Robust Design for ZIF connectors
ZIF connectors are a very popular choice for flexible circuit attachment. The choice allows a seamless method that utilizes the nature of the flexible circuit very well. Most connectors are designed for either 1.0 mm or 0.5 mm pitch, however smaller more dense patterns are also available.
A zif connector is similar to a card edge connector for PCBs. The connector itself accepts a 0.3 mm (0.0118”) thick circuit (most common) and the cable is held in place by friction or in some more extreme cases, by a snap down strain relief. The difference of course is that a rigid PCB once plugged into a connector, is very stable. The flexible circuit on the other hand is by its nature, well, flexible. Let’s discuss further.
After the size of the connector has been chosen there are a few things to consider before jumping into your flexible circuit design. 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? All of these scenarios have been used extensively over the years and most of the issues with each have been resolved. There are always going to be new applications that challenge our design’s robustness, but if you follow the rules that follow in a generic manner, you’ll be able to tackle any situation that arises.
The first and most important message I would like to mention is this: Follow the manufacturer’s specifications for width and tolerance closely! Don’t compromise here. The width and its tolerance are the first thing that will cause grief in any ZIF design. Any compromise on the tolerance will allow the circuit to slide around in the connector causing frustrating intermittent shorts, or worse yet, not allow the cable to fit into the connector at all.
Once the pitch and width are designed properly the overall thickness is the next specification that must be met always. A rigid PCB edge connector is designed to accept a common thickness of .062” material. The most common thickness specification for ZIF connectors is 0.3 mm (0.0118”). This does not match a common thickness of flexible circuits. So we must typically add a polyimide or polyester “stiffener” under the connector pattern to create the proper thickness consistently. See image 1-1.
As you can see in image 1-1 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. In the image above 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”. The thickness requirement of ZIF connectors forces us to do some calculation in order to meet the specification with the materials available. In this case we have two choices and the overall thickness can be adjusted with the polyimide thickness or the adhesive thickness.
Once the specifications, width and thickness have been met within the required tolerance range, the next and last thing we need to do is make that connection as robust as possible. Those of us who have designed flex circuits for any period of time know the orientation of the copper traces relative to the bend areas of the circuit are an important detail. Also, we know that we can’t simply line up edges on top of each other and expect robustness. In this case we’ll have a coverlay opening to expose the copper fingers for the ZIF connector and a stiffener to afford us 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 from misalignment 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.
As you’ve noticed by now, none of these design tips are difficult to put into practice. Sure we have some math to do and must know the application well in order to maintain the integrity of the circuit in practice. But much time will be saved in development if you learn these three tips and utilize them in “every” ZIF connector designed into your products. Of course be sure to consult your flex manufacturer and design comfortably with ZIF connectors.