Conduction vs. Convection in Rework Operations

Estimated reading time: 6 minutes

Rework and repair of densely populated surface mount boards featuring fine-pitch passives, ICs, and connectors can be performed to a high standard. The key lies in selecting the optimum tools and techniques at each stage of the repair process. Controlling the delivery of heat to the rework area is critical to achieve the best results. This article explains the use of conduction vs. convection heating during a manual rework operation.

By Ed Zamborsky, OK International

Value is added to a surface mount assembly at every stage of the production line - from paste printing through final assembly. When the assembly emerges from reflow, the process of adding value is complete. If defects are discovered after reflow, scrapping the assembly can represent a high cost to the manufacturer. Therefore, an effective solution for reworking surface mount assemblies is a sound investment for many manufacturers, especially if the boards or components have a high value. This may be the case for assemblers building large backplane-type products, using expensive multi-layer boards or exotic substrate materials, or those using high-value components such as application-specific devices, large system-on-chip (SoC) die, or cutting-edge packaging technologies including system in package (SiP).

Figure 1. Component removal using conduction heating.

Therefore, to minimize waste and maximize yield, assemblers need high-quality manual rework capabilities. A high level of operator skill also is required, particularly when fine-pitch components or ICs featuring area-array interconnects are used. However, the choice of tools and techniques needed to remove a component from the site of the defect, prepare the board for rework, and replace or renew the component is the critical factor that governs the standard of rework.

Assembly Defects

The defect may be the result of a printing error such as insufficient or excessive paste, or poor paste alignment. On the other hand, a placement error may have occurred, such as component misalignment, incorrect polarity, or excessive placement force - resulting in distortion of one or more leads, or damage to the component body. In some cases, a component may be completely missing from the site, requiring removal of reflowed solder and manual placement of the correct component. Defects induced by the reflow process - bridging, balling, or tombstoning - also contribute to rework.

Component Removal

For the majority of surface mount assembly defects, it usually is necessary to begin by removing the component from the site. The component may be retained and re-used, or discarded if it is faulty or has been damaged.

Figure 2. Convection heating rework system.

Desoldering the component can occur using direct contact with a soldering iron or special rework iron. As a rule, using a soldering iron in conjunction with a surface mount component assumes that the component will not be re-used. Contact from a soldering iron tip can damage and distort fine-pitch interconnects, or the component body of a chip resistor or capacitor. Sudden direct contact by a soldering iron tip at its maximum operating temperature also can deliver a damaging thermal shock to the component, inducing failure in component interconnects and damaging the silicon. This may render the component unusable. However, some soldering irons designed specifically for rework applications allow the operator to vary power to heat the solder joint to the ideal temperature without overshooting (Figure 1). There also are advantages to conductive heating by applying a purpose-designed rework iron. For example, heating effects tend to be localized, which prevent surrounding areas and components from being exposed to high temperatures. This can be beneficial, preventing good solder joints from being re-melted and protecting delicate parts of the assembly from being exposed to potentially damaging temperatures.

Figure 3. Convection heating nozzles for industry-standard components.

A convective heat source, on the other hand, can offer more control over the quantity and intensity of thermal energy applied to the defect site. This energy is naturally distributed throughout a wider area on the board. This may be useful if a large number of solder joints are to be melted to remove a component. Controlling heat delivery to ramp-up the temperature of the solder joint to its melting point can prevent instances of thermal shock. Some manual reflow stations (Figure 2) provide a selection of nozzles that fit standard-size component packages (Figure 3). These allow operators to control the delivery of heat within a localized area close to the I/Os of the target component.

Preparing the Site

After removing a component, the site may be cleared of remaining solder. Using a rework soldering iron, in conjunction with a desoldering braid, is an effective method to remove unwanted solder alloys from PCB pads (Figure 4). The site should be checked for complete solder removal, and the soldering iron and braid should be re-applied, if necessary. If the original solder is removed, miniature portable solder-paste dispensers allow finely metered quantities of paste to be deposited accurately by hand onto selected PCB pads.

Figure 4. Site preparation - removing existing solder using a soldering iron and desolder braid.

On the other hand, the existing solder may be re-used. If this is the case, flux should be applied to the pad area before heating using manual reflow equipment. This causes the solder to flow and provide a smooth, uniform base for subsequent component placement. Whether the chosen method is conduction heating with a soldering iron or convection heating from a reflow station, the site should be cleaned with an approved solvent after cooling and before being re-populated.

Localized Reflow

Following accurate, manual component placement using tweezers or a vacuum pencil - possibly in combination with a magnifying instrument if small or fine-pitch devices are used - solder joints must be re-formed at the points of contact with PCB pads. Convection heating using a local reflow station is a viable option for high-quality results. Instruments offering a selection of nozzles designed to direct heat accurately onto the I/Os of standard component packages allow operators to reflow individual components without affecting other devices on the board. The latest generation of reflow solutions aimed at rework duties can provide enhanced control over the reflow profile and duration. These parameters can be preset to allow reflow of individual devices under automatic control. This leads to enhanced uniformity and reduces the potential for human error. Custom or odd-form components, including radio frequency (RF) shields, also can be reworked using convection heating solutions fitted with a specific nozzle.

Conclusion

Several thousands of boards are reworked each year to correct soldering defects, as well as faulty or misplaced components. Remedial work may be necessary immediately after production, or to repair failed units returned from the field. The selection of tools and techniques for component removal, surface preparation, and component replacement is guided predominantly by decisions related to the heating of solder joints in the rework areas. Ultimately, this results in a choice between heating by conduction vs. convection, depending on whether the existing component and solder deposits are to be discarded or re-used.

Fortunately, the range and capabilities of specialized rework tools has changed with the times to address the demands of modern, high-density assemblies using fine-pitch surface mount ICs, passives, and connectors. Manufacturers and repair specialists can focus on finding tools that place less emphasis on operator skill and judgment, and provide a wide range of options to suit the specialized components that are being designed into boards.

Ed Zamborsky, regional manager, soldering and APR specialist, OK International, Inc., may be contacted via e-mail: ezamborsky@okinternational.com.