Printed circuit boards (PCBs) are the backbone of electronics, providing the foundation for mounting and interconnecting electronic components. However, a bare PCB is rarely usable by itself. Some type of mounting scheme is required to securely install the PCB in a product chassis or enclosure.
One popular PCB mounting technique involves the use of small plastic or rubber feet attached to the underside of the board. These feet, aptly named PCB mounting feet, serve multiple important functions. In this comprehensive guide, we’ll explore what PCB mounting feet are, their different types and materials, installation best practices, and how they benefit electronic assemblies.
With the right choice of mounting feet, you can achieve secure PCB fixing that absorbs shock/vibration, allows for board expansion, simplifies assembly, and improves product reliability. Read on to learn more about this indispensable PCB mounting hardware.
What Are PCB Mounting Feet?
PCB mounting feet are small plastic or rubber supports that attach to the underside of a printed circuit board via self-adhesive, press-fit or screw-on methods. Usually square or circular in shape, these feet sit protruded from the rest of the PCB surface.
When the board is placed inside a device case or chassis, the feet bear the weight of the PCB and maintain a small clearance gap between the board and enclosure interior. This gap prevents the PCB from banging against other components or the enclosure walls.
The feet provide both secure fixing and mechanical isolation for the board. They take the brunt of any vibrations, shocks or stresses on the product that could otherwise damage the fragile PCB or its soldered joints. Mounting feet make up a simple but highly effective PCB mounting scheme.
PCB mounting feet provide secure fixing and shock absorption
Functions of PCB Mounting Feet
Why are PCB mounting feet so prevalent across consumer and industrial electronics? What core functions do they serve?
Securing the PCB
The primary job of mounting feet is fixing the PCB in place. The feet adhere to the board on one side while the other side presses against the chassis surface. This sandwiches the PCB securely between the feet and case interior.
Absorbing Mechanical Stress
The feet act as shock absorbers, flexing to dampen vibrations and stresses. This protects solder joints from fatigue failure.
Allowing Board Expansion
Their flexibility accommodates thermal expansion/contraction of the PCB without distortion or component fracture.
The foot height creates a small gap between board and case. This prevents short circuits and collisions.
Most feet are non-conductive, avoiding any electrical contact between PCB and chassis.
Feet speed up PCB mounting compared to fasteners or other means.
All these benefits combine to help the PCB withstand rugged conditions, extending product lifetime.
For such a small component, mounting feet deliver considerable functionality!
Types of PCB Mounting Feet
Several varieties of mounting feet are available depending on factors like PCB size, weight, required adhesion strength, operating temperature range and cost considerations:
self-adhesive Mounting Pads
- One-piece, flat rubber pad with permanent adhesive backing
- Available in different thickness and adhesion strengths
- Quick installation by peel and stick method
- Limited vibration resistance due to hard rubber
- Rigid plastic legs press-fit into plated PCB holes
- Tight fit secures spacer mechanically without adhesive
- Provides excellent holding strength
- Some designs allow screw-down capability
- Threaded metal or nylon standoff screws into a mounting hole
- Strong adhesion and can be adjusted or removed
- Allows stackability of multiple PCBs if needed
- Most costly option
Dual Adhesive Grommets
- Outer adhesive ring bonds to PCB
- Inner adhesive post attaches to chassis surface
- Cushions shocks and isolates board
- Often used for LCD module mounting
Push-In Mounting Feet
- Press straight into plated PCB holes for simple installation
- Molded ridges provide retention
- No adhesives involved
- Limited holding strength
In summary, self-adhesive feet prioritize ease of use while press-fits and screw-ons offer higher strength at increased cost. Select the variety that best matches your requirements.
PCB Mounting Feet Materials
Mounting feet composition affects their performance and suitability for certain applications:
Made from molded silicone rubber, gum rubber or natural rubber blended with fillers. Properties include:
- Excellent shock absorption and vibration damping
- Stays flexible across wide temperature range
- High chemical resistance
- Low cost
- Limited adhesive bond strength
Thermoplastic Polyurethane (TPU) Feet
Blends polyurethane with rubber elasticity. Offers:
- Abrasion/tear resistant
- Transparency options
- Resilient but lower damping than rubber
- Withstands exposure to grease, oils
A proprietary viscoelastic polyurethane polymer. Provides:
- Superb vibration isolation over 10-2000 Hz range
- High load capacity and durability
- Retains properties from -65°F to 200°F
Made from silicone rubber elastomer. Ideal for:
- Wide operating temperature range (-100°F to 500°F)
- Extreme environments
- High purity applications
- Good chemical resistance
Consider shock damping needs, cost, temperature resistance, and chemical compatibility when selecting foot material.
Key Properties of Mounting Feet
Several performance metrics characterize PCB mounting feet and help determine which type is optimal:
- Hardness – Resistance to indentation. Measured in Shore A, harder feet limit vibration isolation.
- Coefficient of Friction – Grippiness between feet and contact surfaces. Affects anti-slip qualities.
