Reflow Soldering Techniques

The specific thermal differences of the individual mass reflow soldering techniques can be aggravated by the geometrical complexity and profiling of the 3D-MID. 

In the early 1970s, the first companies to apply low cost, mass production techniques to photovoltaics, a technology that had previously been considered an exotic aerospace technology, emerged. These techniques included the use of electroplated and screen printed metal paste electrical conductors, reflow soldered ribbon interconnects, and by 1977, low cost, automobile windshield-style, laminated module constmction. Such processes benefitted from a substantial existing industrial infrastmcture, and have become virtually ubiquitous in the present PV industry. 

Surface mount techniques have also been successfully used to assembly hybrid circuits. The metallized ceramic substrate is simply substituted for the conventional printed circuit board and the process of screen printing solder, component placement, and reflow solder is identical. 

The mass flow or reflow methods are suited for high-volume manufacturing. The entire board is heated and large numbers of components on the board are soldered simultaneously. The two most common of these methods are oven reflow soldering and wave soldering. A third technique, vapor phase reflow soldering, has dwindled in popularity due to environmental concerns regarding the use of the chlorofluorocarbon-based solvents that were key to this process. Now, however, perfluorocarbons are substituted and the technique is still in use. 

Because of safety and environmental concerns and comphance with the Montreal Protocol for the reduction of ozone-depleting chemicals, this soldering technique had fallen out of favor. With the advent of Pb-free soldering, there is increased interest in vapor-phase soldering for SMT applications, but it is likely that it will remain a niche application. Due to its continued diminished status, vapor-phase reflow is covered only briefly. 

Flip-chip solder-bump Chip attachment technique in which pads and the surface of the chip have a solder ball placed on them. The chips are then flipped and mated with the MCM or PCB, and the soldered reflowed to create a solder joint. Allows area attachment. 

Successful soldering is dependent on several factors including suitable, well-maintained, and controlled reflow equipment good-quahty solderable parts a thermally balanced board designed for the reflow process a well-tested and rehable solder paste a proven time-temperature profile and good thermometry techniques. 

Solder Volume. The inabiUty to apply enough solder paste to meet standard through-hole solder-joint acceptabUity criteria is one of the major shortcomings of pin-in-paste reflow. That is why this technique is usually relegated to boards <1.6 mm (0.063 in.) thick. Requisite solder volume is dictated by component pin pitch and the available printing space between component leads stencil thickness, which is generally limited by the smallest or finest pitch components on the PWB and PTH volume and associated component lead displacement volume. 

Solder requirements are the same as for any other process. There are no ahoy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is apphed, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. 

Specifically suited to surface-mount assembly of packages with lead frames, hot-bar soldering has been in use for several years. The technique rehes on a resistance-heated element to push component leads into contact with solder and bonding pads, simultaneously reflowing the solder. Compression of the leads onto the circuit board lands is continued as the heat is ramped down. Upon cooling, the solder solidifies and the heating element is withdrawn from the newly formed solder joints. The heated element is commonly referred to as the hot-bar, although the term thermode is also in widespread use. 

Solder can be applied to the board as a paste, solid pre-form, or solder-coated pad. In all cases, the component must be held down during the soldering process to ensure contact of component leads with bonding pads. Proper gas pressure, temperature, nozzle translation speed, and flux are required to effect joint formation. Otherwise, the same reflow considerations are required for this technique as for any other. Heating ramp rate, solder paste preheating, peak temperature, liquidus duration, etc., must be observed for successful joint formation and joint reliability considerations. 

SPC requires reliable data that can be analyzed either in real time or historically. Visual inspection collects defect data, such as the number of solder joint defects per assembly right after the solder reflow process (either reflow or wave soldering). Some manual and automated inspection techniques also take quantitative measurements of key assembly parameters, such as solder paste volume or solder joint fillet height. To the extent that these data are repeatable, manufacturers use defect data or measurements to characterize the amount of process variation from assembly to assembly or from solder joint to solder joint. When the amount of variation starts to drift outside its normal range or outside its control limits, manufacturers can assess the assembly process and monitor or choose to take action until the process is adjusted to eliminate this drift. Historical analysis of the defect or measurement... 

A second flip chip technique is realized by the use of eutectic Pb-Sn or other low-melting-temperature solder bumps in place of high-Pb solder bumps. These chips can be directly reflow attached to the chip carrier using low temperature. In this application, the chip carrier can be either ceramic or an organic laminate. 

Numerous studies have demonstrated that modest superheat is sufficient to form acceptable solder joints at temperatures that assure the functional integrity of the great majority of printed circuit boards and components utilized [44,70,75,78,79]. There may be some components, such as tantalum capacitors, that are particularly sensitive to temperatures above 210°C. In these cases, techniques such as selective soldering may be appropriate instead of mass reflow techniques. 

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