Interposers are special connecting devices that are used to interface one socket to another. They can be prototyped without tooling charges, and maybe used in both board-to-board and flex-to-board applications. Selecting interposers requires an analysis of six factors: layout, thickness, contact element design, alignment, mating, and compression. Product specifications include pitch, contact count, compressed thickness, target load or contact, working range per contact, average resistance, insulation resistance, self inductance, self inductance, and contact plating type. Typically, the number of mating cycles determines both the plating material and the plating thickness for interposers. Layout, contact element design, and alignment are important parameters to consider when selecting interposers. These connectors are available in standard grid sizes that differ in terms of array pitch. Custom interposers in non-standard configurations with irregular arrays are also available. Typically, the application requirements determine the contact element design. Key parameters include pitch, constant redundancy factor, spring constant, and working range. In terms of alignment, some interposers have holes and slots that match the pins in the motherboard. Other connective devices use frame-to-board alignment techniques. Selecting interposers requires an understanding of mating specifications and features. Most products are designed to mate with FR-4, ceramic, and flexible substrates. FR-4, an acronym for Flame Retardant 4, is a printed circuit board (PCB) material that meets the requirements of UL94-V0, a flammability standard from Underwriters Laboratories (UL). Typically, FR-4 is used strictly as an insulator and without copper cladding. Interposers are also used with ceramic substrates made from materials such as aluminum nitride and beryllium oxide. When used with flexible printed circuits (FPCs) in semiconductor applications, interposers provide self-wiping connections on densely-populated boards. There are many different compression systems for interposers. Considerations include application requirements, space constraints, and number of mating cycles. Clamping solutions may be part of the assembly, or provided by the interposer supplier. Typically, the contact elements for interposers are rated for a compression force of approximately 40 grams (g) per contact. As a rule, the compression force required is a function of the planar variation in the mating surface. Surfaces with greater amounts of warpage require greater amounts of force. In some applications, a stiffening plate or other type of backing is used to improve planarity. Force-deflection curves for interposers provide a valuable reference during product selection.