SCA3000 Assembly Instructions
6
Rework Guidelines
There are several rework systems on the market. Some will heat solder joints directly from the
sides of component package, while others will direct heat on the top of component. Occasionally,
very rough rework methods are used such as hand placement and heating with a soldering iron.
However, these rough methods are not suitable for VTI's MID components. The heat flow of hot-air
convection should be directed at the edge and under the component body directly to the solder
joints and component pad areas. The package has a limited thermal conductivity and thus, a
horizontal flow method, a conduction method, or a conduction heating from the top of the package
to the solder joints should not be used.
Prior to the rework process, PWBs and components should be free from moisture. If necessary,
baking and drying processes should be performed.
The reflow profile for the component removal should be similar to the initial reflow. A carefully
adjusted reflow profile is essential for the successful rework operation. A proper reflow profile
should be measured with thermo-couples. During the reflow profiling, at least the solder joints, the
top of the component, and the bottom of the PWB should be monitored. If there are other
components near the component to be reworked, the temperatures of those should also be
monitored. The bottom side heating of the PWB is recommended in order to reduce the PWB
warpage during the rework operation.
Once the solder joints have been fully melted, the component can be carefully removed from the
PWB. The common rework methods for the component removal can be used, i.e. a vacuum nozzle
etc. It is absolutely necessary to ensure the complete melting of the solder joints before the
component lifting. If the solder joints are not fully melted before the lifting operation, pad damage
may occur on the PWB and the component.
After the component is removed, the pad areas of the PWB should be cleaned using common
rework methods. These include the applying of a rework flux, an excess solder and flux removal
using a vacuum solder removal tool or a solder wick and a soldering iron with a wide chisel tip, and
the cleaning of solder pads with alcohol and brush. The PWB cleaning process should be
performed gently as too high force or a scrubbing motion can cause pad lifting and trace damage.
If the component reuse is desired, it should be carefully inspected for a potential damage, solder
residues should be removed, and the pads should be cleaned. Prior to the placement of a
replacement component on to the reworked PWB, a solder paste should be applied on the cleaned
PWB pads. Suitable methods for applying the solder paste are the usage of a micro-stencil or the
dispensing of the solder paste. If the micro-stencil is used, it should be cleaned after each paste
application to prevent clogging. For the solder paste and the stencil, the same guidelines as for the
initial reflow process should be used.
The accurate alignment of the component is an important process step thought the surface tension
of the solder during the reflow step will help with the self-alignment. The use of a split-vision
alignment system is recommended to ensure the precise alignment of the component to the PWB.
The "Z" placement force should be carefully controlled in order to prevent the solder bridging.
The same reflow profile, as for the component removal, can be used for the re-attachment of the
replacement component as long as all solder joints will fully melt and properly wet the contact
areas. Otherwise, a new and proper reflow profile must be developed and measured from the
solder joints. The reworked PWBs should be allowed to cool to the room temperature. The PWBs
and the components should be inspected after the rework process for possible defects. The use of
X-ray inspection techniques can be used to verify the success of the rework process.
It should be noted, that the performance and the reliability of the reworked component may have
decreased due to the rework operation.
VTI Technologies Oy
www.vti.fi
Subject to changes
TN54
12/15
Rev.C
VTI [ VTI TECHNOLOGIES ]
