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West·Bond is
in the business of designing and producing high quality wire bonding,
eutectic die attach, epoxy die attach, and test equipment. Due to the
incredible number of different bonding applications currently being used
in the industry, we try to avoid involving ourselves in process issues.
Technical support at West·Bond will be happy to give you some general
pointers and ideas to help keep your particular process pointed in the
right direction, but we cannot write your procedure for you. In this
interest we have listed a few of the more common questions along with
their possible solutions. Also, refer to the Keys
to Bonding Success.
It is a detailed list of all the possible contributing factors in a
bonding process. Another very useful reference is George Harman’s book, Wire
Bonding in Microelectronics Materials, Processes, Reliability and Yield. Effectively using the two sources listed
above in conjunction with the FAQ’s
listed on this page should help you to better utilize your wire bonder.
Top Frequently Asked
Questions:
1. Why
is the wire breaking above, instead of at the back of the second bond?
2. Why are the bonds not sticking?
Question: Why is the wire breaking above, instead of at the back of
the second bond?
Answer:
Old
wire - Bonding
wire gradually anneals at room temperature as so loses the work hardening
imparted during its manufacture. That hardness is crucial to the
hardening of the wire and to the shaping of connections during bonding.
The shelf life is generally considered up to 12 months for gold and 24
months for aluminum. Verify the age of the wire being used or replace
with known good (fresh) wire.
Excessive Tool Heat - This problem is commonly
encountered on the deep access, 90° feed, wedge-bonding
machines. With the tool heater coil wrapped around its exterior, a 90°
tool acts like a high temperature oven. As the wire passes through the
tool, it can become fully annealed if the tool heat is too high. Once
this happens, it will be difficult to control the tail length because of
the unknown amount the wire will stretch before breaking. Other problems
that can occur due to excessive tool heat include: wire pulls out of the
back of the tool, inconsistent bonds, and poor feeding characteristics.
Reduce the tool heat to no more than a setting of 100˚ C. If at all
possible, turn it off. Tool Heat aids in creating a bond when it is not
possible to use workstation heat. It is not a necessary element of
successful wire bonding.
Worn Bonding Tool - Replace the tool with a new or known
good version of the same tool. Be sure to use the proper tool for the
wire, part, and pad size you are using. Failing to properly select the
right tool for your process can result in poor feeding characteristics,
loss of tail control, package damage, inadequate looping control, and
worst of all, operator frustration.
Question: Why are the
bonds not sticking?
Answer: This is the million-dollar question.
With so many factors affecting the bonding process it can often times be
difficult to pinpoint the problem. In the vast majority of case when this
problem is seen, it is not the machine’s fault. Usually improper
settings, dirty parts, bad wire, and worn tools are the culprit of bond
failures. If you find yourself up against a wall here are a few things to
try:
1.
Old wire (or incorrect wire) – replace with new or know
good.
2.
Dirty parts – clean parts in a plasma cleaner and try
bonding again. If the plasma cleaner fails to produce a part clean enough
to bond to (it happens) use a clean pencil erase
to burnish the surface of the pads. Then, using isopropyl alcohol, clean
the pads and bond again.
3.
Bad tool (or incorrect tool) – replace with new or
known good. Be sure to use the proper tool for the wire, part, and pad
size you are using. Failing to properly select the right tool for your
process will result in either poor or non-existent bonds.
4.
Incorrect ultrasonic power setting – increase ultrasonic
power if the bond is not deforming nicely. Decrease ultrasonic power if
the bond looks smashed. A properly deformed bond has a width equal to
approximately 1.5 times the wire diameter. See our Wedge Bond Guide.
5.
Incorrect ultrasonic time setting – increase ultrasonic time
if the bond is not deforming nicely. Decrease ultrasonic time if the bond
looks smashed.
6.
Improperly set bond force – increase bonding force
if the bond is not deforming nicely. Decrease bonding force if the bond
looks smashed.
7.
Too much tool heat – as much as heat will aid
you in making nice bonds, it can hinder you. Heat is not a cure-all for
bonding problems. West·Bond’s quality control department carefully and
meticulously checks out each machine before it goes out the door. This
includes setting each machine up and bonding with it to ensure proper
bonding operation. Did I happen to mention that this bonding is done with
1 mil gold wire and a thick film gold substrate at room temperature?
No tool heat or workstation heat is used. This includes all of our wedge-wedge
and ball-wedge bonders. We do this under the premise that if the machines
will bond cold, they will have no problems bonding with heat. If your
process does not required it and your part can stand a little more force,
ultrasonic power, and ultrasonic time, then turn
that heat off!
8.
Tool much workstation heat – see above.
9.
Operator moving during bond – any movement during the
bond can adversely affect the wire bonder’s performance.
10.
Dirty bonding tool – if you are using a
ceramic tipped tool, from any manufacturers, it may have aluminium or gold wire build up on the tip causing
the wire to stick better to the tool rather to the work piece. Using a 5
percent mixture of Sodium Hydroxide and hot water for 15 minutes will
dissolve any soft metals leaving your bonding tool like new again.
11.
The machine – if you have checked all
of the above parameters and are still having problems, see out machine
troubleshooting section. If you still find yourself at wits end, give our
service department a call. Drawing on decades of hard earned industry
knowledge, West·Bond technical support will be happy to give
you some tips and point you in the right direction.
1551 S. Harris Court, Anaheim, CA 92806
Phone: 714.978.1551 Fax:
714.978.0431
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