Guide

Understanding PCB Edge Connectors: A Beginner’s Guide

PCB edge connector

Introduction to PCB Edge Connectors

PCB edge connectors are also called gold finger connectors. The gold-plated pads are designed on printed circuit board edge to fit into special sockets. A special application of these PCBs is to connect them to the motherboard or any additional memory devices connected to a computer. High-speed and high-frequency signal applications adapt this PCB edge connecting technologies to ensure good connectivity of the signals.

The board-to-board connector is preferred for high-frequency signals over the board-to-wire or wire-to-wire. The main purpose of using gold as the metal for PCB edge connector is its durable and high conductive properties. Generally, gold plating is of two types, the first one is soft gold or Electroless nickel immersion gold (ENIG) and the second one is Electroplated hard gold. The thickness of ENIG is generally 1U” or 2U” or 3U” and the thickness is much less when compared to hard gold. Hard gold thickness ranges from 20U” to 50U”.

Designing a PCB edge connector between two edges on a PCB, using hard gold plating improves the conductivity and durability over wear and tear issues over time. This makes the PCB have over 1000 mating cycles for a PCB and connector. Board cutouts are provided to aid the PCB to sit on the motherboard with rigid support.

Bevelling of PCB Board Edge Connectors

The bevelling process is also known as chamfering where the design considerations need to be accommodated according to the manufacturing process and the mating connector on the motherboard.

motherboard

The Standard Bevelling angle is 30 degrees whereas the special manufacturing process and connector demands 20 degrees, 45 degrees, or 60 degrees. These angles are decided in the design phase as the gold finger demands certain clearance from the edge which also should not exceed the limit. The closer thickness of the edge is a minimum of 0.25 mm.  The thickness of the PCB edge connector can be 1.6 mm,  2 mm, or 2.4 mm. The angle of the chamfer design and capabilities varies from one PCB manufacturer to the other. The chamfering process is difficult for PCBs with dimensions smaller than 50 mm x 50 mm. The tolerance of the chamfering process varies from 1 percent to 10 percent and more than that is not recommended by the IPC standards. The image below depicts the chamfering angle calculations by

chamfering angle calculation

Depth of the chamfer L = ((D/2) – (T/2))/t (g * a)

Where D is the thickness of the PCB, t is the edge thickness of the PCB,

‘a’ is the chamfer angle

A notch or a cut is provided for the PCB edge to accommodate the connector design with curved edges or chamfered edges with an angle.

Classification of PCB Edge Connectors

PCB edge connectors or the gold fingers are classified into three types based on the design aspects, they are:

  • Regular gold fingers: Regular gold fingers are the standard gold finger design practice with equal pad lengths and symmetrically spaced on the edge of the PCB. These designs are used in graphic cards, network devices, etc
  • Segmented Gold fingers: Segmented gold fingers have similar arrangements to regular gold finger circuits. The design of a few pads has a small slit on them and is electrically separated in connection.
  • Long – short gold fingers: Unlike Regular or segmented gold fingers the gold pads do not have even height. These are used for applications like USB drivers, memory cards, and card readers.

Regular gold fingers

Advantages of PCB Edge Connectors

  • Gold finger connectors are used to connect cards, PCle, PCI, USB, and storage devices to the other board.
  • Edge PCB connectors are placed on the edge of the board and no need for an external manual connector and saves the cost of the harness
  • The conductivity of the gold finger is large and the hard gold is durable with many mating cycles providing excellent performance.
  • Edge PCB connectors are designed as solder pads and the edges need to be chamfered to attain the fit and finish for the mating purposes. They are mounted on the top of the connectors making electrical contact points.
  • Continuous plugging and unplugging of PCB edge connectors are reliable and this is achieved without any significant damage to the contact points. Once after mating with the connector the rigidness of the PCB tends to provide firm support to the PCB.
  • Though gold is a costly metal for plating, the performance is more with saving the cost of components, and wire harness, and the size of the entire product reduces to a compact form factor.

Applications of PCB Edge Connectors

PCB edge connectors are widely used in a lot of industrial areas, especially they are commonly seen in industrial equipment, computers and laptops. We are a few:

  • Telecom applications for USB and HDMI cables for connecting with Laptop or PCB.
  • Memory devices like SD cards in Camera, graphic cards, etc use an Edge connection.
  • Advanced industries use PCB edge connectors for high-speed and high-precision applications.

circuit board edge connector

Design Rules for Circuit Board Edge Connector

The electrical connections are on the edge of the PCB, therefore certain standards have to be followed to keep up with the design needs. There are a few standard rules that have to be followed irrespective of the application of the PCB edge connector and they are:

  • The spacing between the edge of the connector and the PCB board edge connector must be at least 0.5 mm.
  • Surface mount devices, through-hole vias, and solder pads should be placed at a minimum distance of 2 mm from the gold finger.
  • The chamfering process on the edge of the PCB might expose the inner copper layers and therefore they have to be removed in the design phase to avoid unnecessary contact. The process of removing copper is of two types: full-finger type removal or half-finger type removal. Copper removals help in controlling impedance in gold finger and also provide ESD (Electro static discharge) protection.
  • The height of the common gold fingers should not exceed 40 mm as the circuit board edge connectors on the main board do not require more than this.
  • The Design should have a solder mask opening along the edge of the PCB.
  • Through-hole vias with plated manufacturing process are not supposed to be placed near or on the circuit board edge connector.
  • The clearance of these gold fingers should be set with a certain clearance from the solder mask and silk screen to avoid overlap
  • The edges of the PCB are beveled so the gold fingers must be positioned in the opposite direction
  • The chamfer angle on the edges of the connector is set to avoid breakage and weaker links. Standard designs have an angle of 45 degrees and in a few applications, the angles can be adjusted based on the connector design specifications.

If the design rules are not accommodated in the PCB design, then it might lead to electrical, and mechanical stress. The purpose of using hard gold instead of nickel, or copper, or other alloy is because the nature is to provide more strength and conductivity. The design has to be mentioned with the thickness of the gold. Though gold is one of the most expensive metals it is used because it makes proper contact with the PCB board edge connector and is resistant to corrosion ensuring a longer lifetime for the PCB.

Manufacturing Standards for PCB Edge Connector

Institute of Printed Circuits (IPC) standards provide a certain set of guidelines for designing PCB edge style. IPC standards cover all the properties for manufacturing to ensure quality.

  • The chemical composition of the gold should contain 3 to 15 percent of cobalt depending on the thickness and this ensures rigidity along the PCB edges.
  • The thickness of the gold can vary from hard gold and soft gold. Soft gold is usually 1 U” to 3 U” whereas hard gold is up to 50 U”. Soft gold materials are used for initial prototype testing and also a few applications where the mating cycle is not more. Hard gold is plated and checked with the connecting edges if the mating cycle is appropriate.
  • Optical inspection is done with both an AOI  (Automated Optical Inspection)machine and a manual microlens to ensure the finish and clearance.
  • IPC recommends a tape test where an adhesive tape is stuck and pulled off on the edges of the PCB. This ensures the adhesion of the gold fingers and the substrate is firm and the traces and solder mask also do not peel off.

The gold plating process is completed after the solder mask process. The PCB edge connector pads are plated with nickel for 1 U” to 3 U” for better reliability and adhesion to the gold. The hard gold plating is followed up with a little mixture of cobalt in the composition. The bevelling process is done after the plating of the gold fingers and then sent for inspection.

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