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| A publication of the National Electronics Manufacturing Center of Excellence | February 2004 |
 In today's mixed assembly board, surface mount components represent the majority of all components. However, it is rare to see the complete absence of through-hole components in most assemblies. In some cases, selective soldering can be avoided by using the paste-in-hole process for through-hole components. The paste-in-hole process is not always feasible when the component is temperature sensitive, has too many rows of pins to allow sufficient solder paste deposition, or the board is not designed for paste-in-hole process (e.g., other components may be too close). Whenever the board's secondary side contains devices that cannot go through a wave solder machine, the through-hole components can be selectively soldered without impacting the adjacent components already soldered in the reflow oven. Examples of components on the bottom side that cannot go through the wave soldering process are any components that have solder spheres on the package bottom (PBGA, BGA and CSP). This makes a strong case for the co-existence of throughhole and surface mount packages on the same board (mixed assembly). Continued use of through-hole components like sockets and connectors along with fine-pitch and BGAs means there are no options but to use selective soldering for the foreseeable future. The following are some commonly used selective soldering options, along with some that are not used currently but may be popular in the near future. 1. The most common method to selectively solder throughhole components in a mixed assembly board utilizes nonmetallic fixtures. Board design is critical to reduce the number of iterations needed to come up with the final fixture that will only expose the through-hole components and completely hide the surface mount components on the bottom side. This option can be very expensive. Also, it is easy to run out of storage space for fixtures because about half a dozen fixtures are typically needed for each product. 2. Another method, generally referred to as solder fountains, uses a metallic fixture that covers the solder pot. The solder comes out like a fountain at the designated locations under the through-hole leads. These fixtures also can be very expensive and take more than a month to design and fabricate. The soldering defect levels can be high because the solder fountains change the wave's flow dynamics.  3. Another method, generally known as site-specific soldering, where a robotic carrier moves the board to the solder fountain, is becoming common. A variation of this method is a "dancing" wave, in which the board remains stationary but the solder fountain travels to protruding leads. The solder fountain solders each lead or row of leads at a time. Site specific soldering machines have built in fluxers, preheat and solder fountains and tend to simulate the standard wave soldering process. Such machines are flexible and do not use fixtures but can be expensive.4. Another method uses intense focused light to heat the leads and provides solder by wire feeders. The key disadvantage of this process is that allowance must be made at the board design stage to keep solder mask away from the pads. Otherwise the solder mask close to the through-hole pads tends to burn or discolor. Additionally, such machines are flexible and do not use fixtures but can be expensive. 5. An increasingly popular method for selective soldering is a laser that also uses wire feeders. A built in IR sensor(s) in some of the commercially available systems use high powered diode lasers and can turn off the soldering when the joint achieves the desired solder temperature and volume. Some of these systems also can use the paste-in-hole process eliminating the need for wire feeders. These machines use fiducials for alignment and CAD data for locating the leads and therefore do not need fixtures. The laser can be focused from the top or bottom. Eliminating the need to flip the board before soldering is one advantage of using the laser and wire feed from the bottom. Another advantage is lasers are environmentally more desirable because they do not require hot metal pots. Additionally, laser systems can be less expensive than most other selective soldering options. The disadvantages of laser systems are 1) the processes may be slower and 2) diode laser soldering is a new technology and not everyone feels comfortable being on the leading edge. To select the right solution for your application, examine your product volume, mix, complexity and capital budget. The BeamWorks Spark 100, a Selective Soldering system designed for the SMT assembly industry, is used at the EMPF. The system utilizes a power controlled diode laser with a closed loop temperature and volume control for the soldering of the through-hole components. If you would like to see a demonstration of this system, please contact Jeff Stong at 610-362-1200 Extension 224.  |
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| ACI Technologies, Inc. - - www.aciusa.org - - (610)362-1200 |  |
