Reflow machine

But there are also quality control reasons to smooth out the flow of bottlenecked assembly processes. For example, if the component placement machine has a throughput of 100 circuit boards per hour, but the reflow machine can only process 90 circuit boards per hour, then printed and stuffed boards will spend time in the open factory air awaiting entry into the reflow step. The consequence is a degradation of paste properties, resulting in the increased likelihood of solderability defects and a drop in product yield. Such technical ramifications must also be addressed when considering equipment utilization for an assembly process. 

Conveyors. There are two main conveyor systems used in reflow ovens pin-chain and mesh belt. One is required and both are recommended for any reflow machine. 

Upon exit from the machine, the laminate is interleaved with paper to protect the domes, and then re-reeled. Currently, pick-and-place, screen printer, dispenser, and reflow specialists are working in partnership to produce a fully integrated flowline concept for dome processing and other reel-to-reel applications. The reflow machine is huilt on the same base as the pick-and-place machine (Figure 2) for placement of sticky foils onto PCBs. 

Figure 2. Reflow machine built on a pick-and-place base.

Reflow machine (RF-430-N2, Japan Pulse Laboratory Ltd. Co., Japan)... 

A number of assembly-related issues must be addressed with odd-form components. First, it is necessary that correct pad dimensions be designed on the circuit board. Also, the stencil must have the correct aperture size to print an adequate quantity of solder paste. The pick-and-place machine may require custom tooling in order to handle these components. Lastly, odd-form parts are typically larger and heavier. Therefore, it is possible that they wiU not readily self-align while the solder is molten during the reflow process. 

The physical plant costs are generally similar for both a Pb-free assembly process and a Sn-Pb line. The two possible differences would be electricity usage, which may be greater with the Pb-free line due to the higher soldering temperatures, particularly with respect to a reflow process, and the need for an inert atmosphere capability for the soldering machine. 

Just as in the reflow soldering oven, there are multiple solder-wave preheater styles, but only two are in prevalent use radiant preheaters (direct and indirect IR) and forced-air convective preheaters. Both are effective and both have their advantages. In fact, the best configuration is a combination of the two. Some wave-soidering machines can be equipped with both top and bottom preheaters. This can be advantageous for thermally massive boards. 

In most factories, defect levels at the wave step are higher than those for the oven mass reflow process. The defects are related to poor process setup, poor process control, inadequate PWB design, or any combination of the three. Although wave soldering has been around for a long time, it is stiU not very well understood, due mainly to varying machine configurations and number of process variables. 

The vapor reflow soldering machine is composed of three main subsystems conveyor, reservoir vessel, and heaters.The vessel has cooling coils surrounding it, placed well above the level of the liquid in the reservoir.These condense the vapors, returning the majority to the reservoir, as shown in Fig. 47.31. 

When massive, densely populated boards are introduced to the vapor reflow oven, vapor collapse can occur—a condensation rate that outpaces that of vaporization. The result is that the internal atmosphere of the oven thins dramatically to the point that it cannot sustain adequate reflow. Vapor-phase machines that rely on immersion heaters are prone to this phenomenon. More recent machines include massive heating element housings that provide sufficient thermal inertia to preclnde this problem. 

Hot-gas soldering relies upon a heated stream gas heated to effect reflow. This noncontact, directed-energy method is most suited to bonding surface-mount components. Although hot-gas soldering has been around for years and after numerous machine offerings, it is not a popular method for soldering despite its evolutionary improvements. Instead, it has made its mark in the area of component rework—removal of previously soldered devices from a circuit board and replacement of same. 

One disadvantage of this soldering method is that the thermal energy is not well localized. Most machines typically emit a hot-gas jet too large to be isolated to reflow only the device of interest. The gas jet, once impinged upon the board and component leads, is deflected and its backwash can be problematic. It may cause unwanted reflow of previously formed joints, especially on closely spaced adjacent components. This problem is typically overcome by the use of baffles that are either applied to adjacent components or by a singular baffle that confines the gas jet to the component to be soldered. 

Inspection systems normally are dedicated to one type of measurement capabihty solder paste, pre-reflow, or post-reflow inspection. For example, systems for solder paste measurements do not normally also make component placement measurements. The cost of combining different measurement capabilities into one system would typically make that system prohibitively expensive. More importantly, to reduce manufacturing costs, manufacturers want to implement linear, sequential production lines where an assembly always flows in one direction and goes through each machine only once per assembly side. So automated inspection systems fall into three major categories ... 

Automated production requires a workpiece carrier that can be used all the way along the production line. Consequently, the design is such that the carrier can be conveyed by the printed-circuit board conveyor to all the machines in the process chain. The appropriate number of workpiece carriers is required if production is to be continuous. An interface for energy supply and communication between the automatic machines and the workpiece carrier is necessary, particularly for the dispensing unit and the automatic placement machines. The electronics built into the workpiece carrier render it unsuitable for reflow soldering, so a handling step has to be included so that the MID can be lifted on to a separate, passive workpiece carrier for this process. This transfer can be manual or by means of an industrial robot such as the Scara robot. 

FIG. 33 Representative SMT assembly line. The tools depicted in the photograph are (a) stencil printer, (b) turret-style placement machine, (c) line-pitch gantry style placement machine, and (d) reflow furnace. (Courtesy of Universal Instruments Corporation). 

---