Circuits Assembly Online Magazine - Integrating AOI and Selective Soldering

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A breakthrough machine for PTH mass soldering combines AOI and selective soldering in a single inline unit.

Wave soldering remains the most prevalent, efficient method for mass soldering plated-through-hole (PTH) components. Although mass soldering of PTH components, either by wave or dip soldering, has improved dramatically, the process generates one of the highest defect per million opportunities (DPMO) rates in electronics assembly. Inherent characteristics of the process render it far from perfect. Thus, post-wave manual inspection and hand touch-up have been common practices.

Pb-free implementation will not only make this situation more severe, but will also aggravate the quality risks associated with manual touch-up. The tighter process window and higher process temperatures for Pb-free soldering greatly increase the risk of overheating adjacent components or damaging the board during manual touch-up and repair.

Meanwhile, AOI has gained acceptance in SMT as a technology that provides objective results, enhances first-pass yield and enables traceability. However, AOI has not been commonly used for PTH inspection in mass-soldering lines where it is most needed because an operator is typically required to perform manual touch-up. The highly subjective nature of manual inspection and repair is accepted practice, however, as is lack of repeatability (due to operator variability) and lack of product traceability.

Increased use of tiny surface-mount packages such as µBGA, CSP and flip chip, together with expanded use of area-array SMT components, continues to challenge board-level assembly. At the same time, packaging roadmaps forecast that PTH components will continue in power devices and connector applications for the foreseeable future, albeit at diminished volumes. This continued convergence of SMT and through-hole components means more mixed-technology assemblies, which in turns means greater attention needs to be paid to PTH solder joint quality.

Due to the physics involved with an unlimited supply of flowing liquidous solder, all forms of PTH mass soldering - including wave, dip and aperture pallet soldering - result in various solder defects. Most common are bridging, insufficient solder, opens and miscellaneous solder-related defects. Wave solder defects can result from insufficient wetting force, component shadowing, excessive lead protrusion or solder splashes. While dip soldering can closely equal wave soldering throughput rates, solder defects can result from inadequate keepout area, board warpage or restricted solder peel-off.

A method that has gained popularity for soldering mixed-technology assemblies is the use of masking, or aperture wave pallets. PCBs are mounted in pallets that have cutouts to direct the flow of liquidous solder to the individual sites of PTH interconnections. The pallets are run through a wave solder machine so that the wave only makes contact with the PTH interconnections and not the SMT components that have been previously reflow soldered. While this method substantially reduces bridging of SMT components, increased PTH solder defects can occur due to pallet design confines such as limited adjacent component clearance, pallet thickness and draft angle resulting in shadowing, inadequate solder contact and limited vertical hole fill.

Manual Inspection and Repair

Manual touch-up with a soldering iron has always been a risky undertaking at best. Hand soldering quality is completely determined by operator skill level and the efficiency of the soldering iron. From a repeatability standpoint, all solder joints should be made with the same temperature with dwell time regulated for variations in component thermal mass.

The variable human factors associated with manual PTH inspection and repair generally result in inconsistent solder quality and unpredictable throughput. Problems associated with manual touch-up include lack of repeatability, operator dependant subjective inspection criteria, operator fatigue, non-traceable results, risk of board damage and scrap, high cost for solder wire and replacement solder iron tips, operator training, high labor costs, large floor space and low productivity.

One common shortcoming of manual inspection and repair is that repetitive touch-up is often carried out on a continual basis without providing feedback for resolution of wave soldering problems. The subjective nature of manual inspection and repair conducted concurrently by several operators rarely results in statistical process control (SPC) monitoring of solder defects. Repetitive touch-up data should be continuously tracked and a root cause analysis conducted to determine if the wave soldering process is in control, or if a specific component or pallet design is causing quality problems. Post-wave repetitive touch-up has a direct correlation to improper pallet design, poor wave optimization or inadequate thermal profiling.

Impact of Pb-Free

The higher process temperatures required for Pb-free soldering greatly impact assembly, especially in manual touch-up and repair processes. The higher thermal excursions required for rework increase the potential for damage to components and boards. High thermal mass PCBs with area-array SMT packages and PTH components in particular will create significant challenges for Pb-free rework and repair.

While Pb-free solder alloys require higher processing temperatures due to the nature of their wetting properties, many PTH components can be damaged if their internal threshold temperature is exceeded by either rapid or excessive heating of the PCB during soldering or manual touch-up. This is especially true in Pb-free repair since the process temperatures are typically 30° to 40°C higher than the SnPb temperatures for which most PTH components were designed.

