Bonding Flip Chips on to Flexible Circuit Boards
As the result of a unique adhesion soldering process, Würth Elektronik is able to put flip chips on different circuit board substrates – even on flexible materials
Niedernhall, February 2011 – Time and again, a major
challenge lies in attaching integrated circuits (ICs) to highly integrated
circuit board substrates in a space-saving manner. Würth Elektronik took this
challenge on and found an ideal solution with the ESC (encapsulated solder
connection) process. The chips are soldered and at the same time glued
'face-down" in their exact position. The ESC process is suitable for the most
diverse substrates, be they fragile glass materials, FR4 or even flexible
materials such as polyimide foils or LCP (liquid crystal polymer).
Each substrate represents a separate challenge. Depending on the substrate,
extremely tiny pitch distances of less than 100 µm can be realized. Using
thermal compression bonding technology, the upside-down chip (flip chip) is set
onto an anisotropic adhesive containing microscopic soldering particles. Briefly
heating the adhesive causes the solder particles to melt and results in a true
soldering connection between the chip's contacts and the substrate, which
creates the electrical contact. The simultaneous hardening of the epoxy adhesive
also sets the flip chip firmly into place. The entire process is called flip
chip bonding. “An additional 'underfill process' is not required. In particular,
the high exactness of placement and the finely dosed bonding force make it
possible to process even extremely brittle substrates without any problems,"
explains Roland Schönholz, who is responsible for bonding technologies and
Lasercavity at Würth Elektronik in Schopfheim, Germany.
At first glance, it seems contradictory to place relatively brittle ICs on
flexible circuit boards. The ESC process, however, masters this challenge as
well. The right chips are the prerequisite for successful bonding of the flip
chips. These chips should have gold stud bumps, galvanic bumps or similar means
of contact to serve as contacts. Würth Elektronik is able to manufacture the
gold bumps itself for small amounts of individual ICs or also for small runs.
The ESC process gives the user great latitude in selecting the correct
substrate – anything is possible, from brittle to flexible. The decisive factor,
however, is that the final surface on the substrate can be soldered. "We are
able to demonstrate that we have experience working with all common surfaces
such as chemically applied silver, chemically applied zinc, ENIG (electroless
nickel immersion gold) or ASIG (autocatalytic silver immersion gold)," says
Roland Schönholz, "because numerous tests have already proven the reliability of
the new construction and connection technology." In addition, flip chip bonding
has already been proven in real life in connection with the innovative
Lasercavity method, which Würth Elektronik is pioneering. With the bonding of
flip chips, the circuit board specialist is continuing its technological
leadership and enables users to further miniaturize its products and to achieve
a maximum of integration while maintaining reliable functionality. /p>
About Würth
Elektronik Circuit Board Technology (CBT)
- Würth Elektronik
is the leading PCB manufacturer in Europe, producing a wide spectrum of PCBs
- from standard technologies through to pioneering system solutions - at
manufacturing plants in Niedernhall, Rot am See and Schopfheim. The PCB
specialist delivers application specific solutions in all technologies and
is a driving force behind new technical innovations, for example, in the
field of embedded active and passive components through Lasercavity and
FLATcomp. The extensive PCB portfolio ranges from double sided PCBs and
multilayers in all conventional technologies through to technically
demanding PCBs such as HDI, flex-rigid and polymer technologies
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