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

Subject to changes

12/15

www.vti.fi

TN54

Rev.C