FormFactor ( FORM) plans to offer 5 million common shares after filing a registration statement with the Securities and Exchange Commission. The company expects to grant the underwriters an overallotment option to buy up to 750,000 additional shares. Goldman Sachs, Morgan Stanley, Citigroup and Thomas Weisel are underwriting the offering. FormFactor makes products that are used to test semiconductor wafers. Shares of FormFactor fell 59 cents, or 1.5%, to $39.52.