SMT Spring Finger Contacts for PCB - Doigts de contact ressorts CMS pour PCB - Compelma

The What, Why and How of Spring Finger Contacts for PCB Grounding and Connection

Catalogue of Standard Spring Finger Contacts

What is a Spring Finger Contact?

A spring finger contact is a small mechanical part used to make connection or grounding on electronic projects. These contacts are made to support harsh environment (humidity, temperature, and vibrations). Spring finger contacts are widely used on on-board electronic projects. Especially for automotive projects (dashboard, headlight) but also on electrical bikes projects (batteries connection), medical applications (sensors) and military project (weapons or vehicles).Spring finger contacts are a type of connector used to make electrical contact with a printed circuit board (PCB). PCB spring finger contacts provide an easy, low-cost way to ground circuit board and to prevent EMI noise and static electricity on an electronic assembly. These spring parts are designed to make reliable contact between two PCB or from a PCB to the frame, in harsh conditions (vibration and humidity). Spring fingers can be used both for grounding and connection purposes. Spring finger contacts come in a variety of different shapes and sizes. In this blog post, we will explain the what,why and how of spring finger contacts for PCB grounding and connection.

What is the composition of a Spring Finger Contact?

Core material:

Beryllium Copper (CuBe), standard and most common. This material is a kind of bronze.

o  Excellent mechanical characteristics (>300k cycles)
o  Good thermal conductivity
o  Good electric conductivity (electrical resistance is few milliohms)
o  Easy forming

Stainless Steel, less common

o  Lower mechanical characteristics than Beryllium Copper (CuBe)
o  Lower electric conductivity

Other metals and alloys can be used. However, not using standard material might lead to higher costs and worst characteristics (as electrical than mechanical characteristics).That is why we recommended to use as much as possible Copper and Beryllium (CuBe) as core material for your spring finger contacts (for standard contacts as well as for your custom parts).

Inner Electroplating (mandatory/standard):

Ni (Nickel)

o  Protect the inner core (CuBe) from oxidation
o  Ensure a high adhesion force between the electroplating layers (with the optional gold electroplating for example)
o  Withstand high friction level
o  Good weldability on PCB

Outside Electroplating (Optional):

Au (Gold)

o  Higher resistivity to bad exterior environment (humidity, temperature)
o  Higher conductivity (gold is a highly conductive material)

Sn (Tin)

o  Good weldability
o  Poor resistance to oxidation, especially in high humidity environment

Other electroplating possible on demand

We strongly suggest to use standard as much as possible

About Optional Surface Treatment

The spring finger contacts can be delivered without surface treatment on the outside (Au/Gold plating). Indeed, the price of the raw material (gold) is high and volatile comparing to the raw material CuBe (Copper Beryllium) used for the core of the spring contact. In some cases (depending on the volume and the design), the gold surface treatment can even double the cost of the spring finger contact (due to the raw material cost).

How are manufactured spring finger contacts?

Machine for spring finger contact manufacturing - Machine pour contacts ressorts - Compelma

The manufacturing of spring finger contacts starts with metal strips of several thousand of meters of Copper Beryllium (the raw material for the core of the spring contact). The thickness of the strip is 0.08mm or 0.1mm depending on the design of the spring finger contact.

Then this strip of material goes into a specific machine with tens of tools arranged in circle. Each tool does a specific operation (a blending, a cut, or a fold) in a specific order.

After the different operations the spring finger contact has its final form. The next step is the electroplating.
Firstly, with Nickel (Ni), and if required, with gold (Au).Once the spring finger contact is electroplated,it is packaged on reel for assembly on SMT process (automatic assembly on PCB).The standard global lead time from order to delivery is 8 to 12 weeks (if there is no stock). That is why you should anticipate as much as possible all of your electronic project using this kind of spring contact.

How are spring finger contacts certified and controlled?

Spring finger contacts are widely used on many different fields of electronic. That is why site production are certified IATF16949 (strict automotive norm) and processes are certified ISO 9001. Final spring contacts are compliant to RoHS and Reach norms.

Compression tester for Spring Finger Contact - test de compression de Contact de masse ressort - compelma
Compression Tester
Gold electroplating inspection of spring finger contacts - Inspection du revêtement de surface Or d'un contact de masse ressort - Compelma
Au Electroplating Inspection
2D Auto Inspection System for Spring Finger Contacts - Système de contrôle 2D pour les contacts de masse ressort - Compelma
2D Auto Inspection System

Compelma Conflict Minerals Policy

Conflict minerals are minerals mined in conditions of armed conflict and human rights abuses, such as Tantalum (Ta),Tin (Sn), Gold (Au), Tungsten (W). The profits from the sale of these minerals finance continued fighting in War, and control of lucrative mines becomes a focus of the fighting as well. Compelma, as the citizen of the world, supports human rights, and embraces its Corporate Social Responsibility. We keep paying attention to this issue and do due diligence to ensure that Compelma and Compelma’s suppliers avoid using conflict minerals.

Why are used the spring finger contacts (SMT):

Spring finger contacts are the perfect way to make low voltage electrical connections in all types of small, printed circuit board (PCB) applications. They are easy to use. Spring finger contacts can be used for antenna feeds, grounding, or preventing EMI noise and static electricity.

