Figure 1 shows an example of a "shield" made of copper foil tape
which is connected to the ground pins of an IC package with short
wires. Sometimes the copper foil tape is not necessarily acting as a
shield but instead to improve the grounding under the IC such as in
single and two layer boards. Used either as a shield or to improve
grounding, such a structure can act as a troubleshooting aid for a
number of PCB problems. Examples of such problems include:
- Improving ground on single or two layer PCBs. Single and two
layer PCBs are still quite common in electronic equipment, especially
consumer equipment such as washing machines and other home appliances,
DVD players and other consumer electronic equipment, switch mode power
converters and supplies, and many other cost sensitive applications.
Often on PCBs of this nature, significant ground noise voltages can be
impressed between the ground pins of the IC device potentially causing
operational problems. This is especially true of high frequency impulse
noise due to ESD and EFT (electrical fast transient).
Such noise can corrupt signals, cause latch-up, and even burn out ICs.
Adding a copper tape plane on top of the IC package can improve the
ground between the IC pins and fix problems. If a known problem is
helped by this approach, some work is needed on the board layout.
- Reducing inductive and capacitive coupling from noise sources
such as switch mode power converters and ESD to the IC package itself.
In the case of ESD, sometimes the tip of an ESD simulator used for
compliance testing can be close enough to an IC package to couple
enough E and or H field noise directly into the IC package to cause
problems. This is especially true for small products with a plastic
housing where the tip of the simulator may be placed on a metallic
feature (connector or even just a piece of decorative trim) very close
to the IC package. Current generated by a time changing E field is
i = C*dv/dt
- i is the generated current
- C is the capacitance between the IC package and ESD simulator tip in this case, and
- dv/dt is the rate of change of voltage between the IC package and ESD simulator tip
For only 1 pF of coupling capacitance, a change of only 2 kV in a
nanosecond (a very mild ESD event) yields a coupled current of two Amperes
to the IC package chip and lead frame. In this case, the copper foil is
acting as a shield to intercept the E field and shunt it around the IC
package to the board ground (which is hopefully robust enough itself to
Construction of such a copper foil shield requires multiple low
inductance connections to the board ground, often using the IC pins
themselves. A flat pack, such as shown in Figure 1, has exposed leads
which makes grounding of the foil very easy. The grounding connections
must be very short, just a few millimeters, to work for ESD. Packages
such as ball grid arrays that do not have exposed connections require
the copper foil to be connected directly to the board ground which can
be a challenge while trying to keep the connections short. The more
connections and the shorter they are, the better.
Although not always a practical fix, construction of an IC package
shield from copper foil tape can be a great troubleshooting aid on PCBs.
Construction of an IC package "shield" from copper foil tape and
multiple short wires can be a good troubleshooting aid for PCBs. The
shield is easiest to implement on ICs with exposed connections, such as
a flat pack, but can be successfully implemented on other package types