Crosshatching

What’s Crosshatching in PCB?

 

Crosshatching (or Cross Hatch) of PCB planes refers to a technique wherein certain planes or other large areas of copper within a PCB is cross hatched copper, like a latticework in a screen door. The requirement for cross-hatching a plane is used in flex- and rigid-flex implementations where they can provide a number of benefits. Cross hatches are quite important in flex/rigid-flex PCBs, where the hatch ground pattern plays dual roles as a structural support and a ground element.

 

 

The amount of flexibility gained when cross hatching planes is directly related to the percentage of copper removed. For instance, if your application has four conductive layers and only one plane, and the crosshatch pattern is only removing 30% of the copper on the plane layer, there will be very little flexibility gain. However, if you have several planes that are going to be cross hatched, and the hatch pattern removes 70% of the copper in those planes, there will be a noticeable improvement in flexibility.

 

History in Rigid Boards

 

As noted above, a hatch ground plane has regular openings at regular intervals. The actual process of creating a cross-hatched plane occurs in the CAD or CAM system, where the region to be hatched is the filled area with a series of regularly spaced lines much, like the traces plotted in signal layers. Then, the area is edged with a thin border trace that connects the ends of the lines that form the crosshatch. Connections, such as power or ground connections, are made to this cross-hatching in the same way that they are made in solid planes.

In the earlier days of the multilayer PCB fabrication process, the final step of inner layer processing involved roughening the copper surfaces so that they would adhere tightly to the resin in the prepreg system during lamination. This step was necessary because the copper surfaces as they emerged from the DES (develop, etch, and strip) process were very smooth. In fact, they were so smooth that it was difficult to create a strong bond between the resin used to laminate the PCB and the copper. As a result, if the copper surfaces weren’t roughened, delamination would occur between the laminate and the solid copper planes of the PCB. This same problem occurred with component mounting pads on outer layers resulting in pads coming loose from the PCB while soldering during rework.

 

Crosshatching in Flex and Rigid Flex Circuits

 

While cross-hatching is rarely used in rigid PCBs these days, it does have practical application for both flex and rigid-flex circuits. These applications come in two areas for flex and rigid-flex circuits:

• Controlled impedance in flex regions: Using a hatch ground is a good method for providing the reference plane required in controlled impedance routing for high speed digital (HSD) boards. The hatch ground provides wider, more manufacturable dimensions while retaining the flexibility of the circuit and assembly. It should be noted that cross-hatching reduces the amount of copper under a transmission line, which decreases the capacitance and raises its impedance.
• Structural support for flex regions: Using a hatch ground provides structural support needed for a dynamic or static flex ribbon without increasing the rigidity of the copper layer. on a two-sided flexible circuit. The layer can still be used for controlled impedance routing creating undesired rigidity, or the ribbon can be permanently deformed.

 

 

There are Hatch Width (HW) and Hatch Pitch (HP) values, which are important in hatch impedance calculations.

 

In order to calculate a trace width that results in the correct impedance, it is necessary to use a modeling tool that accounts for the missing copper in the crosshatched plane. Because the impedance for a given trace over a hatch ground region is higher than that over a solid ground region, the inductance of the trace needs to be decreased to maintain controlled impedance. Therefore, we would want to make the trace a bit wider as this will reduce the trace’s inductance and increase the total capacitance with respect to the hatch ground. Both effects will contribute to set the impedance to the correct value.