Laminate Material Types

There are many different laminate types, and many different IPC classifications for laminate, but lets just go over a few of the more common terms/classifications.

FR-4

People usually refer to an FR-4 epoxy as just a difunctional epoxy laminate that has a low Tg of about 110-130°C. We prefer to break the FR-4 group into three basic categories though. The first being a “difunctional” epoxy as just stated. Second being a “modified difunctional” epoxy which has a Td range of about 135-160°C. And the final being a “multifunctional” epoxy system that has a Td of 170°C and higher.

FR-4 is still one of the most common epoxy systems used today because of it’s economics and versatility. Most applications can incorporate this epoxy system if carefully selected.

Polyimide

Polyimide is a very robust laminate that is very good for resisting chemical and environmental influences as well as being very dimensionally robust, (low CTE). The military and “down-hole” industry still prefer polyimide even today because of these characteristics. Unfortunately their are a few down sides to Polyimide. Fist being that it is very hygroscopic and special care needs to be taken when manufacturing and populating the PCB. Another being cost, which can be upwards of double the price of materials compared to a traditional FR-4. And lastly is the brittleness. This material unfortunately is quite brittle and manufacturing it comes at an additional cost especially in the drilling/machining departments. There are many types of polyimides on the market today that combat the general negatives of using a polyimide, so discussing your application with your manufacturer can yield better results and lower costs with this product.

PTFE, ( Teflon® )

PTFE, (Polytetrafluoroethylene) is most commonly known by the DuPont trademarked name of Teflon®. PTFE is most commonly seen in high frequency RF applications like microwave PCB’s. The product has many positive attributes for high speed PCB designs including a very low dielectric loss factor and a very good dielectric constant, (Dk). PTFE, unfortunately has it’s drawbacks of course. The largest ones being the cost, manufacturability, and dimensional instability. Some of these issues are addressed by laminate manufactures using different fillers like ceramic to assist in reducing these issues, but these changes only reduce the manufacturing headaches, it doesn’t eliminate them.

One of the biggest issues with manufacturing a PTFE PCB is the metallization of the hole wall. Keep in mind the beauty of PTFE is that it doesn’t allow items to stick to it. This is why the most commonly know application for it is for non-stick pots and pans. Unfortunately that attribute is not the most conducive for trying to metallize the material. Special techniques to make the surface a little more porous is incorporated in order to “seed” the metal. Lastly the drilling and machining of the part has to be critically observed since the material is fairly soft compared to FR-4 materials and is susceptible to gouging and tearing.

Lastly, linked below is a great paper written by Chet Guiles from Arlon. This is a non-biased deep overview on laminates used in PCB’s.


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