This paper deals with an operation sequencing problem for assembly of Printed Circuit Boards which contain sequence precedence relations. A heuristic whose objective is to minimize operation time is proposed. The efficiency of this heuristic is tested, and it is shown to be nearly optimum while requiring only limited computational time. (1) Sequencing among groups of parts is solved by a branch and bound method. (2) Sequencing within a group of parts is solved by a nearest neighbour method.