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Position Accuracy Machines for Selective Soldering Fine Pitch
Position Accuracy Machines for Selective Soldering Fine Pitch
Selective soldering is a robust soldering process. This paper explores the different process steps and critical parameters.
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Authored By:
Gerjan Diepstraten
Vitronics Soltec B.V.
Oosterhout, Netherlands

Summary
The drive towards fine pitch technology also affects the soldering processes. Selective soldering is a reliable soldering process for THT (through hole) connectors and offers a wide process window for designers. THT connectors can be soldered on the top and bottom side of boards, board in board, PCBs to metal shields or housing out of plastic or aluminum are today's state of the art.

The materials that are used to make the solder connections require higher temperatures. Due to the introduction of lead-free alloys, the boards need more heat to get the barrels filled with solder. This not only affects the properties of the flux and components, but the operation temperatures of solder machines become higher. A nitrogen tunnel wave solder machine requires a temperature control in the tunnel to prevent overheating. Advanced systems are available that insert cold nitrogen.

The closed tunnel wave soldering process has a wide process window and is not sensitive to small changes in environmental conditions. The same counts for wave solder machines that have nitrogen blanket systems over the wave. Improved preheaters will bring sufficient heat in the assembly and exhaust systems are adequate enough to maintain required process conditions. The nitrogen will improve the soldering and minimize dross amounts at these elevated solder temperatures.

Selective soldering is a different process. Compared to wave soldering there are additional process parameters that are affected by the higher temperatures. Solder joints have to be made close to SMD pads or components. An off-set of 0.5 mm may result in solder skips or re-melting SMD components. The higher temperature may cause warpage of the board, which also affects the position accuracy of the solder nozzle. All materials will expand at higher temperatures, but not all expansion coefficients of the materials used are equal. This not only introduces stress, but also may create off-sets.

Conclusions
Selective soldering is a robust soldering process when the process parameters are addressed and under control. In the different process steps we identified critical parameters:

Flux process - In this first process there are no elevated temperatures and accuracy is established by:

1. Accurate x, and y position of the robot;
2. Flux flow measurement; and
3. Verification of the flux flow direction.

Preheat process - No special requirements for board handling or placement accuracy. The temperature of the print is measured with a pyrometer to guarantee flux activation and prevent overheating.

Dip soldering process - Several requirements need to be fulfilled to have a consistent dip process.

1. Position of the PCB in the gripper has to be defined: A. mechanical fix, or B. fiducial recognition
2. Position of the nozzle plate must be defined: A. machine point verification with fiducials, or B. mechanical fix of PCB/gripper with nozzle plate
3. Z-direction is guaranteed by stand-off pins on the nozzle plate.

Drag soldering over select wave - The requirements for a consistent drag process are:

1. Position of the PCB in the gripper has to be defined: A. mechanical fix, or B. fiducial recognition
2. Machine point of the nozzle should be verified: A. fiducial x, and y position B. z-height verification with a sensor
3. Z-position of the PCB is critical during soldering. Special attention is needed to make sure that there is no excessive warpage. Special tooling in the gripper or warpage compensation (measured with laser) by software are methods to compensate for a lack of flatness.

Initially Published in the IPC Proceedings

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