When using a soldering iron, it is critical that the tip temperature remain constant. Because a soldering iron generally does not recover heat loss fast enough, however, touch-up and repair operators attempt to compensate by using higher set temperatures, typically 380° to 440°C. These already high solder iron temperatures will be even higher for Pb-free. The problem: a soldering iron tip - with a relatively low mass - is heated by a heating element - with relatively low power - and as heat is transferred to each consecutive joint, the tip temperature decreases (Figure 1). How well a soldering iron puts back the heat lost at the tip and how long the tip remains on the joint ultimately determines the actual joint temperature. Although manufacturers are developing better irons, accurate, repeatable results are impossible to achieve manually. In addition, traceability is nearly impossible. Finally, skilled hand-soldering operators will need to be retrained and re-equipped when implementing a Pb-free process.

FIGURE 1: Typical solder iron temperature degradation.

Factors challenging manual inspection and touch-up in a Pb-free environment include:

Changes in visual inspection criteria.
100% operator dependence.
Operator fatigue.
Potential false calls.
Soldering iron design limitations.
Higher working temperatures.
Operator skill retraining requirements.
Greater tendency for lifted pads and board damage.
Higher direct labor costs.
Higher costs of additional factory floor space.
Higher running costs (solder tip wear, solder wire, scrapped boards, etc.).
Lack of repeatability or traceability.

The philosophy behind in-process inspection is that it catches a defect at the process center that produced the error, where it costs less to correct the error in terms of time, labor and material in the assembly process. In-process inspection also facilitates root cause analysis with regard to the source of the defect so that the offending process can be immediately rectified before more defects are generated. When inspection is not integrated into an assembly line, it is often supplanted by offline manual inspection and repair that is decoupled into "islands of inspection," resulting in longer reaction time to correct an offending upstream process, such as wave soldering.

Using AOI for in-process inspection can detect defects (with the exception of faulty components), thereby providing high value-add at low cost to improve and maintain process yields, and functioning as a feedback tool for immediate process improvement. AOI provides inspection coverage for defects such as solder bridging, missing or displaced components, and wrong components, so that all defects that occur during assembly are detected. To this extent, AOI affords several advantages over in-circuit test (ICT) in that programming time is generally hours versus days, plus the use of dedicated fixtures is eliminated.

One of the pitfalls of AOI is false calls, or pseudo errors, including false alarms or bogus defects that typically occur as a result of the behavior of the software algorithms in combination with the imaging process. For AOI to function as a meaningful tool, the frequency of false calls must be reduced to an acceptable minimum while maintaining maximum system throughput.

A 99.5 to 99.7% first-pass yield for AOI inspection can result in as many as 15 to 25 pseudo defects for a board containing 5,000 SMT solder interconnections. This relatively high frequency of false calls is caused by the difficulty in discriminating between the considerable number of different SMT component sizes, shapes and interconnect configurations. Through-hole solder joints, on the contrary, always look similar regardless of the component type when viewed from the solder source side of the board. This results in a false call rate for repairable PTH soldering defects such as bridges, insufficient solder and opens, that is 10 times lower than typical SMT pseudo defects.

Approximately 99% of all AOI installations are used for SMT applications such as post-print, post-placement or post-reflow while PTH applications account for less than 1%. This is somewhat of a revelation as world-class, in-process quality for SMT assembly typically ranges between 50 to 200 DPMO while PTH assembly is considerably higher (2,000 to 3,500 DPMO). While it is uncommon to use AOI for PTH inspection in mass soldering lines, that is where it is most needed due to higher DPMO rates.

Selective Soldering

Selective soldering permits the user to optimize the soldering process to the pin level versus the compromise techniques in flux application and contact time with mass soldering. Since flux deposition, nozzle height, solder dwell time and peel-off parameters are fully programmable and can be optimized for individual components, it is an ideal method for repairing PTH solder interconnections.

The ideal system for Pb-free repair should provide uniform heat distribution and incorporate features to prevent the repaired component, and adjacent components, from overheating. Selective soldering meets this requirement by delivering constant, localized temperature via an unlimited supply of recirculating liquidous solder. The wettable solder nozzle is ideal for PTH repair since peel-off parameters are fully programmable and can precisely remove a solder bridge, or re-solder an insufficient or open solder joint (Figure 2). It also maintains the lowest possible working temperature - maximum of 285°C for Pb-free - which replaces the hand soldering working temperatures of 380° to 440°C.

FIGURE 2: Wettable selective soldering nozzle with constant temperature.

An added advantage of the wettable solder nozzle is that the keep-out area around the nozzle is significantly less than the adjacent component clearance of aperture wave pallets or the keep-out area of dip soldering. The wettable solder nozzle can repair PTH solder joints as close as 0.5 to 1.0 mm between a PTH pad and an adjacent SMT pad, equal to hand soldering and significantly less than typical aperture pallet clearances of 2.5 to 3.0 mm and dip soldering keep-out areas of 3.0 to 5.0 mm.

