Create robust, durable overlap ZIF connections on flexible circuits in four steps

Create robust, durable overlap ZIF connections on flexible circuits in four steps

November 17, 2015 in Design Tips

Because their design withstands the demands of flexible circuits, Zero Insertion Force (ZIF) connectors are a popular choice for attaching these circuits to other electronics.

A ZIF connector is similar to a card edge connector for printed circuit boards (PCBs), but takes into account the specific connection needs of flexible circuits. The most common connector accepts a 0.3 mm thick circuit and is held in place by friction or a clamp down strain relief. The principals of overlap ZIF design apply to circuits with typical 1.0 mm and 0.5 mm pitches, as well as smaller, denser patterns.

At Tramonto Circuits, we’ve designed a four-step process that can guide any overlap ZIF design. It starts with knowing the application.

Step 1: Understand the application

Before designing a ZIF connection, it’s critical to understand the application. Will the circuit route directly into the connector? Will it make a slow, soft bend into the connector? Or will it make a hard, sharp bend due to the lack of physical space? These questions help designers identify any unique connection challenges that might occur or affect performance.

Step 2: Follow manufacturer specifications for width and tolerance

Following the connector manufacturer’s specifications for width and tolerance is a key part of ZIF design. Don’t compromise here, because width and its tolerance are the first things that will cause trouble in any ZIF design. Any difference in tolerance will allow the circuit to slide around in the connector, causing intermittent shorts or a cable that doesn’t fit into the connector at all.

Step 3: Design to thickness specifications

Once the pitch and width are designed properly, the overall thickness is the next essential specification. The most common thickness specification for ZIF connectors is 0.3 mm (0.0118”). This does not match a common thickness of flexible circuits, which is .006” – .010”. To create the proper thickness, it’s typical to add a polyimide or polyester “stiffener” under the connector pattern. See image 1-1.

ZIF Stack-up

Image 1-1

In the image, the thickness of the polyimide ZIF stiffener was left off purposely. This allows the thickness to be adjusted to the construction of the circuit, whether it’s a single-sided circuit as is depicted above, or a double-sided or multi-layered circuit. The thickness requirement of ZIF connectors forces us to do some calculation in order to meet the specification with the materials available. The thickness of the polyimide stiffener would need to be either 0.007” or 0.008” to meet the thickness requirement of 0.0118” +/-.001”. Another option, not shown in the image, would be to adjust the adhesive thickness.

Step 4: Employ overlap for the most robust design

Once the specifications, width and thickness have been met within the required tolerance range, the final step in ZIF design is to make the connection as robust as possible. The orientation of the copper traces relative to the bend areas of the circuit are an important detail.

Robust design goes beyond simply lining up edges on top of each other. Instead, it uses a coverlay opening to expose the copper fingers for the ZIF connector, and a stiffener to create the proper thickness. If we line up the stiffener edge with the coverlay edge with the trace to pad junction, we will almost certainly see fractures of the trace at the pad junction. The most robust design calls for the pad to overlap the coverlay opening, and the ZIF stiffener to overlap the pad/trace junction. This allows the circuit to be inserted without fear of broken traces during insertion. It also protects the circuit in dynamic applications or those times when a sharp bend is necessary to insert the flex into the connector. See image 1-2.

 ZIF Stiffener Overlap

Image 1-2

With these four steps, designers can build a robust, durable ZIF connection for flexible circuits. For more about how Tramonto Circuits designs to application, click here.

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