Reflow processes defects

Assembly requirements also place constraints on the circuit board layout. A very high part density requires a large number of apertures in the solder paste stencil, which can cause the stencil to become locally too flimsy to control the solder paste deposit. A surface-mount circuit board with a very wide range of component sizes and package configurations may require multi-thickness stencils to properly control the paste deposit. Solder paste printing quality is a determining factor in solder-joint defects observed after the reflow process. 

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. 

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... 

Defect detection where rework is easiest, before component placement and solder reflow Process characterization during the lead-free conversion with minimal program tuning... 

When inerted reflow processes were introduced to attach fine-pitch components (i.e., < 0.5 mm) to boards, companies reported substantial improvements in defect rates. One longitudinal study [15] reported a 50% reduction in wetting defects, and bridging also declined. It was demonstrated... 

If desired, plasma oxide films can be doped much as the plasma nitride film we discussed earlier. In fact, doping with boron and phosphorus has been carried out as an alternative to the standard atmospheric-pressure thermal CVD process for BPSG.11 12 The latter process has the drawbacks of high defect density and poor thickness uniformity, so it was hoped that plasma BPSG would be an improvement. However, there are differences in the films in terms of H2 and N2 content, and their effect on reflow temperature, intrinsic stress and passivation effectiveness had to be examined. 

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. 

An adjunct to the surface-mount process, this method, sometimes referred to as intrusive reflow, allows the soldering of some through-hole (solder-tail) parts into plated-through holes on the circuit board during SMT oven reflow. This process can eliminate or reduce the need for wave soldering—a step prone to defects. 

A potential defect is, in the manufacturing process, a deviation from a norm, that may or may not be a defect at the end of the manufacturing process This category needs to be understood. An example is a pre-reflow misaligned chip component. This component may or may not self-align in the reflow oven. Another example is an insufficient paste volume that may not end up as a defective solder joint at the end of the manufacturing process. 

After reflow, the main objective k to identify defects so that repair action can take place. For thk manual vkual inspection, AOI post-reflow, or automated x-ray inspection, can be used. Even with good process control, defect levek are hkely to increase, especially for higher-complexity boards. If defect levek are higher, then a good inspection strategy becomes more valuable. 

Visual inspection after the reflow and wave-soldering process steps can also be jnst a qnick scan for obvious defects to detect a process condition outside of control limits. In this case, the operator visually inspects for solder bridges, large solder balls or solder splashes, lifted leads, and a... 

Pre-reflow AOI systems also have the ability to inspect 2-D solder paste however, this ability is utihzed only to inspect a small percentage of the solder paste deposits combined with the component misalignment measurements. Component misalignment measurements cover the passive components, whereas the solder paste measurements cover deposits for BGA, CSP, or fine-pitch QFP devices. Therefore, these systems are placed within production lines after the pick-and-place systems for passive devices but before the pick-and-place systems for the larger area-array and leaded devices. These systems serve the same purpose as those only meant for component placement measurement, both detecting defects and monitoring measurements within control limits to discover process drift as early as possible. 

Application. These post-reflow AOI systems generate only attribute data. For instance, these systems detect the existence of a solder bridge between two joints or the absence or presence of a toe fillet on solder joints. But they do not measure the height of the heel fillet or the amount of solder in the solder joint. These systems typically do not measure how far a component is misaligned from its proper placement, but instead simply determine whether or not the component is misaligned more than a predetermined amount. These attribute data are not as useful for process control, and thus manufacturers use these systems strictly to detect defects. Normally these systems do, however, warn of a condition where the same defect has occurred several times consecutively or within a specific number of assemblies, indicating that some part of the process needs adjustment. 

They afford real-time process control of all three process steps— paste printing, component placement, and solder reflow— to lower defect rates and rework costs. 

Nitrogen is preferred for controlled atmosphere soldering [46,47]. It is the least expensive gas to produce. Nevertheless, the use of a N2 atmosphere extols a significant cost penalty on the assembly process. Specially equipped furnaces are required to support a N2 reflow environment. Additional plant costs are incurred for providing the N2 supply as well as to transport N2 gas to the furnace location. Residual O2 levels less than 100 to 200 parts per million (ppm) and, preferably, in the range of 20 to 50 ppm, can significantly reduce Pb-Sn solder defects that occur during assembly. 

In addition to termination-material or compatibility issues, there are several other component material-related concerns, the most important of which is damage due to moisture uptake when processed at the higher Pb-free assembly temperatures. The most prominent defect is delamination due to warpage or popcoming, a phenomenon whereby the moisture absorbed by a component on the manufacturing floor is converted to steam when exposed to the solder reflow-temperature cycle. Many components are downgraded in moisture-sensitivity-level rating when used with lead-free process conditions. 

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