Automated Inspection and Repair

The best that most standalone AOI systems can do is inspect defects out of a process rather than preventing them, which contradicts Six Sigma quality principles. A proprietary new system combines AOI technology with an automatic selective soldering system that has the ability to inspect, correct and verify soldering. Called AOI+R, it is a fully automated optical inspection and repair system for PTH and mixed-technology assemblies in a single machine. The system is automatic and closed-loop, reducing the problem of human subjectivity and error.

Immediately following mass soldering, boards are inserted into the system, where each PCB is automatically scanned. When a defect is found, an automatic command is sent to the selective soldering module where the board containing defects is automatically programmed and immediately repaired. No additional programming or operator intervention is required. The exact position and defect type determines the automatic repair program. A wettable selective nozzle that is covered with a nitrogen blanket completes the repair process. The temperature and quality of the repaired joint is superior to that of a hand soldering iron. After repair, the PCB can be either selectively rescanned to confirm defect correction or sent for further post-wave processing. Repaired boards exit the system only when a "pass" condition is guaranteed and are transported back to the inline workflow process for near "zero defect" mass soldering. All process data for the inspection and repair processes are documented for quality control and traceability.

When multiple mass soldering lines are in operation, inspection can be performed on each line using an inline AOI+ scanning module (AOI+iL), after which boards that pass inspection continue through the module and move down the line. Meanwhile, boards containing defects are transferred to a centrally located AOI+R machine for repair. When a repairable defect is detected, the appropriate repair command is generated and networked via data transfer to the AOI+R machine. Boards containing defects are then transferred to the offline machine, which functions as a centralized repair cell requiring only one operator. Defective boards are scanned via bar code and manually loaded into the machine. The defect and repair command is sent to the selective repair module that conducts the repair. Immediately following repair, the board is automatically rescanned for correction validation. Boards that have a non-repairable defect are identified and stacked separately for manual unloading.

This closed-loop system provides full product traceability and board tracking throughout inspection and repair processes. Unlike manual inspection and touch-up, the need for segregated board handling and traceability concerns are eliminated.

The operational differences between manual inspection and touch-up and the new machine can be illustrated by evaluating a typical PTH assembly in terms of cost, throughput and defect rates (Table 1). Only the manual inspection and touch-up time are reviewed as the wave solder system and secondary operations prior to wave soldering are non-limiting operations. Subjective inspection criteria will often generate false calls, leading operators to overcorrect by touching up good solder joints, which substantially increases the potential damage to components or the board.

In this example, five operators are required to manually inspect and touch-up 1,000 boards per 8 hr. shift. To process a larger volume of boards, such as 3,000 per 24 hr. workday, several more operators are required resulting in additional floor space, added cost, longer process times and increased variation in process quality.

By implementing AOI+R, the nonuniform demand condition is eliminated from post-wave inspection and repair and work-in-process inventory is reduced (Table 2). Upstream process response and wave-soldering corrective action are effectively speeded, thereby enhancing process yields, improving quality and reducing scrap (Table 3).

Conclusion

Mainstream in-process quality rates that are historically 2,000 to 3,500 DPMO after wave soldering can be reduced to below 200 DPMO using the new system. The remaining defects that cannot be repaired by the machine: missing or skewed components or insufficient destination side vertical hole fill. Yet over 85% of post-wave defects are repairable such as solder bridges, excess solder or bulbous joint, insufficient solder and solder splash.

Finally, an AOI+R module can be integrated as a subsystem within a wave solder machine. This forms a new concept in mass soldering, "progressive wave technology," consisting of chip wave, finishing wave and selective wave, all in one machine. This three-wave process permits boards to be processed within a true closed-loop system before exiting the machine. In line with Six Sigma principles, progressive wave technology does not repair boards but rather corrects defects internal to the process center and eliminates non-value-added end-of-line inspection and touch-up.

References

International Electronics Manufacturing Initiative, "2004 iNEMI Board Assembly Roadmap," June 2005.
Bob Klenke, "Lead-Free Selective Soldering: The Wave of the Future," SMTA International Proceedings, September 2002.
Reinhard Klein Wassink, "Soldering in Electronics," Electrochemical Publications, 2nd edition, pp. 107, 1989.
Ron Daniels and Bob Klenke, internal ITM Consulting document, May 2003.

Mark Cannon is president and COO, ERSA GmbH (ersa.de); mark.cannon@ersa.de. Bob Klenke is a principal consultant at ITM Consulting (itmconsulting.org). Phil Zarrow is president and principal consultant, ITM Consulting.