Prepreg is an insulation layer. It is a dielectric material that is sandwiched between two cores or between a core and a copper foil in a PCB, to provide the required insulation. You can call it as a binding material as well. It either binds two cores or a core and a copper foil.

As the name suggests, prepreg is a glass fibre weave/cloth impregnated with a resin bonding agent. The glass fibres are interwoven to form a glass fabric. This glass fibre weave is partially dried to form a B-stage material. All prepregs are B-stage materials

By the combination of certain additives and catalysts, under a chemical process, a specific section of a prepreg can be transformed into a conductive region. This is called selective conductivity. This phenomenon finds its purpose while drilling holes into the prepregs for connecting the top and the bottom layers. However, the rest of the part of the prepreg remains to be an insulator and protects the circuit board from short circuit which is the basic functionality of it.

There are different types of prepregs that are incorporated in PCBs depending on the thickness and other requirements. The prepregs are available in Standard Resin (SR), Medium Resin (MR), and High Resin (HR) depending on the resin content they hold.

The thickness of the PCB depends on the type of prepregs used. It must be noted that the required thickness can be achieved by a combination of prepreg plies. Prepreg pliers are thin sheets of prepregs placed on top of one another. This helps in achieving the desired thickness of the PCB.

Other than PCBs prepregs are also implemented in aerospace components, automotive parts, A/C ducting, machinery, tooling, honeycomb and foam panels.

Annular Ring

An annular ring is the area on the pad that surrounds a through via. The width of these rings can affect your design and manufacturing considerations. Annular rings can indicate the reliability of your HDI manufacturer based on the precision of the drills creating the vias.

Annular-Ring

The width of the rings determine how much space the board can shift, or a drill can move before electrical connectivity is impacted. A wide ring would ensure that even if the drill moves off center, the via is still within the pad and connectivity is maintained.

A small ring would put the via at risk of touching the edge of the pad, which is called tangency, or no longer being encircled by the pad at all. This is called an annular breakout. A class 2 annular ring is at least 1 mil wide. In a class 3 annular ring, the annular ring must be at least 2 mil or more in measurement.

PRO TIP!

You can read more about annular rings here, or check design manufacturability of your board using our Better DFM tool.

Annular breakouts

Annular breakouts occur when the drilled via is not fully encircled by the pad. This can happen when the lamination process causes the PCB to shift and move away from where parts (pads,traces, etc.) should be. This is a problem when drilling, as annular breakouts can occur. Breakouts of 90 to 180 degrees can occur.

A 90-degree annular breakout
A 90 degree breakout is when the section of the via outside of the pad is 1.4 x radius of the via in length. This is allowed in Class 2 annular rings as long as the conductor junction width is wide enough.

Annular-Breakout-Deg90

A 180-degree annular breakout
A 180 degree breakout is when the width of the breakout is the same as the diameter of the via. This is not allowed in a Class 2 nor Class 3 annular ring.

Annular-Breakout-Deg180

Teardropping

Teardropping is used to fix breakout vias and to reinforce connections. When vias breakout at the junction of the conductor and the pad, adding copper around the via can reestablish the broken connection. This will fix the short and ensure the signal can still reach where it needs to go.

Teardropping

Yield and Drill To Copper

Yield is another component of PCB manufacturer reliability. Yield refers to the amount of boards made to fill an order. This includes boards with mistakes or discrepancies that are replaced. If a customer orders 10 boards but the shop needs to make 20 boards in total to fill the order, yield is 50%. This can be due to problems with the board, like difficulty drilling and plating vias, machine error or human error, like dropping a board.

Drill to copper is the distance from the edge of the drilled hole to the copper feature (pad, pour, trace, etc).

A 90 degree breakout is when the section of the via outside of the pad is 1.4 x radius of the via in length. This is allowed in Class 2 annular rings as long as the conductor junction width is wide enough.

Annular-Breakout-Copper-Trace