Copper peel strength, which is typically denoted in units of force per length of material (N/mm), is a mechanical material parameter that is often misunderstood relative to some of the other material parameters. Alone, it would be intuitive to think that a material with higher copper peel strength is better than one with a lower value. But tradeoffs among a circuit material’s various other properties must also be considered. For example, some materials with good copper peel strength cannot survive lead-free soldering because of other material attributes like CTE or Tg. Also, materials with lower modulus (softer materials) will typically exhibit higher peel strength numbers: during testing, the soft materials elongate and hold together longer, increasing the bond-fracture area and raising the peel strength number. In contrast, a more rigid material which has just as good bond will yield a clean break in the bond-fracture area during peel strength testing, lowering the peel strength value.
Lead-free soldering has received a great deal of attention due to environmental concerns. There are numerous ways to test a circuit’s lead-free soldering quality, including by running a material under test through a conveyor-belt-based, lead-free-solder reflow oven multiple times and checking for changes in mechanical attributes. The lead-free soldering temperature can vary, but is typically around +260°C. Some materials will perform well in multiple lead-free soldering cycles when making a simple single-layer PCB, but will perform poorly when the same approach is used for a multilayer construction. The fabrication process can have an impact on the capabilities of some materials to withstand the lead-free soldering process. In general, materials that do best in multiple lead-free soldering cycle testing are materials with high Tg, low CTE, and high Td. Reinforced hydrocarbon/ceramic RO4350BTM circuit material from Rogers Corporation ( www.rogerscorp.com) is an example of an extremely robust lead-free-capable RF/microwave PCB material. It handles lead-free processing effectively due to a CTE of 50 ppm/°C, Tg