Pushing Force in SMT & COB

Pushing force is an important setting in both SMT placement and COB die bonding. When working with flexible PCBs or rigid-flex circuit boards, you need to control this force carefully to avoid damaging the board, keep the placement accurate, and make sure the product stays reliable over time.

Pushing Force in SMT (Surface Mount Technology)

In SMT assembly, pushing force is simply the downward pressure the machine applies when placing a component onto the solder paste on a flex PCB. Controlling this pressure helps the solder form properly and prevents damage to the flexible board.

Why pushing force matters in SMT

Purpose: Ensure the component’s solder pads or leads make full, even contact with the solder paste so that after reflow, the solder wets properly.

If the force is too low:

• Poor contact

• Floating pins

• Risk of component tilt or open joints

If the force is too high:

• Solder paste is squeezed out

• Solder shorts or bridging

• On flex PCBs, the substrate may warp, twist, or deform

Why flex PCBs require gentler force

Flexible circuits are thin, elastic, and easily deformed. Excessive pressure can stretch copper, distort pads, or cause misalignment. Therefore, an SMT factory must apply lighter, more controlled force and ensure the pressure is evenly distributed.

Typical force-control methods used by SMT factories

Soft landing control on the placement head to ensure smooth, low-pressure component seating.

Vacuum carriers or FR4 stiffeners to keep the flex PCB flat and absorb downward force.

Optimized placement force settings:

• Typically 0.3–0.8 N for small components

• Even lower force for LEDs, sensors, or COB dies

• Higher precision for fine-pitch ICs

Proper force control directly improves placement accuracy and reduces the risk of component damage, especially on flexible materials.

Pushing Force in COB (Chip-on-Board)

In COB assembly, pushing force refers to the bonding pressure applied during die attach when a bare die is placed onto the substrate using adhesive or conductive epoxy.

Effect of incorrect die bonding pressure

If the force is too low:

• Insufficient adhesive spread

• Air bubbles (voids) under the die

• Weak attachment and reliability issues

If the force is too high:

• Cracked die

• Adhesive overflow into pads, causing electrical shorts

• Risk of uneven die tilt

Additional challenges with flexible substrates

When the substrate is flexible, excessive bonding force can introduce bending or twisting during placement. This deformation may later cause:

• Wire-bond loop height variation

• Bonding angle inconsistencies

• Long-term mechanical stress on the die

This is why precise force control is essential in any COB process performed on flex PCBs or thin rigid-flex sections.

Why Pushing Force Control Defines SMT Factory Capability

For an SMT factory, controlling pushing force is not just a machine parameter—it reflects the factory’s ability to handle advanced electronics assembly:

• Higher placement accuracy

• Lower risk of component or die damage

• Reduced solder defects

• Better wire-bond consistency

• Higher first-pass yield

• Improved long-term reliability on flexible and rigid-flex PCBs

Well-controlled pushing force is especially important for miniaturized components, fine-pitch ICs, pressure-sensitive LEDs, sensors, and COB dies.

FAQ

1. What is pushing force in SMT?

Pushing force in SMT refers to the vertical pressure applied by the pick-and-place head when placing a component onto solder paste. Proper force ensures even paste contact and accurate component seating.

2. Why is pushing force important for flexible PCBs?

FPCs are thin and easily deformed. Excessive pressure can warp the substrate, distort pads, or cause solder bridging. Controlled, gentle force prevents mechanical damage and improves placement accuracy.

3. What happens if SMT pushing force is too high?

Too much force may squeeze out solder paste, create shorts, crack components, or deform a flexible PCB. It can also lead to alignment issues during reflow.

4. What happens if SMT pushing force is too low?

Low force can cause floating pins, poor solder wetting, tilted components, and open joints. Components may not stay stable during reflow.

5. How do SMT factories control pushing force on FPCs?

Factories typically use soft landing control, vacuum carriers or FR4 backing, and optimized placement force settings—often around 0.3–0.8 N for small components.

6. What is pushing force in COB die bonding?

In COB, pushing force is the bonding pressure used to attach a bare die to the substrate with adhesive or epoxy. It must be precisely controlled to avoid cracks, voids, and adhesive overflow.

7. Why does pushing force affect wire bonding quality?

If the die is tilted or the substrate bends due to excessive bonding force, wire-bond loop heights may vary, leading to weak or inconsistent bonding.

8. How does proper pushing force improve yield?

Accurate force control reduces solder defects, prevents component damage, stabilizes die bonding, and ensures consistent wire bonding—directly improving first-pass yield and long-term reliability.