In the past lead frames for IC chips have usually been given a thin coating of gold or silver on the areas to which the conductive gold wires are to be attached so as to give a good bond between the wire and the lead frame. Recently, however, there has been a move to the use of aluminium wire to replace the expensive gold wire and as a result there is a need to coat selected areas of the lead frame with aluminium so that a good bond can be achieved with the aluminium wire.
Thin aluminium coatings are generally applied by vacuum deposition in which the item to be coated is placed in a chamber under a very high vacuum and a piece of aluminium is heated so that it evaporates and the evaporated molecules then condense onto surfaces surrounding it, such as the surfaces of lead frames which are placed in the chamber.
There are however, problems and one problem which arises particularly in Hong Kong and in areas of high humidity is the difficulty of adequately removing water molecules from the vacuum chamber. Thus even the presence of stray molecules may lead to less than perfect results in the aluminium deposition. When the resulting lead frames are checked, if any imperfection is seen, the blank carrying it is discarded. Thus, since one cannot separate out the defective frame and still apply automatic production, the imperfection results in the loss of a blank carrying not only the single defective lead frame but perhaps ten to twenty others which are in fact perfect but which are all still part of the same metal blank. Losses are therefore magnified and represent a serious increase in costs.
The invention has therefore been made with this in mind and aims to provide an improved aluminium deposition machine for lead frames.