Laminates

There are many types of Printed Circuit Board (PCB) laminates in use and they range from very low cost materials used in single sided boards to expensive high performance materials that can satisfy the most demanding requirements. Increases in soldering temperatures will be encountered when using popular Lead-Free solders; this means that laminates may degrade. Low cost substrates such as paper reinforced phenolic-based materials are less likely to survive increased soldering temperatures than more expensive boards. However, it has been found that glass reinforced epoxy laminates such as FR4 type materials are normally perfectly acceptable for Lead-Free processing. Newer types of laminates are also generally compatible with Lead-Free solders and soldering temperatures. The general rule is that the lower the quality material used the greater likelihood of a problem.

At the soldering temperatures required for Lead-Free alloys, circuit board laminates can be above their glass transition temperatures (Tgs). This means that their polymers become more rubber-like and are increasingly likely to sag and warp. One way of minimising this type of problem is to use laminate materials with higher glass transition temperatures. Conventional FR4 laminates typically have Tgs in the region of 135°C to 150°C, but there are many newer types of materials with Tgs in the region of 170°C to 180°C or even higher and these may need to be considered if warping is likely to be a problem. The transport of boards during soldering and the provision of a centre support may be required to minimise sagging. The higher Tg laminates are also more chemically resistant and may require the circuit board manufacturers to modify their fabrication processes in order to maintain plated-through-hole reliability.

Most PCB laminates are heavily flame retarded via the use of brominated resin systems. Two groups of brominated flame retardants are restricted under the RoHS Directive; however these are uncommon in circuit board fabrication. In the case of the widely used FR4 type laminates, the flame retardant is based on tetrabromobisphenol A (TBBPA). This particular flame retardant is not restricted by the RoHS Directive. However, there is a general move away from brominated flame retardants and laminate manufacturers are working to produce laminates that are both Lead-Free compatible and bromine-free.

An example of a printed circuit board bowing during wave soldering (courtesy of Soldertec)

Summary

• Lead-Free elevated temperatures can cause problems with laminates
• These can be overcome by careful material choice