Medical Design Briefs - September 2021 - 20

TECHNOLOGY LEADERS Electronics
device application. In these instances,
it's prudent for medical OEMs to work
closely with their PCB microelectronics
assembly management to determine
appropriate assembly methodologies.
Wire bonding goes back to the transistor
age and has been used to connect
a silicon die to its package, most recently
to such packages as BGAs. However,
now that dramatically increased functionality
is in demand for medical
miniaturization, multi-tier wire bonding
is entering the process to provide
such extra functionality.
Today's complex microchips are
designed to provide that needed functionality.
Com bining a single die with
multi-tier wire bonding captures that
additional functionality. Simultaneous -
ly, this combination significantly
reduces chip count, thus decreasing
printed circuit board real estate.
There can be two, three, and four levels
of wire bonding, in some cases called
stacked wire bonding (see Figure 3). Also,
multi-tier wire bonding offers OEMs a
solution when the number of inputs/
outputs (I/Os) are far beyond the traditional
ones that are used in the single
wire-bonding application.
Meanwhile, CoB provides an extra
measure of space on the PCB because
device packaging is not involved, and
the chip or die is directly attached to
Mold-frame
(fan-out area)
Chip
the board. Since a package is much
larger than the die, itself, the form factor
is significantly decreased. A prime
feature of CoBs is their flexi bility, which
allows interconnection changes to be
made.
For higher thermal medical de -
vice applications, CoBs provide
high levels of heat dissipation
through the use of various epoxies.
Moreover, fanning out of CoB is
easy and not complicated for medical
device PCB designers. Fanning
out means assigning optimal paths
from the die bond pad to the substrate
pad.
When CoB fanout is optimally de -
signed, traces coming out of the die can
be optimized, using a straight and short
path toward the substrate pad. Plus,
those interconnects can be more optimal
compared to wire bonding. Again,
this is one advantage CoB has over wire
bonding for medical devices and other
applications.
Also, for high-speed, high-frequency
medical device applications, wire bonding
isn't as effective due to the capacitance
and inductance it creates. CoB
operates more optimally in higherfrequency,
high-speed ranges because it
has less capacitance or inductance built
in while traveling along the forward and
return current path.
Die attach closely resembles CoB. But
its application is more extensive since
this process, as the name implies, attaches
a die to a package, substrate, rigid or
flex circuit, another package, or even a
die attached to another die. There are
three different die attach methods:
epoxy, eutectic, and solder.
In the epoxy method, a fine and
extremely precise dispenser very accurately
dispenses the epoxy. Eutectic die
attach uses an aluminum or gold metal
layer. Solder attach is a common type
of die bonding due to better thermal
conductivity of the solder material
itself. After die attach is completed,
wire bonding is performed to create
the joint that connects the die to the
substrate.
The combination of a flip chip and
BGA packaging is a third major technology
applied during microelectronics
assembly (see Figure 4). This combination
reduces the conventional BGA
package by one-third to one-fourth the
original size. However, all necessary connections
are efficiently accommodated
in that space. As a result, less real estate
is used in a miniature flex or rigid-flex
circuit board.
In this instance, the medical OEM
customer and PCB microelectronics
Redistribution
Flip Chip
Solder Stop
Dielectric
BGA
Interconnect
(Solder ball)
Fig. 4 - A BGA and a flip-chip interposer. (Source: Thorsten Meyer, Wikipedia, with minor modifications in red)
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www.medicaldesignbriefs.com
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assembly management must be on the
same page. Both must have an understanding
of new advances in the redistribution
of layers (RDL). In the case of a
flip chip, in particular, RDL conveys signals
from a die out to pins that solder it
to the circuit board and through the
BGA, effectively reducing the amount of
real estate involved.
In summary, an EMS company that
provides PCB microelectronics assembly
should be knowledgeable about newly
emerging medical products like wearables,
implantables, insertables, and
ingestible electronic devices, as well as
all the nuances associated with them.
Because this is an emerging area, considerable
research still must be performed
to ensure that these devices produce
desirable results without creating harmful
effects in the human body.
This article was written by Zulki Khan,
President and Founder, NexLogic Technologies,
Inc., San Jose, CA. He can be reached
at zulki@nexlogic.com. For more information,
visit http://info.hotims.com/79417-345.
Medical Design Briefs, September 2021
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Medical Design Briefs - September 2021

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