As electronic components become smaller and the space between the components decreases to less than 0.5 mm, the opportunity for creating solder "bridges" or short circuits be tween adjacent contacts increases. A more-precise method of applying and controlling both the heat and the solder volume is needed, therefore, in the realm of miniature and microminiature selective soldering. Diode-laser heating technology, coupled with solder-bearing lead technology, offers the electronic assembler a new tool.
Current methods
The electronics industry effectively uses mass-reflow-soldering techniques to bond the majority of through-hole and surface-mount components to its printed-circuit-board (PCB) assemblies. Some temperature-sensitive electronic components, however, cannot tolerate the high temperature peak of 230?C for one minute, typically encountered in the mass-reflow-soldering process, without suffering damage. These components are soldered off-line using hand or other semi-automated soldering techniques. This process is commonly known as "Odd Form Soldering" or "Selective Soldering" and constitutes as much as 10% of all electronic assembly work.
null
Hand soldering is slow and highly dependent on operator skill level to achieve a good joint. Attempts to use semi-automated forms of selective soldering have been partially successful. Semi-automated selective-soldering techniques include hot bar, the common soldering iron, and microflame. All three heat sources require intimate contact with the components to create a solder joint. These heat sources are typically paired with a wire-solder-feed system for controlling the wire-solder-feed rate.
Hot-bar reflow soldering uses a temperature-controlled metal heating element that can be pulsed to the desired reflow soldering temperature or maintained at a constant temperature. Hot-bar reflow soldering suffers from several major drawbacks, including mechanical deformation of the electronic-component surface and reduced thermal transfer due to warping and flux buildup on the heating-element surface. In some cases, the hot-bar contact surface must be cleaned as often as every 500 to 1000 cycles.
The soldering iron also uses a metal heating element for a constant-temperature heat source. Solder in the form of solder wire is hand or semi-automatically fed into the joint area. To prevent a reduction in heat transfer, the soldering-iron heating element or tip must be constantly wiped by a mechanical scrubbing mechanism to remove flux and solder oxide buildup. This repeated oxidation-removal process eventually results in the replacement of the heating element.
Microflame heating uses a miniature gas-feed flame to create the necessary soldering heat. Microflame is capable of generating very high temperatures with a large amount of heat, sufficient to melt a variety of metal alloys. This process works well with more-continuous applications such as brazing, which uses brazing alloys that typically melt above 450°C, compared to soldering in the range of 180°C. The microflame is generally turned on using an electric discharge. This arc-discharge mechanism can potentially damage sensitive electronic components. Moving the microflame in and out of the joint area provides a rough way to turn on and off the heat source without having to extinguish and relight the microflame.
Laser alternative
"FlashSoldering," which uses diode-laser technology, is a new, noncontact selective-soldering process that offers the electronic-component assembler a highly controlled method for soldering a variety of temperature-sensitive miniature and microminiature components. FlashSoldering applications include making miniature magnetic components such as single and multiple toroidal transformer packages, LAN filters, low-power dc-dc converters, single or multiple form coils and inductors, and joining fine magnet wires to high-speed data connectors. Printed-circuit-board applications include joining flexible printed circuits and other miniature electronic components to flexible or rigid mounting surfaces.
The FlashSoldering system comprises four components. In the case of the magnet-wire application, the first component is an "electronic contact" that locates and retains very fine insulated copper magnet wires during the soldering process. For flex-circuit and other electronic-component applications, the "electronic contact" is the mating surface, which could be another flex circuit or a rigid PCB.
The second component is the insulated magnet wire for the magnet-wire application, flex-circuit pads for the flex-to-PCB application, and component leads or feet for the electronic-component application.
FIGURE 2. Cross-section view of Fig. 1 as photo graphed by a backscanning electron microscope shows the very thin "fuzzy" copper-tin intermetallic ring surrounding the copper wire, indicating a strong bond between the wire and solder.
The third element is a precise amount of solder. The solder is plated on one or both contacts or mechanically attached to one contact in the form of a solder-bearing "nugget" that may also contain flux. Solder also can be applied in the form of dispensed solder paste, but requires more volume compared to a solid solder-bearing "nugget."
The fourth component is a diode-laser heat source. The growing commercial availability of low-cost, highly controllable diode lasers with wavelengths in the 810 to 980-nm infrared range and self-contained programmable power supplies makes noncontact selective soldering economically feasible.
Pulsed Nd:YAG lasers with an operating wavelength of 1064 nm have not proved very successful for soldering miniature electronic components due to their high peak power and short pulse duration. These characteristics cause solder plating or solder-paste expulsion and vaporization. The base materials tend to become pitted and covered with tiny solder balls that can cause shorting in the final assembled product.
To illustrate the diode laser as a reliable, controllable soldering heat source, consider the magnet-wire application previously mentioned. The diode laser simultaneously removes the insulation and solders the wire to the contact without damaging or contacting the wire. In the process of FlashSoldering a 100-µm-diameter, polyurethane-nylon insulated copper magnet wire to a surface-mount contact without using any flux, the low peak power (less than 11 W) of the diode laser and the short heating time (less than 0.25 s) prevent the copper wire from being dissolved by the tin, thus ensuring a reliable solder joint (see Fig. 1).
An analysis of a FlashSoldered joint by Sandia National Laboratories (Albuquerque, NM) showed no embedded carbon particles from the insulation material (see Fig. 2). Additionally, the copper-tin intermetallics surrounding the copper wire and the surface-mount contact were less than 1 to 2 ?m thick, indicating a strong bond.
Overall, the diode laser offers the electronics industry a new, noncontact selective-soldering process for producing and assembling a variety of miniature electronic components and interconnection systems. FlashSoldering with diode lasers improves process reliability by preventing shorts caused by solder bridging. The short soldering time improves joint integrity by minimizing copper dissolution by tin. In addition, FlashSoldering can reduce the labor and maintenance costs associated with other forms of selective soldering.