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Key Steps in BGA Rework Assembly Issues Rework Establish Temperature Profiles Clean Mount Locations Mount Devices Most manufacturers agree that ball grid array (BGA) devices have undeniable benefits.However, some of the problems in this technique remain to be discussed further, rather than being implemented immediately, because it is difficult to trim the solder ends.Interconnect integrity of the BGA can only be tested by X-ray or electrical test circuit methods, both of which are expensive and time consuming.Designers need to understand the performance characteristics of BGA, which are similar to those of earlier SMDs.The PCB designer must know how to modify the design when the manufacturing process changes.For manufacturers, there is the challenge of dealing with different types of BGA packages and the resulting process changes.To improve yield, assemblers must consider establishing a new standard for handling BGA devices.Finally, BGA rework personnel may be the key to more cost-effective assembly.The two most common BGA packages are plastic BGA (PBGA) and ceramic BGA (CBGA).The PBGA has fusible solder balls, typically 0.762 mm in diameter, which collapse to a 0.406 mm high solder joint between the package and the PCB during reflow (typically 215 ° C).CBGA uses non-melting solder balls on components and printed boards (in fact, its melting point is much higher than the temperature of reflow soldering). The diameter of solder balls is 0.889 mm, and the height remains unchanged.A third type of BGA package is the tape ball grid array (TBGA) package, which is now increasingly used in high performance assemblies where lighter and thinner devices are required.On polyimide carrier tapes, TBGA can have more than 700 I/O leads.The TBGA can be processed using standard screen-printed solder paste and conventional infrared reflow methods.Assembly Problems The big advantage of BGA assembly is that, if the assembly method is correct, the yield is higher than that of conventional devices.This is due to the fact that it has no leads, which simplifies the handling of components, thus reducing the possibility of damage to the device: the use of tin-absorbing webbing can safely and effectively remove residual solder paste.The BGA reflow process is the same as the SMD reflow process, but BGA reflow requires precise temperature control and an ideal temperature profile for each component.In addition, most BGA devices are self-aligned on the pad during reflow soldering.Therefore, from a practical point of view, the BGA can be assembled with the apparatus for assembling the SMD.However, since the solder joint of the BGA is invisible, the application of the solder paste must be carefully observed.The accuracy of solder paste application, especially for CBGA, will directly affect the assembly yield.SMD devices are generally allowed to be assembled with low yield because they are quick and cheap to rework, while BGA devices do not have such advantages.In order to improve the first pass rate, many large-scale BGA assemblers have purchased inspection systems and complex rework equipment.Inspecting solder paste application and component placement prior to reflow is more cost effective than inspecting after reflow, which is difficult and requires expensive equipment.Careful selection of the solder paste is required because the composition of the solder paste is not always ideal for BGA assembly, especially for PBGA assembly.The supplier must be made to ensure that their solder paste does not create solder joint voids.Similarly, if a water-soluble solder paste is used, care should be taken to select the package type.Because PBGA is sensitive to moisture, preconditioning measures are taken prior to assembly.It is recommended that all packages be fully assembled and reflow soldered within 24 hours.Leaving the device out of the antistatic protective bag for too long can damage the device.CBGA is not sensitive to moisture, but care should be taken.The basic steps for reworking a BGA are the same as those for reworking a traditional SMD:
Establish a temperature profile for each component; remove the component; remove the residual solder paste and clean the area; mount a new BGA device.In some cases, BGA devices can be reused; reflow soldering.Of course, these three main types of BGA require slightly different adjustments to the process.For all BGAs, the establishment of the temperature profile is of considerable importance.No attempt should be made to dispense with this step.If the technician does not have the right tools and is not specially trained, he will find it difficult to remove the residual solder paste.Too frequent use of poorly designed desoldering braid, combined with poorly trained technicians, can result in damage to the substrate and solder mask.Compared with traditional SMD, BGA has much higher requirements for temperature control.The entire BGA package must be heated gradually to reflow the solder joints.If the temperature, temperature ramp rate, and hold time (2 ° C/s to 3 ° C/s) are not tightly controlled, reflow will not occur at the same time and may damage the device.Establishing a stable temperature profile for BGA removal requires some skill.Designers do not always have information for every package, and trying to do so can cause thermal damage to the substrate, surrounding devices, or floating pads.Technicians with extensive BGA rework experience rely primarily on destructive methods to determine the appropriate temperature profile.Drill holes in the PCB so that the solder joints are exposed, and then attach thermocouples to the solder joints.In this way, a temperature profile can be established for each solder joint being monitored.The technical data show that the PCB temperature profile is based on a component-filled PCB, which uses new thermocouples and a calibrated recording element, and installs thermocouples in the high and low temperature areas of the PCB.Once the temperature profiles are established for the substrate and BGA, they can be programmed for reuse.The BGA can be easily removed with some hot-air rework systems.Generally, hot air of a certain temperature (determined by the temperature profile) is ejected from the nozzle to reflow the solder paste without damaging the substrate or surrounding components.The type of nozzle varies depending on the preference of the equipment or technician.Some nozzles make hot air flow on the top and bottom of the BGA device, some nozzles move hot air horizontally, and some nozzles spray hot air only on the top of the BGA.Some people also like to use a nozzle with a hood, which can directly concentrate the hot air on the device, thus protecting the week.
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