss in high frequency bands. In addition, ensuring the reliability of mounted multiple RF chips requires low CTE and high Tg to minimize warpage and prevent solder cracks after the heating process that includes reflowing in mounting.

Multilayering substrates is also an effective method to make RF modules even smaller. To realize thinner substrates, it is necessary to reduce the width and thickness of wires for impedance matching or to further lower the Dk.

Resonac’s HS200 (D) achieves low dielectric properties, low CTE, and high Tg at remarkably high levels by optimally compounding thermoset resins of modified polyimide (PI) and soft segments, and fillers.
The table “Physical property comparison of substrates” compares the properties of HS200 (D), conventional RF module substrates with two different grades (Material A and B), and Resonac’s FC-BGA substrate E-705G which is well known for its low CTE. Resonac’s HS200 (D) offers superior balance with its high Tg, low CTE, low Dk, and low Df, particularly with its Dk lower than conventional products. HS200 (D)’s low Dk properties enable substrates to be thinned without fine wiring.

Physical property comparison of substrates

• *1) 200 µm thickness
• ※ The data shown are representative values showing examples of the measurement and calculation results, and do not guarantee quality.

Product Features

Low transmission loss

Resonac’s HS200 (D) demonstrates 64% lower transmission loss at 40 GHz than its FC-BGA substrate E-705G.

Transmission loss in strip line

• ・Core thickness : 400 μm
• ・Prepreg thickness : 250 μm (1037 x 5 ply)
• ・Copper foil thickness : 18 μm
• ・Copper foil roughnesss : 1 μm (Rz)
• ・Inner copper foil roughness : 1.5 μm (Rz)

 

Material	HS200(D)	E-705G
Transmission Loss (dB/cm) @40GHz	-0.47 (-64%)	-1.29 (0%)

• ※ The data shown are representative values showing examples of the measurement and calculation results, and do not guarantee quality.

Low warpage

Resonac’s HS200 (D) displays almost the same amount of warpage as its FC-BGA substrate E-705G which is well known for its low CTE.

Warpage performance

• ・Substrate: 3-2-3 construction
• ・Package size: 40 x 40 mm
• ・Core thickness: 800 μm
• ・Bu thickness: 20 μm
• ・SR thickness: 19 μm (SR-FA)
• ・L1,4,5,8: 12 μm Cu 65%
• ・L2,3,6,7: No copper
• ・Die size: 20 x 20 mm
• ・Die height: 775 μm
• ・Underfill thickness: 60 μm (CEL-C-3730series)

• ※ The data shown are representative values showing examples of the measurement and calculation results, and do not guarantee quality.

High reliability

The biased highly accelerated stress test (bHAST) indicates the excellent reliability of Resonac’s HS200 (D).

bHAST result

• <Material>
• ・Core : HS200 0.1mm(Copper foil : 12μm)
• ・Prepreg : GH-200(D) 1037N73
• <Test condition>
• ・Pre condition : 60ºC/60%RH/48hr+260ºC reflow×3cycle
• ・Treating time : 300hr
• ・Measurement Condition : 130ºC/85%RH, DC 3.5V

• ※ The data shown are representative values showing examples of the measurement and calculation results, and do not guarantee quality.

Update date: 30th January, 2024

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