In the world of PCBs, designers are consistently in search of the perfect balance. Whether they are perfecting their design to balance functionality and performance or connectivity and depanelization, in either case, balance is the essential component.
For example, a design might achieve the highest level of performance, but it may still fail in terms of DFM or DFA. While there will always be trade-offs in the field of PCBs, striking that perfect balance for your design should be the goal of every PCB designer. And this extends to connectivity. Although trends such as IoT promote the use of wireless data transfer, there are still many cases where physical connections provide the best balance between functionality and manufacturability. And, one of the best physical options available is gold fingers.
What are Gold Fingers?
Aside from its aesthetics and rarity that make gold one of if not the most sought after elements in the world, gold has some electrical attributes that set it apart for electronics. These include its high thermal conductivity of 310 W/(mK) at room temperature–about 20 °C–and resistivity of 0.022 µΩ-m. Additionally, gold is not susceptible to oxidation, which can be a significant problem for copper. These properties make gold highly prized for use in electronics and on PCBAs.
Although gold conductors are excellent for trace routes, copper is a superior choice.. But complex or compound systems where direct mechanical interface between boards is necessary require exposed conductive surfaces that will not oxidize. Gold fingers provide a means of achieving connectivity without the need for cabling. Gold fingers are specially plated gold connectors situated at the edge of circuit boards that typically insert into other boards.
The choice to use gold for these connecting points requires balancing the considerations listed below.
PCB Gold Finger Design Considerations
- Ease of manufacturing
Gold Finger Plating Processes
The most common plating option for gold fingers is “hard gold”. This involves applying a thin layer of “hard” gold (an alloy of gold and cobalt or nickel) which is good for cases where the connectors will go through removal and insertion many times during the board’s operational lifetime. However, this method is too costly to use for the entire board. The more common electroless nickel immersion gold (ENIG) plating is more cost-effective and solder-friendly, but too thin and mechanically weak for a connector surface.. Therefore, the best option is usually a compound process with ENIG for the majority of the board surfaces and a layer of hard gold at the gold fingers only.
How to Best Use Gold Fingers for Your Design
Gold fingers can aid your design’s performance and reliability; however, it will also increase manufacturing costs. Therefore, prior to opting for these connectors you should adopt and follow guidelines to ensure compliance with industry standards and that your CM employs a process that will meet your performance and reliability requirements.
DFM for HDI Printed Circuit Boards
Relevant IPC Standards
- ★ IPC-A-600J Acceptability of Printed Boards
- ★ IPC 6012E Qualification and Performance Specification for Rigid Printed Boards
- ★ IPC-4556 Specification for Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Plating for Printed Circuit Boards
PCB Gold Finger Design Guidelines
- Do not use solder masking or silk screening in the gold plate area.
- Always point gold fingers away from the board center.
- No vias can be used in the gold finger area.
- You may use features up to a depth of 25 mm on both sides of a gold finger plate.
- Gold fingers should include between 5 to 10 % cobalt to aid with rigidity.
- Thickness should be carefully considered as insertion into some connector is required. Gold fingers should be no thinner than 2 microns and no thicker than 50 microns.
- Consider a chambered board edge to facilitate smooth insertion.
|Tempo's Custom PCB Manufacturing Service
When opting to use gold fingers, you should work closely with your CM to make sure that your design intent is a top priority and that the manufacturing process meets industry standards. At Tempo Automation, the industry leader for building boards of all levels of complexity fast and precisely, we will work with you to ensure that your boards meet the highest quality standards.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on CAD files or how to incorporate your design into a CAD format, contact us.