Connecting a Flex Circuit to a Rigid PCB: Five Methods

What the Interface Has to Do

Where a flexible circuit meets a rigid board, the design has to satisfy three requirements at once. It has to make a reliable electrical connection at a fine pitch, it has to survive the mechanical movement of the flex without loading the joint, and it has to remain reliable over the life of the product. Most flex failures in the field occur at this interface rather than in the middle of the circuit, which is why the choice of method matters more than the choice of flex material.

There are five established approaches, and they are not interchangeable. Each has a pitch limit, a current capability, a tooling cost and a mechanical behaviour, and the right one follows from the product.

1. Zero Insertion Force Connectors

A zero or low insertion force connector accepts the flex tail as a plug. The contacts are opened by a lever or a slider, the tail is inserted, and the mechanism closes the contacts onto the exposed conductors. The connector is usually surface mounted on the rigid board.

Its advantages are assembly simplicity, the absence of any thermal process at the joint and serviceability, because the flex can be unplugged and replaced. Its limitations are the contact resistance, which is higher than a soldered joint; the number of mating cycles the contacts will tolerate; the insertion force the operator must apply; and the fact that the connection depends on mechanical contact pressure, which means contamination and wear both affect it. It is the usual choice for a display, a keyboard or a sensor in a consumer product where the joint is assembled once.

2. Hot Bar Soldering

A hot bar machine presses a heated thermode onto the flex tail and the pads on the rigid board while solder paste or a solder coated pad provides the joint. The heat is applied locally for a short time, which limits the thermal load on the surrounding components.

It gives a soldered joint with a low contact resistance and a good current capability, and it suits moderately fine pitches. The challenges are process control and planarity: the thermode must heat the joint evenly, the flex must be held flat, and the rigid board and the flex must be supported so that the pressure is distributed. It is common in camera modules, battery connections and display boards.

3. Anisotropic Conductive Film

An anisotropic conductive film is an adhesive loaded with small conductive particles. It is placed between the flex and the board, and heat and pressure are applied. The particles are trapped between the opposing pads and conduct vertically, while the adhesive insulates laterally, so the film conducts in one direction only.

The method is capable of very fine pitch, it works at a relatively low temperature, and it produces a mechanically bonded and sealed joint, which is why it became the standard for display connections. The requirements are a precise bonding machine, controlled temperature and pressure, and a pad surface that is clean and flat. Once bonded, the joint cannot be serviced, which is the principal drawback.

4. Direct Soldering with a Stiffener

The flex tail can be soldered directly to pads on the rigid board, either by hand or in a reflow process. The critical requirement is a stiffener on the flex under the solder area, because the polyimide is too flexible to hold the pads flat and the joint will be stressed by any movement.

It is the cheapest method and it can carry significant current, which is why it appears in battery connections and in power boards. The disadvantages are the manual work, the thermal load on the flex, and the fragility of the joint if the tail is not properly strain relieved. It is a good choice where the flex does not move after assembly and the current is high.

5. Rigid Flex Integration

The most reliable interface is no interface at all. In a rigid flex construction the rigid and flexible sections are laminated into a single board, so the connection is part of the copper structure rather than a joint between two parts.

This eliminates the connector, the solder joint and the failure mode that comes with them, and it removes the assembly step. It is also the most expensive option, because the whole board has to be built on the rigid flex process, and it is the least serviceable. It is the right answer for a high reliability product with a long life, or for a product where the assembly volume is high enough that the removal of the connector pays for the more expensive substrate. Our notes on flex PCB assembly describe how these constructions are built.

Choosing Between Them

• Pitch: anisotropic film and hot bar reach the finest pitches, connectors are limited by the contact geometry, and hand soldering is limited by the operator.
• Current: direct soldering and rigid flex carry the most, connectors and anisotropic film carry less.
• Mating cycles: a connector is the only method that can be unplugged and reconnected repeatedly.
• Serviceability: connectors win, anisotropic film and rigid flex are effectively permanent.
• Environment: a soldered or bonded joint tolerates vibration and temperature cycling better than a contact interface, which can oxidise and loosen.
• Cost: direct soldering is the cheapest at low volume, rigid flex the most expensive, and the connector sits in between once the assembly labour is counted.

Design Rules at the Interface

Provide a stiffener. Any flex area that is soldered or bonded needs a stiffener under the pads, and a flex tail that plugs into a connector needs one so that it can be inserted and so that the contacts are not loaded by the film.

Strain relieve the tail. The tail should be anchored to the enclosure or the board by a feature, a clamp or a length of tape, so that a pull on the flex is carried by the anchor rather than by the joint. The tail should approach the connector straight, without a lateral pull, and the bend should be outside the joint area with a generous radius.

Keep the bonding area clean. The exposed conductors on a flex tail are the contact surface, and contamination, oxidation or flux residue affects the contact resistance. The pad finish and the cleaning process have to be specified with the interface in mind.

Design the pad geometry to the process. The pad width, the spacing and the length required by hot bar or anisotropic bonding differ from those of a connector. The flex and the rigid board have to be designed together, from the interface outward, rather than each separately. Our notes on PCB design and layout cover the layout rules.

Reliability Testing

The interface should be qualified with the test that represents its worst case: a flex cycling test that bends the tail repeatedly with the joint under load, a thermal cycling test, and a combined test where the flex is moved at temperature. Contact resistance is measured before, during and after, because the failure mode is often an increase in resistance rather than an open circuit.

A pull test on the tail and a twist test on the joint complete the mechanical picture, and a vibration test covers the condition that a connector is most likely to fail. Our PCB assembly group builds these interfaces, and our notes on PCBA testing describe how the checks are structured.

FAQ

Which method is most reliable? Rigid flex integration, because there is no joint, followed by a soldered or bonded joint. A plug-in connector is the least reliable over time but the most serviceable.

Can a flex tail be soldered by hand? Yes, provided the tail has a stiffener and the heat is controlled, but the resulting joint is more variable than a machine bonded one and should be inspected carefully.

What is the finest pitch achievable? Anisotropic conductive film reaches the finest pitches and is used in display connections. Connectors are limited by the contact design and hot bar by the thermode and the paste printing.

Why does a flex connection fail in the field? Usually because the tail was not strain relieved, so the joint carried the mechanical load, or because the connector contacts oxidised or loosened.

Do I need a stiffener on every flex tail? On any tail that is soldered, bonded or inserted into a connector, yes. The stiffener is what makes the pads flat and the joint stable.

Conclusion

Connecting a flex circuit to a rigid board is a mechanical design problem with an electrical requirement. Choose the method from the pitch, the current, the number of mating cycles and the environment, provide a stiffener and a strain relief in every case, design the pad geometry around the process, and qualify the interface with a test that moves the flex rather than one that merely measures the resistance on the bench.