- Compression Force Deflection (CFD) – Force required to compress a material to a defined deformation. Related to shock cushioning capability.
- Adhesion Strength – Bonding force between adhesive feet and PCB surface, quantified in psi.
- Operating Temperature – Feet material should maintain integrity across electronics’ temperature range.
- Flame Resistance – Important for safety compliance. Measured by UL 94 flammability standard.
- Outgassing & Water Absorption – Outgassed vapors and moisture absorption can impair electronic components if excessive.
Consider the target levels for each property based on your PCB application’s requirements and operating conditions.
PCB Mounting Feet Size
Mounting feet come in a range of sizes to match different PCB dimensions and weights. Common sizes include:
- 3 to 6 mm height – For small, lightweight boards
- 6 to 10 mm height – General purpose size range
- 10 to 15 mm height – For larger or dense boards
- Over 15 mm – Maximum heights for heavy boards in high vibration environments
The feet area or diameter can range from:
- 4 to 8 mm – For minimal PCB intrusion
- 10 to 18 mm – Standard footprint size
- 20 to 25 mm – Used singly or in pairs for extra large boards
Aim for three or four evenly spaced feet around the PCB perimeter. Too many feet can cause board bowing while too few can allow excess flexing.
Installation Best Practices
Follow these guidelines when attaching PCB mounting feet:
1. Determine Foot Positions
Place feet near board edges and symmetrical across the center point. Avoid component interference.
2. Clean PCB Surface
Remove any grease, solder flux or debris from PCB footprint area to maximize adhesion.
3. Follow Foot Specifications
Adhere to instructions for attachment pressure, temperature and cure times.
4. Apply Even Pressure
Use a roller tool or press to apply consistent pressure and flatten adhesive feet.
5. Allow Proper Cure Time
Let adhesive fully cure before stressing feet. Insufficient cure weakens bond.
6. Apply Thread Locking Compound
For screw-on standoff feet, use thread lockers to prevent loosening from vibration.
Following the manufacturer’s specifications for foot installation ensures optimal bonding strength, positioning and reliability.
Benefits of Using PCB Mounting Feet
Why choose mounting feet over other PCB retention methods like clips, screws or rails? Here are the main advantages:
Simple Installation – Just peel and stick adhesive feet for fast assembly compared to fasteners.
Lower Cost – Feet are an affordable mounting solution, saving on machined brackets or specialty hardware.
Reduce Stress – Absorbs mechanical shocks/vibrations instead of transferring those forces to solder joints.
Tolerate Thermal Expansion – Cushion feet accommodate expansion/contraction of PCB without distorting or fracturing the board.
Avoid Short Circuits – Feet establish safe clearance between conductive PCB surfaces and case walls.
Reusable – Removable press-fit and screw-on feet allow easy PCB removal without damage.
Lightweight – Minimal added weight, important for mobile and aerospace applications.
For most electronic devices, mounting feet provide a simple yet effective method of securing PCBs.
PCB Mounting Feet – FQA
Here are answers to some frequently asked questions about PCB mounting feet:
How thick should mounting feet be for a given PCB?
As a general guideline, foot thickness should be 1-2% of the longest PCB dimension. For example, a 6 inch long board would use 0.06-0.12 inch thick feet. Smaller PCBs need a higher foot thickness to board length ratio for stability. Larger boards can use slimmer feet proportional to their size.
Are adhesive mounting feet removable?
Most adhesive feet forms a permanent bond with the PCB surface and are not designed to be removed once applied. However, some acrylic-based adhesive feet can be detached with sufficient force, although this may damage the PCB footprint area in the process. For removable mounting, opt for press-fit or screw-on feet instead.
Can I use just two mounting feet instead of four?
You can get away with using just two mounting feet on smaller, lighter PCBs. However, four feet spaced symmetrically around the board perimeter provides the most secure, stable installation. With only two feet, the board is more prone to pivoting, flexing or detaching from impacts. Four points of contact prevent teetering and reliably fix the PCB position.
How are mounting feet different from rubber bumpers?
While rubber bumpers can also be attached to PCBs, their function differs from that of mounting feet. Bumpers serve mainly as protectors that protrude past the edge of the board to absorb side impacts in handling. They do not provide the same downward pressure retention of feet that adhere to both PCB and chassis. The two can be used in conjunction for maximum impact protection.
Can I mount stacked PCBs using longer mounting feet?
Yes, elongated mounting feet allow securing a vertical stack of two or more stacked PCBs in proper alignment. Use screw-on standoffs with enough height to span the PCBA stack, applying light pressure between boards to maintain proper spacing without overly bowing or deforming the assembly.
PCB mounting feet play a deceptively important role in securing board installation while mitigating stresses that could impede reliable functionality. The variety of foot shapes, sizes and material properties allows matching feet characteristics to the specific needs of the electronics application. Proper usage of mounting feet improves quality and longevity of PCB assemblies. With knowledge of available options, you can select and implement the optimal mounting feet for your project.