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Thermal Imaging Looks at Laser Rework and Repair
December 31, 1969 |Estimated reading time: 8 minutes
Thermal imaging can be combined with laser rework to enable precise, noncontact solder joint heating and to develop and test process specifications for rework.
By Larry Sirois
As circuit boards become increasingly populated with ball grid arrays (BGA), chip scale packages (CSP), direct chip attach (DCA) and other microcomponents, temperature monitoring is essential for process analysis and development in rework. Thermal imaging is a method of determining surface temperatures by measuring the infrared (IR) energy emitted by a component or package: the higher the energy radiated, the higher the surface temperature. Energy level differences are represented by the color of the device being imaged. Each color is calibrated to a specified temperature.
Thermal imaging can be combined with laser rework to enable precise noncontact solder joint heating without overheating components and surrounding interconnects. Additionally, thermal imaging can be used in developing and testing process specifications for rework, which would be difficult to accomplish using thermocouples alone.
Thermal Imaging Thermal imaging permits thousands of data points to be analyzed by viewing colors assigned by software to represent specific surface temperatures. A special camera measures IR radiation; the radiation emission being viewed on an LCD display or loaded into a computer for analysis. Surface temperature ranges correspond to specific colors of the visible spectrum. Uniformity and temperature separation at any given time can be displayed, recorded and measured. Specific temperature points may be shown using a cross hair guided by a PC mouse control.
While the accuracy of absolute temperature depends on surface emissivity, thermal imaging can depict temperature variation differences accurately for any material. Thermocouples, though useful in developing reflow profiles, are subject to placement, attachment, size of weld, quality variation and calibration. Measuring surface temperature with installed thermocouples can be difficult because they shield the area to be measured and cannot be depended on to give repeatable, accurate results. Solder joint temperature must be measured by attaching the thermocouple using traditional methods. Ramp rates, maximum temperature and process time requirements need to be specified to achieve the desired results. The profile for a rework procedure typically is the result of an effort to recreate the original assembly process.
Laser ReworkLaser rework is an effective alternative to hot air convection soldering for applications that require precise process control or have physical constraints that prevent the use of shielding devices to protect adjacent components. The process uses a computer-controlled laser to heat only those specific locations in which solder reflow is required during rework, thereby removing tooling costs and process delay. Because no tooling or nozzle is used, this noncontact heating process results in an unobstructed view of the rework area. Originally developed for circuit assembly in PCs, the technology is gaining use in the telecommunications industry where substrate real estate is highly populated with components, and soldering interconnects is a particular challenge. By changing path, speed and intensity, the laser is programmed for different component types. Thermal imaging then is used to monitor rework process effects.
Continuous Wave Yttrium Aluminum Garnet (CW YAG) heating. The CW YAG laser has a beam diameter of approximately 3.0 to 4.0 mm, allowing the operator to heat a specific component without affecting adjacent components or the surrounding substrate. With a wavelength of 1,064 nm, the laser heats the solder joints simultaneously. For FR-4 substrates, solder joints absorb 75 percent of the heat and 25 percent is absorbed by FR-4 material.
Once completely isolated during normal operation, the laser is considered Class 2, and neither safety glasses nor special facilities are required to operate the equipment under normal operating conditions.
Beam positioning. Computer-controlled positioning mirrors control the laser's path. They can be programmed in an infinite variety of paths. Five reflectors control the path in the X and Y axes. X and Y positioning mirrors are programmed for each component type, and the paths are stored in the computer interface. The distance the beam travels and the time it takes the path to be completed are programmable, providing the flexibility to modify the velocity of the laser beam path.
Visual alignment laser. Because the YAG laser is in the IR spectrum, it is invisible to the human eye. By coupling a visible helium neon (HeNe) laser with the YAG laser, it becomes possible to show the path during programming. The X-Y positioning mirrors deflect the HeNe laser, giving the programmer an exact visual representation of the YAG's path.
Temperature monitoring. To remove or replace a component successfully during rework, the YAG laser must be controlled so as not to overheat the components by applying heat too rapidly. The objective is to simulate the thermal profile of a reflow oven and maintain thermal uniformity across the component while preventing adjacent components from being overheated or reflowed. To accomplish this, a third optical device is coupled with the HeNe and YAG lasers: a pyrometer.
By reading the 3.5 μm light wave emission, the pyrometer continually monitors surface temperature every 10 milliseconds during the process for optimal process control. As the information is fed back to the computer, the power to the YAG laser is adjusted. Continuous process monitoring through a closed-loop thermal regulation system ensures a repeatable, reliable process. The distinction to be made here is that the pyrometer monitors the component being removed and replaced, while thermal imaging provides an ongoing visual temperature indication for the component and surrounding interconnects.
