Cores, Prepregs, and Dielectrics

FR-4 is commonly used to describe the dielectric material that makes up PCBs. That's a problem since FR-4 stands for Flame Retardant level 4, and hundreds of possible laminates meet that specification "FR-4" only describes the way the material behaves when exposed to flame.
Mark Hughes
Designer

The simplest common stack up consists of two layers of copper separated by a fiberglass/epoxy core.  The fiberglass provides strength and rigidity and the epoxy provides an adhesive force that bonds the fiberglass to the copper.  Other materials, such as Teflon and polyimide, are available but are often outside the price range of most projects, so they are not discussed here.

The fiberglass layers range in thickness from <1 mil to >50 mils thick (made by stacking thinner layers of fiberglass) and provide mechanical strength and dielectric medium for the surrounding copper layers.

Isola 3070 has a very tight weave with little room for epoxy resin.

It takes time to attach copper layers to one another, so to move quickly, most PCB manufacturers have a stockroom full of an assortment of prefabricated two-layer FR-4 stacks in common copper weights and core-thicknesses.  You can save time and money by selecting a core that your manufacturer commonly stocks.

Prepregs

Prepreg” is short for “pre-impregnated.”  Prepreg is a thermally activated epoxy that is used to bond two layers of copper or core material together.  Often the copper has been etched at this point, so there are height variations of 1-6 mils between the top and bottom of copper foil.  To prevent cavities, (or measling), fabricators have to use a material that has a lot of epoxy, which doesn’t allow for a tight fiberglass weave like in the core material. 

Isola 106 is a very open weave with plenty of room for epoxy resin.
Isola 1080 allows a reasonable mix between epoxy and fiberglass

Epoxy

The epoxy is mixed with the fiberglass to create the prepregs.  It is activated at specific temperatures under carefully controlled pressures.  Excess epoxy is forced out of the side of a panel during assembly.  Your fabricator will select prepregs with specific resin contents for your design, based on a variety of factors, including the thickness of adjacent copper layers.

Considerations

Fiberglass and Epoxy have different material properties and different dielectric constants.  Most Dk values presented in datasheets are an average of the Dk of the epoxy and the Dk of the fiberglass.  But your traces will experience a slight change in impedance as they travel over each warp and weft yarn in a board.  For impedance controlled traces, your best bet is to use a “flat-glass” or “e-glass” core material that has a tight fiberglass weave and a low amount of epoxy and ensure your traces are coupled to the copper on the other side of the core.

Construction

To make a four-layer board, your manufacturer would:

  • Start with two pieces of two-layer copper-clad core material, place prepreg between the cores, or
  • Start with a core, place prepreg and copper foil on either side. 

Then the stack would be placed in a hot-oil hydraulic press for several hours.

The hot-oil lamination press at Royal Circuits in Hollister, Ca.

Any number of core layers can be attached during a lamination-cycle.  However, if even one of the layers shifts a bit too much during the process, the entire panel will have to be thrown out. 

To save money on your design, try to use common core sizes and limit the number of lamination cycles.  You can communicate directly with your fabrication house to decide on the layer stack early in the design process.

Typical overall board thicknesses are 0.8 mm (32 mil), 1.6 mm (63 mil), and 2.4mm (95 mil).

Recent Posts

Basics of Back Drilling – Webinar

Back drilling technology reduces reflections in high speed nets. It is the preferred fabrication method in high-speed designs. In this webinar recording, learn the process of back drilling, main advantages, design parameters and more. Download the presentation slides here > Backdrilling_Presentation Slides
Read More

Build a Better Via Stack

Build a better via stack by thinking about how your board is assembled. Cap vs. Foil Construction Printed Wiring Boards are constructed in one of two ways described as “Cap” or “Foil” construction.  The construction method determines via layer options and lamination cycles. Foil Construction In foil construction, fabricators start by drilling, electroplating, and etching… View Article
Read More

Circuit Analysis – Part 4: RC-Filters, LEDs & Buzzers

Before you layout a circuit, it’s generally a good idea to understand what is happening in your schematic.  And before you lay out a schematic, it’s generally a good idea to understand what is happening in your block diagram. For additional information on circuit analysis, read the previous posts in this series: Part 1, Part 2 and Part 3. When a… View Article
Read More

Share

Share on twitter
Share on pinterest
Share on facebook
Or use this public share link