Examples of use cases:

Power Connector:

Battery contact
o For Electric Bikes
o Consumer Equipment (Electrical Household Equipment)
o Camera(Digital Still, On-Board Equipment, Drone)
o  Cellular Phone

Grounding
o Contact between PCBs
o Contact between PCB and Frame

Vibration Proof Connection
o Automotive Dashboard
o On-Board Equipment

Signal Connector

Antenna Spring Connection
o Cellular Phone Antenna
o GPS Antenna
o Automotive Antenna

Mechanical Connector

Vibration Proof Connector
o Automotive Equipment
o Military Project and Weapon Equipment
o Aeronautic Equipment and Sensors

Plug And Play Setup
o Modular Connection
o Battery Connection
o Wire Clampers

How are spring finger contacts used?

Spring finger contacts can be used on different setup and configurations. Depending on the shape and the design of the spring contact, the electrical contact is made vertically or laterally on the PCB.

Different setups of spring finger contacts - Différentes application de contacts ressorts CMS - Compelma

These kinds of spring finger contacts can handle over 300 000 compression cycles. However, the maximum recommended compression rate is 40%. It means that for a 10mm height spring finger contact; the recommended maximum compressed height is 60mm. Over 40% compression the spring finger contact might be definitively deformed. So, you will lose the spring effect of this contact and the electrical contact might be lost. To help you to determine the best compression rate and the corresponding compression strength: each spring finger contact is delivered with a graphic showing the Compression Strength Vs Compression Distance.

Compression Distance Vs Compression Force - Spring Finger Contact - Compelma

Spring finger contacts are made to be surface mounted. That is why you can see on the internet these parts named: SMT spring finger contacts (SMT: Surface Mounted Technology).To permit SMT process assembly (automatic assembly on PCB), the spring finger contacts are packaged on reel. There are 2 standard sizes of reel for these spring contacts: 7’’ or 13’’ diameter. The size of the reel and the number of spring contacts on each, depends on the sizeof the spring finger contacts packaged. It starts from 500 contacts/reel for the biggest spring contacts, to 5 500 contacts/reel for the smaller ones.

Spring Finger Contacts on a finger - Contacts de masse ressorts sur un doigt - Compelma

For prototyping steps, these spring finger contacts can be delivered on bulk packaging. That way, you can manually test different designs of spring finger contacts.For each spring finger contact, there is a recommended PCB layout recommended. In main cases it is at least 0.5mm much than the base of the spring finger contact. For each design of spring finger contact, we indicate the recommended PCB layout recommended.

PCB Recommended Pad Dimension - Dimensions de pad recommandées sur le PCB - Compelma

Once on the PCB, the spring finger contact are welded, following a specific temperature and speed process. This process is detailed on the datasheet of each spring finger contact. The Nickel contained on surface of the spring finger contact makes welding easier. You can use standard welding material (as Copper and Tin alloys).

Welding of SMT Spring Finger Contact - Soudure des contact de masse ressorts CMS - Compelma

What are the limits of the spring finger contacts?

The thickness of the strip of CuBe material used to manufacture these parts is 0,08 or 0,10mm (depending on the design of the spring finger contact). So, these spring contacts cannot support high voltage and electrical tension. We recommend not to go below 10A/part. These finger contacts are also sensitive to violent (quick and high) variation of tension.

The maximum height of our standard spring finger contact is 13mm. Some custom spring contacts can go below this side. For example, we have developed a custom contact of 22mm height (used on 1 automotive project and 1 medical application).

From a mechanical point of view, maximum compression for spring finger contacts is 40%. For custom spring finger using a thicker material (ex: 0.15mm) the recommended compression rate might be lower than 40%. Indeed, if the material is thicker, the spring elasticity will be worse than with a thinner material.

Mechanical simulation of spring finger contact - Simulation de compression de contacts ressorts - Compelma

Mechanical simulation can be done on custom made spring finger contacts, to evaluate the elasticity and the recommended compression rate.

How to resume spring finger contacts?

Spring finger contacts are an alternative to EMI Gaskets (Loaded Silicone or Metallic Gaskets) for EMI shielding and grounding. These spring finger contacts are also used as connectors. Spring finger contacts are developed for highly efficient manufacturing process. Indeed, these contacts are SMT friendly and packaged on reels (7’’ or 13’’ depending on the spring finger contact reference).

The design of the spring finger contacts allows more flexibility on electronic projects. Especially for electronic projects with thin and light weight requirements.

Most of the spring finger contacts (SMT) are manufacturing in CuBe (Beryllium Copper), Au (Gold) electroplated outside, and Ni (Nickel) electroplated inside. These surface treatments on the spring finger contacts have different advantages:

-         Better electronic conductivity
-         Higher resistance on high humidity and temperature
-         Better weldability on the PCB

Design of the spring finger contact can be standard,or custom-made following your specifications.

What about spring finger contacts for your electronic project?

Spring finger contacts can be used in all types of small, printed circuit board applications. But it can be hard to find the right ones for your specific application. You need a reliable, low-cost way to ground your circuit boards and prevent EMI noise and static electricity.

It seems like every time you need spring finger contacts, you must search through a million different products to find the right ones. And once you do find them, they might not be compatible with your specific application.

That is why Compelma is here to help you to solve all your spring finger contact needs. We have a huge selection of contacts that are proven on many industry fields and electrical projects. Plus,we have established production capacity to support you from prototyping to mass production. As example we are providers of the biggest automotive suppliers (big volumes of standard parts) as well as the military industry using small amount of specific spring contact. To complete our standard range, we offer a wide range of customization options so you can get just what you need for your electric project.

We can provide prototype parts using soft tooling. It means no cost for a tooling, but a price/part higher. This is the ideal solution to validate your design before engaging money on a mass production tooling to get a low price/part.

Do you want to go further? Let's have a talk on our tchat or by our form below

Catalogue of Standard Spring Finger Contacts

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Bureau d'étude CEM et thermique Compelma - EMI and thermal management