Theoretical and practical aspects of the miniaturization of lead frames by double sided asymmetrical spray etching

Normally etched lead frames are produced by symmetrical double-sided spray etching. Therefore they have a symmetrical lead cross-section. Producing lead frames by asymmetrical etching, give the possibility of further reduction of the pitch. While the bonding flat is produced at the topside only, the most vertical etching can be done from the other side. Compared with symmetrical etching, the etching time is prolonged. This can be compensated by increasing the etching speed by use of higher spray pressures and etchant temperatures. Two additional ways for the dimensional reduction of the pitch are thinner sheets and higher etch factors to diminish the undercut. Based on Weißinger’s theoretical model which is assuming an isotrope-etching behaviour of the lead frame material, this paper shows how the lead cross-section will develop under different etching conditions. Some experiments on a high pressure/high temperature spray etching machine verify the theoretical findings.