Phil,
For aluminium wedge bonders try K&S, Palomar, or Delvotek. Wedge bonding to
nickel, immersion gold is a reasonable process. I have not tried it over three
layers of photo imagable dielectric though, but can see no fundermental reason
why it shouldn't work. Bonding takes place at room temperature, so the Tg of the
material isn't so important for bonding. A three layer built up board is going
to limit your choice of suppliers at the moment, but you may be able to find one
who can give you a gold bondable surface. Single and multi chip BGA substrates,
for example can be gold wire bondable. This may get round the need for new
equipment and a slower bonding process. Some suppliers may be able to produce a
board with fewer built up layers, depending on their technology. There are many
alternative ways of solving the problem and you need to look at the whole
picture.
If you do go to wedge bonding there are some things to consider. One major
difference with wedge bonding is that the tool must be kept in line with the
wire, unlike a capillary, which can bond at any angle. The two approaches to
achieving this are either to rotate the bonding head, or to rotate the
substrate. This need for rotation is the main reason for the slower throughput
of wedge bonding, compared with ball. The choice of machine will depend on which
part, the head, or the substrate, is the easier and quicker to rotate. If the
substrates are always going to be small, 1.5 x 1.5" is fine, then pick a bonder
with a rotating workholder as these can be faster. If the substrates are bigger,
or are going to be panalised several up, then you will need a bonder with a
rotating head. The layout of the bond wires and attach pads on the board may
need to be modified compared with a ball bonded design. Wedge bonds tend to be
narrower than ball bonds, but are long. Problems can occur at fine pitches if
the wires are at extreme angles.
regards, Jeremy Drake
---------------------------------------------------------------------------------------------------------------------------------------------------------------
Please respond to Phil Hersey <[log in to unmask]>
To: [log in to unmask]
cc: (bcc: Jeremy J Drake/HQ/CUK/Celestica)
Subject: [TN] seeking aluminum wedge bonding advice
I just found out that I can replace a $120 LTCC substrate with a $1.95
microvia PCB. Trouble is the PCB comes with nickel/flash gold so cannot be
wire bonded using our gold ball bonding equipment. Evidently it can be
bonded using ultrasonic aluminum wedge. This MCM has over 700 wires in a
1.5"x1.5" area (one of the ASIC's has 220 wires itself)- military
application.
Does anyone have experience with al. wedge on such boards? Construction is
175C Tg FR4 core with 3 layers of photovia on top (photoimageable epoxy
plated up with copper). Will it work? Who makes the best bonder?
Phil Hersey, Hytek Microsystems
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