|Figure 1: Typical solder fountain system used to quickly and reliably remove and replace large through hole components.|
Rework departments often encounter large through-hole components on high-mass circuit boards, or components with heat sinks. The solder fountain machine is ideal for this type of rework. With the introduction of lead-free solders, the use of solder fountains has increased.
Solder Fountain Pros and Cons
A solder fountain can transfer large amounts of heat to a specific area very quickly. A normally laborious and time-consuming procedure with a desoldering tool is completed quickly and with relative ease.
However, the strength of the solder fountain system is also its chief danger. Solder can overflow or spatter, which presents the possibility of serious injury. Safety is the primary concern when using one of these machines.
|Figure 2: Solder height should be set at 1.50 to 3.0 mm (0.060 to 0.120 in.) above the lip of the nozzle.|
A solder fountain, by virtue of direct contact and heat transfer by molten solder, transfers a considerable amount of thermal energy to both the circuit board and the component. Printed circuit board materials are susceptible to damage such as layer delamination, copper delamination, separation of pads or barrels from inner layers, burns, solder mask damage and warpage. If several circuit boards require rework, a few should be carefully evaluated until a reliable procedure is established.
Solder Fountain System
Think of a solder fountain as a mini wave-soldering machine without a conveyor. Similar components - a solder pump and solder reservoir, various interchangeable nozzles of different sizes, and controls for solder flow height - can be found on the system. See Figure 1.
Solder Height Adjustment
|Figure 3: Once full solder reflow has been achieved, removal of the component is completed using an extractor tool or a gloved hand.|
Solder height should be set at 1.50 to 3.0 mm (0.060 to 0.120 in.) above the lip of the nozzle. Ideally, the leads of a component should be immersed and wetted without having the wave exert any upward pressure on the circuit board. The solder fountain table surface should be parallel to the nozzle surface.
Components and leads on the bottom side of the circuit board may cause the circuit board to be uneven, a condition that must be corrected. Insufficient immersion will prevent proper heat transfer and reflow. Excess pressure will cause solder to surge up through holes and to spill onto the top side of the circuit board - a potentially hazardous situation for the operator and circuit board. See Figure 2.
Solder temperature adjustment varies depending on various factors, including the melting point of the solder, and will be higher for lead-free alloys. Normal setting for tin/lead solders is 260 degrees C, 500 degrees F). During heavy use, solder temperature may cycle between 250 to 270 C (480 to 520 F).
The heaters should react quickly to normal drops in temperature. Heaters may overshoot the preset temperature when vigorous activity is suddenly halted. Operators must be alert to temperature fluctuations that exceed preset standards.
This adjustment can be used to precisely control operations of a repetitive nature, or in instances where a circuit board exposure heat must be strictly controlled.
A variety of removal tools exist to help extract the component once reflow has been achieved. The extractor tool should provide the operator a good grip, but should not unduly damage the component during removal. However, sometimes a tool is not workable and the operator will need to use a gloved hand. See Figure 3.
Circuit Board Pre-heat
The standard recommendations for pre-heat is 1 to 4 hours at 65 to 120 degrees C (150 to 250 degrees F). The requirements of temperature and time for pre-heat depend on the board construction, age and exposure to the atmosphere.
Prior to rework the operator should straighten any component leads which may prevent the easy removal of the component, and mask the area surrounding the component to be removed if protection is needed. An operator may need to mask components, with high temperature tape or high-temperature flexible mask. The mask may need baking to provide the proper cure prior to reflow.
Some final tips: 1) be meticulous in masking and be sure to preheat the circuit board adequately prior to rework, 2) allow the solder fountain enough time to heat up properly and make sure to keep the system properly cleaned and maintained (including the interchangeable nozzles) and 3) once the component is removed or replaced, immediately drop the solder fountain to prevent overexposure.