Printed Board Laminates

07 June 2006

In percentage terms the major cost of the single-sided and plated through-hole board is the base laminate. FR4 laminate has been discussed in a previous issue of EM&T. Care must be taken over material selection in terms of cost and mechanical and electrical requirements. The material selection should also be discussed with your PCB manufacturer as often he can make suggestions that can alternatively reduce cost.

The move to lead-free assembly has raised issues over the ability of existing products to stand up to the higher temperatures. In many cases the materials can survive increased temperatures. It is often the case now that suppliers will quote time to delamination rather than Tg as a reference for applicability.

Copper Thickness
Basically the base laminate consists of a rigid base with a copper foil. The copper foil will be either 1/4, 1/2, 1, or 2oz material. The oz relates to the copper weight in oz/ft squared. For example 1/2oz would be 0.018 microns, 1oz equals 0.036 microns and 2oz would be 0.072 microns thick. In the case of singles and double-sided non-plated through-hole boards the thickness would normally be 1 or 2oz material as no additional copper is plated to the circuit during manufacture.

FR2
The base is composed of multiple plies of paper impregnated with flame retarding phenolic resin. The major advantage of the material is the low cost, good electrical and punching performance.

FR3
The same as FR2 but the paper is impregnated with an epoxy resin providing better electrical and mechanical properties. The material is used in TV, computer and telecommunication applications.

CEM1
The material is a composite with paper as the core and woven glass cloth on the outer surfaces - each impregnated with epoxy resin. The material can provide the punching characteristics of the previous materials and many of the electrical and mechanical advantages of FR4 laminate.

CEM3
This material again has woven glass cloth on both outer surfaces but with non-woven glass in the core. It has epoxy resin and is more suitable for through plated holes than CM1.

FR4
By far the most often specified product produced from multiple layers of woven glass cloth impregnated with epoxy resin. It has superior properties to the other laminate sheets with a higher price tag. The glass transition (Tg) temperature of the material is between 125-140°C. This laminate is mainly used for plated through and multilayer functions. It is used in all types of electronic products including military space and advanced computer systems.

FR5
Basically the material is the same as FR4 but with a modified epoxy filling which improves the Tg to between 150-165°C. This improves the thermal stability of the product. Generally used where a product is required to work at higher temperature.

There are many variations of laminate from different suppliers, both local producers and suppliers from the Far East. It's important to know specifically what you produce and the assembly process requirements are before specifying one product.


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