Soldering assembly recommendations

AN1235

10/14

Doc ID 7272 Rev 8

5.3

Underfilling

Underfilling is not essential for Flip Chips. These devices can do without an underfill if the

process temperature does not exceed 175 °C and if the process time is short (typically 5

minutes).

5.4

Manual rework

Flip Chips are able to tolerate one repair in addition to the two reflows mentioned in

Section 5.2.

As for other BGA type packages the use of laser systems is the most suitable form for Flip-

Chip repair. Manual hot gas soldering is acceptable but iron soldering is not recommended.

For leaded Flip-Chip manual rework the maximum temperature allowed is 260 °C (lead-free

compatibility) and dwell time must not exceed 30 seconds.

For lead-free Flip-Chip manual rework, the maximum temperature allowed is 260 °C. The

typical soldering profile of Figure 9 can be used.

5.4.1

Rework procedure

Remove the device

Rework process starts with the removal of the device. To remove the device, heat must be

applied to melt the solder joints so that the component can be lifted from the board.

Large area bottom side preheaters may be used to raise the temperature of the board. This

may help to minimize warping of the board, and minimize the amount of heat that must be

applied on the component.

Top heating may be applied to the component by using a laser or a convective hot gas

nozzle. Nozzle size must be selected to match the component footprint appropriately. After

top heating has melted the solder, vacuum is applied through the pick-up nozzle, and the

component is lifted from the board.

The heat should be carefully directed at the component to be removed to avoid adjacent

components solder joints being reflowed. Shielding, control of gas flow from the nozzle, and

accurate temperature control are the key parameters.

Removing solder

Next step is cleaning the solder from the work site. Due to space constraints and the need

for accurate temperature control, automatic tools are recommended.

Typically, site cleaners consist of controlled non-contact gas heating and vacuuming tools.

The objective is to remove the residual solder from the site without damaging the pads,

solder masks or adjacent components, and to prepare the site for the application of a new

component.