Deciphering datasheets for high-frequency circuit materials: Page 6 of 7

January 07, 2013 //By John Coonrod, Rogers Corporation
Data sheets for printed-circuit-board (PCB) materials carry a great deal of information. Understandably, these materials are the foundations for many circuits, and they are characterized by many different parameters, some related to applications, some to fabrication issues, some to environmental and mechanical concerns.
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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

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