You're correct. You should expect to see an intermetallic layer in a proper solder connection. Try leaving the component at temperature for a longer period.
Growth of intermetallic compounds on Cu surfaces [Klein Wassink]: d = 10^3(exp(-5*10^3/T)t^1/2), where: d = Thickness of IMC in um T = Temperature in K t = time in seconds
For more on growth of intermetallics try: * Metals Handbook Volume 6: Welding, Brazing, and Soldering; EF Nippes, ASM International, 9e, 1983, [0871700123] * Soldering Handbook, PT Vianco, Sandia National Laboratories, American Welding Society, 3e, 1999 [0871716186]
If incresing the time at soldering temperature doesn't help, you could have a component solderability issue.
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