User`s manual

12 rabbit.com Rabbit Hardware Design Overview
2.5 Through-Hole Technology
Most design advice given for the Rabbit 4000 assumes the use of surface-mount technology. However, it is
possible to use the older through hole technology and develop a Rabbit 4000 system. One can use a Rabbit
4000-based Core Module, a small circuit board with a complete Rabbit 4000 core that includes memory
and oscillators. Another possibility is to solder the Rabbit 4000 processors by hand to the circuit board.
This is not difficult and is satisfactory for low production volumes if the right technique is used.
2.6 Moisture Sensitivity
Surface-mount processing of plastic packaged components such as Rabbit microprocessors typically
involves subjecting the package body to high temperatures and various chemicals such as solder fluxes and
cleaning fluids during solder wave and reflow operations. The plastic molding compounds used for IC
packaging (encapsulation) is hygroscopic, that is, it readily absorbs moisture. The amount of moisture
absorbed by the package is proportional to the storage environment and the amount of time the package is
exposed to the humidity in the environment. During the solder reflow process, the package is heated rap-
idly, and any moisture present in the package will vaporize rapidly, generating excessive internal pressures
to various interfaces in the package. The vapors escaping from the package may cause cracks or delamina-
tion of the package. These cracks can propagate through the package or along the lead frame, thus expos-
ing the die to ionic contaminants and increasing the potential for circuit failures. The damage to the
package may or may not be visible to the naked eye. This condition is common to all plastic surface-mount
components and is not unique to Rabbit microprocessors.
Rabbit microprocessors are shipped to customers in moisture-barrier bags with enough desiccant to main-
tain their contents below 20% relative humidity for up to 12 months from the date of seal. A reversible
Humidity Indicator Card is enclosed to monitor the internal humidity level. The loaded bag is then sealed
under a partial vacuum. The caution label (IPC/JEDEC J-STD-020, LEVEL 3) included with each bag out-
lines storage, handling, and bake requirements.
The requirements outlined on the label only apply to components that will be exposed to SMT processing.
This means that completed board-level products that will not be subjected to the solder reflow processing
do not have to be baked or sealed in special moisture barrier bags.