Professional Rework Concepts

Bernd Monno (retired)
FINETECH GmbH & Co. KG, Wolfener Str. 32/34 Haus L, D-12681 Berlin / GERMANY
Tel. +49 30 936681-0, Fax +49 30 936681-44,

When reworking BGAs, CSPs and Flip Chips, their removal and resoldering is always necessary, even when the problem lies with the solder connections between the component and the board and not the component itself. Touching up, as with QFPs and PLCCs, is not possible. Applications of Area Array Packages continuously show peculiarities.

• High number of interconnections demand very narrow substrate tracks which make them fragile.

• The high density of interconnections makes it necessary that many layers are used in the substrate in order to spread out the tracks vertically. This results in expensive substrates.

• Most of the solder connections between the component and the substrate are placed among, and close to, narrow tracks. For the precise placement of these components optical placement systems are needed. Soldering cannot be carried out with soldering irons and reflow profiles must be accurately reproducible.

• The amount of solder for each connection is relatively large, especially in the case of "soft" balls and for C4 Flip Chips.

In most cases, solder residues have to be removed from the substrate after component removal and new solder applied in the form of solder paste. These features require the following steps to be followed when replacing Area Array Packages:

• Melt solder and remove component,
• Remove solder residue,
• print or dispense solder paste and
• solder new component.

Relative complex equipment has been developed for these tasks. More or less accurate vision systems, such as cameras and microscopes, are usually incorporated. There is heating of the substrate from below and the component and solder site from above with radiated heat and hot gas. Most machines rely on PC controlled heaters to be able to repeat profiles that have proved successful.

Some rework machines now have solder residue removal capabilities. One tool uses hot air to apply local heat and vacuum to remove the solder. The board is moved with a motorised XY table to cover the pad area in a meandering path. After the pad layout has been specified and communicated to the PC the PCB is moved to the start position. According to the manufacturer’s information the actual suction process takes up to 600ms per pad; ie several minutes overall.

A similar, manually moved, tool also takes several minutes (manufacturer’s statement: 2 to 3 minutes).

Both systems state that they do not make physical contact with the board. The distance between the substrate surface and the suction nozzle has to be so small that, especially in the case of Flip Chip rework, any gap can no longer be controlled.

Height changes, caused by locally heating the top surface of printed circuit boards, are of such a magnitude that the likelihood of actual contact, even when removing larger solder residues, cannot be excluded.

Finetech has, right from the start, enthusiastically welcomed the challenges of reworking Area Array Packages. With simple and precise Vision Alignment Systems (placement accuracies in the range of µm), Finetech has dominated the Flip Chip Bonder market. The COMISS hot gas system has provided the best possible reproducible temperature profiles, while making it independent of the response limitations of the heaters. For too long Finetech has left the problem of solder residue removal to users, who have bravely struggled with vacuum irons and solder braid. One particular user even developed a heating profile, which ensured that during the removal of a BGA almost all solder would remain attached to the component. This result has not been able to be replicated reliably in other applications.

At long last Finetech has developed special suction nozzles and patents applied for. These tools are used in the Reflow Arms of Fineplacers.

A suitable program of the hot gas system COMISS ensures that the solder residue of the first row of the array melts. The gas flow is selected so that the hot gas is largely absorbed by the vacuum system. This ensures that only solder in the immediate vicinity of the nozzle is heated and liquid solder is not blown away from the rework site.

By means of the air table of the Fineplacer the printed circuit board is moved under the tool in such a way that the pads are heated and the solder sucked away row by row. The whole process requires only one movement and typically takes 40s, including preheating. The elastic strip (1) sweeps the solder residues off their pads and unto non-solderable surfaces, where the vacuum can easily remove them. This enables the removal of even very small quantities of solder residues, such as occur with C4 Flip chips.

Illustration 1 shows such a tool in action. The majority of the pads are already cleared; the red arrow indicates the direction of travel of the PCB.

When nitrogen is used as heated gas the process results in smooth and clean finished pads even without the application of flux.