Bottom heating. A large capacity convection bottom heater is used to pre-heat the board prior to applying the laser to the top. This reduces the temperature differential between the top and bottom of the board, thereby reducing thermal stress and preventing localized warping in the rework area. The bottom heater is programmable and can be adjusted from 100° to 300°C.
Combining Thermal Imaging with Laser ReworkReworking defective components on circuit boards is affected both by localized heating of the component itself and generalized heating of the assembly. In both cases, heat dissipation is essential to ensure against damage and to form a proper solder joint. Contributing to the complexity of heat dissipation are such factors as the ground plane and the density of the component and the board. Thermal uniformity, adjacent component temperatures and bottom heat uniformity are related factors that have a bearing on where the thermocouples are placed and, ultimately, the success of the rework effort.
Thermal imaging provides the rework technician with the ability to not only depict and analyze temperatures in real-time, i.e., during the process, but also to quantify and calculate performance results to ensure and document criteria being met for a particular rework procedure.
Top heat uniformity (localization). Soldering BGAs to the surface of a printed circuit board (PCB) requires precise control of applied heat to maintain uniformity and reduce rework defects, such as solder bridging due to component warping. Typically the specification of ±10°C is required for most applications. To measure this, thermocouples need to be added to the surface being measured using high-temperature epoxies or high-temperature tape. In the procedure of mounting the thermocouples, surface conditions are affected, and errors in the readings are introduced. Any small movement in the thermocouple can dislodge the device and result in a significant difference in temperature measurement. This is not an acceptable error rate in the effort to prove the performance; thermocouples need to be incorporated for every data point that is required.
Thermal imaging, on the other hand, requires no tooling or changes in the surface to be measured and can gather thousands of data points instantaneously (Figure 1).
Figure 1. Thermal imaging depicts temperature differences during rework.
Bottom heat uniformity (globalization). Bottom heater uniformity during rework is important for reliability as problems of localized warping can occur if localized heat is applied from the top and bottom of the circuit board. By heating the PCB using the bottom heater only, the heat distribution can be analyzed. For small, localized bottom heaters, areas of potential damage because of heat stresses are identified quickly. For large-area bottom heaters, "hot spots" and thermal distribution may be identified for each application. This method also can evaluate the rework system performance.
Figure 2. Thermal imaging of a QFP during laser rework shows the heat dis-sipation pattern on the left and bottom of the component.
Ground plane heat absorption and dissipation. Components react to applied energy based on internal PCB structures. Ground planes spread the heat from the component being heated in nonuniform distributions (Figure 2). This may account for areas in which component solder joints reflow earlier in the process than others, regardless of the heat application. For hot air rework stations, thermal process control by thermocouples in an air stream will not compensate for variation in ground plane geometry. For BGA components, ground plane dissipation may cause component sides to collapse first, resulting in solder shorts or misalignment. Thermal imaging provides constant indication of the heating process.
Adjacent component temperatures. Rework specifications often call for process considerations applicable to components adjacent to the rework area. Typically, adjacent components need to maintain temperatures under 180°C to avoid solder joint melting. Attaching thermocouples to the surrounding area of the rework site with glue or solder to a lead usually tests this specification. Adding thermocouples affects the accuracy by increasing the mass at the location(s) being measured. Variances also are likely, depending on the interface between the thermocouple and the surface of the device, which can lead to a false "passing" or "failing" with regard to the specification.
Thermal imaging finds the variations in adjacent component temperature around the component, and determines if a process will affect adjacent components.
ConclusionWhile thermal imaging remains a useful tool in determining parameters for rework procedures and evaluating rework equipment, thermocouples still are required for determining absolute temperature of solder joints in rework profile development. However, laser rework enables an unobstructed thermal image of PCB surfaces during rework. This technology is valuable in gaining rework information about heat dissipation, adjacent component effects and component uniformity. As component densities and PCB complexities continue increasing, process analysis tools for rework become even more valuable. Using thermal imaging for measuring rework process effects increases measurement accuracy, improves data collection reliability, and decreases the time required to validate system performance and rework criteria.
Larry Sirois, thermal product manager, Americas, may be contacted at ViTechnology LLC; 179 Ward Hill Ave., Haverhill, MA 01835, (978) 372-1230; Fax: (978) 372-1767; E-mail: lsirois@vitechnology.com; Web site: www.vitechnology.com.