AuSn thin film solders can be used for the bonding
of two components with planar surfaces. The planarity of
the surfaces should be better than one third of the solder
thickness. Otherwise, gaps and voids will result. This
planarity condition can easily be fulfilled in the case of
bonding small dies to a substrate. E. g. as in the case of
bonding laser diodes or photo diodes to a submount. The
main advantage of a thin film solder compared to solder
preforms is its ease of use during the soldering process.
The solder is already attached to one of the two parts to
be assembled. Therefore, the cumbersome handling of
small solder preforms is eliminated. This saves a lot of
money for the die bonding process. Further, the thickness
of the solder joint is reduced drastically. This reduces the
thermal resistance of the solder joint dramatically. Also, a
nice solder meniscus is formed nearly without squeezing
excessive solder out from under the die.