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44 SMT Magazine • April 2015 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. introduction First, the impact of temperature will be discussed for the separate process steps and for machine tooling. In the experimental part measurements are done to verify the accuracy that can be achieved us- ing today's selective soldering machines. Dedicated tooling is designed to achieve special requirements with respect to component position accuracy. Flux unit A selective soldering pro- cess has three process steps: fluxing, preheating and sol- dering. During the first step, the fluxing of the solder side of the assembly, the board and machine parts have an ambi- ent temperature. A high-fre- quency drop-jet device is able to apply very fine droplets. The device is mounted on a robot that is moving in an x, y direction to shoot fine droplets on those spots where the printed circuit will meet the liquid solder. The position of the flux is critical. Not only should the flux be on the soldering area to clean the board and support the wetting, but there should not be flux anywhere else on the board. It can be critical if non-activated flux is mixed with solder paste flux residues close to the soldering spot. Any non-activated flux on the assembly may cause electro-migration in the field when it is exposed to humidity and a bias. Flux that is not applied correctly may affect reli - ability during the lifetime of the product. Flux amount and position should be con- trolled to avoid field issues. The robot in the machine should move the drop-jet to the right spot. These locations are imported from the CAD-files or by teaching the cold print by a camera. preheat unit After the flux is applied, the board is trans- ferred to the preheating unit. This can either be an IR lamp device or a forced convection heater. The board is heated to a topside temperature of 120°C. A typical FR-4 board has a coefficient of thermal expansion (CTE) of 14 ppm/°C in x- direction and 17 ppm/°C in y-direction at tem- peratures from ambient to the T g (glass trans- mission temperature) of the board. The heating will extend the board length. An FR-4-print is heated to 120°C (ΔT = 100°C). The length of the board is 250.00 mm. Af- ter preheating the board will have a length of 250.35 mm. Not only should the expan- sion of the print be consid- ered, but also the warpage. Due to the heat and mass of components the warpage might give problems in the z-position. This deviation in the z-axis can be measured with laser sensors. The offset data is used to modify the robot position towards the solder nozzle to have a con- sistent contact time/immer- sion depth all over the print. If not compensated, warpage may give open joints (no contact or con- tact with solder too short) or bridging (contact time too long). An alternative to compensate warpage is to have dedicated tooling vacuum or mechanical holders installed in the gripper that keep the board flat. robot Gripper After preheating, the board is picked up by the robot, which then moves the print to the soldering station where the board has to be positioned towards the solder. But first, the position of the board in the gripper has to be defined. There are different ways to define the print location: • Fiducial camera: A camera can recognize fiducials on the print and calculate the off-set of the fiducials to a reference point. pOSiTiON aCCuraCy MaCHiNES FOr SElECTivE SOlDEriNG OF FiNE-piTCH COMpONENTS continues FeAture A high-frequency drop-jet device is able to apply very fine droplets. the device is mounted on a robot that is moving in an x, y direction to shoot fine droplets on those spots where the printed circuit will meet the liquid solder. the position of the flux is critical. " "