Server backplane connectors

01

Match the data rate, impedance and pair count

Amphenol's ExaMAX datasheet describes a backplane connector system for 25 Gb/s to 56 Gb/s, with 92 Ω nominal impedance, 2 mm pitch for high density or 3 mm pitch to allow quad routing and lower PCB cost, and a 12 to 96 differential pair node range; its traditional mother-daughter part table runs from 12 to 72 differential pairs in 2-, 3-, 4- and 6-pair columns. It lists support for PCI Express Gen 1–5 (2.5–32 Gb/s), CEI-56G-MR-PAM4 (56 Gb/s PAM4), SAS 1.1–4.0 (3–24 Gb/s) and SATA (1.5–6 Gb/s), among other standards. [1] Samtec, which also supplies ExaMAX, rates the board-to-board version at 64 Gbps PAM4 (32 Gbps NRZ) and the Flyover cable version at 112 Gbps PAM4, both PCIe 6.0/CXL 3.2 capable; it states the ground placement is engineered for 92 Ω and addresses both 85 and 100 Ω applications, with 24–72 pairs per connector and 4 or 6 pairs per column in its 2.00 mm EBTM headers. [2]

A Molex Impulse part sheet (1736311016, an orthogonal daughtercard receptacle) lists 112 Gbps PAM-4, 90 Ω impedance, full shielding, 6 pairs per column, 16 columns and 96 differential pairs (192 circuits) at 2.00 mm pitch. [3]

Editorial calculation: if the 2.00 mm pitch is the column-to-column spacing, a 6-pair column layout gives 25.4 ÷ 2.00 × 6 ≈ 76 differential pairs per inch of connector length, and a 6-pair layout at 3 mm gives 25.4 ÷ 3 × 6 ≈ 51; confirm the column pitch and pairs per column on the supplier's drawing before comparing density. [3] [1]

Editorial note: the reviewed connectors are specified at 90 Ω or 92 Ω nominal while server channels are designed at 85 Ω or 100 Ω, so ask for insertion loss and crosstalk data at your impedance and data rate rather than comparing headline Gb/s alone.

Give the supplier the data rate and signalling (NRZ or PAM4), the interface standards to meet (PCIe generation, SAS, Ethernet, CXL), channel impedance, number of differential pairs, low-speed and power contacts, architecture (traditional backplane, coplanar, orthogonal or cable), board thicknesses, guide and keying needs, mating cycles, operating temperature and quantity.

02

Check press-fit mounting, mating force and mating cycles

Both ExaMAX sources specify press-fit termination: Amphenol gives finished PCB holes of 0.36 mm for high-speed signals and 0.5 mm for ground pins, an average press-fit insertion force of 12 N max per contact and tin over nickel on the press-fit tails; Samtec gives 0.36 mm and 0.50 mm plated through-holes. Amphenol lists mating force of 0.38 N max and unmating force of 0.10 N min per contact with a mating wipe over 2 mm and 1.1 mm of X and Y capture; Samtec lists 0.36 N max and 0.12 N min per contact and a 2.4 mm contact wipe. [1] [2]

Amphenol rates signal contacts at 0.5 A each with a 30 °C temperature rise and operation from −55 °C to +85 °C, and states Telcordia GR-1217-CORE qualification; the Molex Impulse sheet lists 0.75 A per contact, 150 V AC (RMS)/DC, 0.762 µm minimum gold on the mating area, −55° to +85 °C and a durability of 200 mating cycles maximum. [1] [3]

Editorial calculation: with 0.38 N max mating force per contact, a 192-contact connector of the Molex size would need up to about 192 × 0.38 ≈ 73 N to mate if it used ExaMAX-level contact force; ask each supplier for the total mating force of the exact part. [1] [3]

03

Storage-backplane drive connectors: SFF-8639 (U.2) and TE's SAS receptacle

Storage backplanes use a different connector family. SFF-8639 (Rev 2.2, 2024) defines a six-lane multifunction connector with 68 contacts at 1.27 mm and 0.80 mm pitch that can carry SATA, SATA Express, SAS, MultiLink SAS or up to four PCIe ports, with a three-level mating sequence (0.50 mm steps) for hot plugging into a powered backplane. Its requirements include 1.5 A continuous per power pin and 500 mA per signal pin, 30 mΩ maximum signal contact resistance, 1000 MΩ insulation resistance at 500 V DC, 500 V AC withstand, 59 N maximum mating and 6 N minimum unmating force, and durability of 500 cycles for backplane and 25 cycles for cable receptacles. [4]

TE's product specification 108-51109 for its SAS 3.0 receptacle (SFF-8680 and SFF-8639 types) lists gold over nickel contacts, thermoplastic housings, 30 V DC, operation from 0 °C to 55 °C, a 500-cycle durability test and 50 g, 11 ms mechanical shock; it gives 12 N minimum unmating force per SFF-8639, while SFF-8639 Rev 2.2 states 6 N, so confirm which revision the part is qualified to. [5] [4]

When requesting quotations, ask each supplier for the product specification, signal-integrity test report at your data rate, PCB footprint and press-fit application specification, and samples.

04

Signal wafers, ground shields, plating, press-fit tails, housing and guide pins

Ground structure and shielding are the rows that set impedance and crosstalk: Samtec engineers the ExaMAX ground placement for 92 Ω, and the Molex Impulse part is fully shielded. [2] [3]

05

Pair count, pitch, current, mating force, cycles and plating for a backplane connector

Mating cycles show the widest spread among the parts reviewed, 200 maximum on the Molex Impulse sheet against 500 for an SFF-8639 backplane receptacle, so state the cycles your service plan needs. [3] [4]

06

Send the channel, pair count and mounting data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Channel: [data rate; NRZ or PAM4; standards; impedance]
Pairs: [differential pairs; low-speed; power]
Architecture: [backplane; orthogonal; coplanar; cable]
Mounting: [press-fit; board thickness; hole sizes]
Mechanical: [mating cycles; guides; keying]
Environment: [temperature]
Quantity: [sets]

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