FPC Stiffener SMT Sequence: Avoid These Costly Mistakes in Flexible PCB Assembly

In FPC (Flexible Printed Circuit) design, the stiffener application sequence is crucial—but often overlooked. A small mistake in the sequence can cause issues ranging from failed SMT (Surface Mount Technology) placement to complete product scrap. But why does this happen? It could be due to an improper bonding order of the stiffener, which can lead to a host of problems such as board warping, component lift-off, and even total failure after the SMT process.

Today, we’re uncovering the mystery behind the correct stiffener application sequence to help you avoid these “hidden traps” in your design process.

Siffener Bonding Mistakes for SMT Boards

Case 1: Stiffener on the Component Side Causes Uneven Solder Paste During SMT Printing

When the stiffener is designed on the component side, it can create an uneven surface, making SMT placement impossible due to inconsistent solder paste distribution.

Solution:

Avoid designing stiffeners on the component side. If necessary, perform SMT first, then apply the stiffener using 3M double-sided adhesive to prevent issues and reduce the need for rework.

Case 2: Excessive 3M Adhesive Area Causes Bubbling, Shrinkage, and Yellowing

Applying too much 3M adhesive before SMT can cause issues like bubbling, shrinkage, and yellowing after exposure to high temperatures.

Solution:

Segment the adhesive design to prevent warping and bubbling after SMT. Alternatively, consider performing SMT first, followed by adhesive application to ensure better results.

Case 3: 3M Adhesive Between Pins of an SMT Component

Placing adhesive between the pins of an SMT component can cause the component to lift during the soldering process, leading to placement failure.

Solution:

Avoid placing any adhesive between the pins of SMT components. Ensure the stiffener is positioned away from these critical areas to prevent component lifting.

Case 4: Excessively Thick FR4 Stiffener Prevents Proper SMT Placement

If the FR4 stiffener is too thick, it may prevent proper SMT placement after attachment.

Solution:

Choose to perform SMT first, then apply the stiffener. The FR4 stiffener can be attached using 3M double-sided adhesive to ensure it doesn’t interfere with the SMT process.

Case 5: Steel stiffeners positioned on the component surface interfere with solder paste deposition during SMT processing, resulting in uneven solder distribution and component placement failure

Solution:

Follow the “SMT first, then apply the stiffener” method. Ensure that the stiffener is attached using 3M double-sided adhesive to avoid soldering issues.

How to Determine Whether to Apply the Stiffener Before or After SMT

The key to the stiffener bonding sequence is whether the stiffener and SMT pads are on the same side.

When Stiffener and SMT Pads Are Not on the Same Side

If the stiffener and SMT pads are on different sides of the board, you can proceed normally by selecting the "apply stiffener first, then SMT" sequence when placing the order.

When Stiffener and SMT Pads Are on the Same Side

This situation requires more attention, as the bonding order will depend on the type and position of the stiffener.

（1）FR4 Stiffeners (thickness ≥ 0.4mm):

Choose “SMT first, then stiffener” and apply using 3M9077 adhesive.

（2）FR4, 3M9077, Steel Sheet, or PI Stiffeners (thickness < 0.4mm):

If the stiffener is at least 20mm away from the SMT pad, you can apply “stiffener first, then SMT.”

If the stiffener is within 20mm of the SMT pad, apply “SMT first, then stiffener” using 3M 9077 adhesive.

（3）3M468 Adhesive (Non-Heat Resistant)

Always choose “SMT first, then stiffener” as 3M468 is not suitable for use during the reflow process.

（4）Discoloration of 3M9077 Adhesive

If discoloration (yellowing) of the 3M9077 release liner occurs during reflow, opt for the “SMT first, then stiffener” method.

FPC + SMT Board Design Considerations

1. Avoid Using Stiffeners on the Component Side: FR4, steel sheet, 3M adhesive, and PI stiffeners should be avoided on the component side whenever possible.

2.Selecting the Right Adhesive: When applying adhesive before SMT, use 3M9077 high-temperature adhesive. Yellowing, bubbling, or shrinkage after high temperatures is normal and does not affect the quality.

3. No Stiffener Between Chip Leads: Never place a stiffener between two rows of chip leads. If absolutely necessary, it should avoid the component housing. Mark the design as “Stiffener to be applied after SMT” and use 3M adhesive for bonding.

In FPC design, even a small mistake in the stiffener application order can result in significant project failure. By following the guidelines above, you can prevent the stiffener bonding sequence from becoming the “hidden bomb” that derails your project.