Unlike surface mount components with leads around the perimeter (i.e. QFP, SOIC), Ball Grid Array (BGA) packages contain a matrix of solder spheres on the bottom side.  From a design standpoint, one of the benefits that a BGA offers versus a part with leads is having a greater number of connections over the same amount of die size. 

The disadvantage, however, is that those connections are now hidden making rework a more complicated process requiring specialized equipment.  This article details the steps of reworking a BGA device on a circuit board using an AT-GDP Rework Station.  A Pb-Free profile has been applied during soldering and desoldering processes.

Device and PCB

There are several sources for how a reflow profile graph should look like.  The most common are solder paste and component BGA measures 31mm x 31mm (1.22” x 1.22”) and contains 421 spheres (Figure 1).  Solder spheres measure 0.6mm in diameter with a pitch of 1.0mm.  Printed Circuit Board (PCB) measures 229mm x 305mm x 1.52mm thick (9“ x 12” x 0.060”). 

Desoldering

After securing the circuit board on the machine’s stage, an operator installs a vacuum pick-up tip and nozzle suitable for the BGA device.  Implementing machine optics, the nozzle is aligned over the component.  A pre-established profile is selected from the software’ library and the Start icon is selected.  At this point the process is hands-off.  The rework station automatically drives the nozzle down to a board and covers the BGA.  The machine then activates the vacuum pick-up tip so as to remove the component once reflow is achieved and the heating cycle is initiated.  Figure 2 shows a nozzle covering the device during reflow.  

Heating is precisely controlled by the software (Figure 3).   It mimics an original reflow profile with heat applied from both the top and bottom sides of a circuit board.  The Source of bottom heating is Quartz IR while the top side is forced air or nitrogen convection.  Upon completion of reflow, the machine lifts a BGA off a board and moves the nozzle up along the Z axis to its starting point.

Figure 3: Screenshot of a reflow profile

 

Residual solder will remain on the circuit board after removing a component.  It will be irregular in shape and in some cases may look like a Hershey’s kisses candy due to the surface tension of molten solder as the part comes up.  As a result of its uneven shape and volume, solder must be removed prior to soldering a new component (Figure 4).  This may be accomplished by using a vacuum de-soldering tool or more commonly by wicking solder with a copper braid and soldering iron. 

Alignment, Placement, and Soldering

Once PCB pads have been cleaned to remove residual solder, the component may be aligned, placed, and soldered to the circuit board.   After the BGA has been picked up by the rework station, the next step involves transferring tacky flux or solder paste to spheres of a device. 

Both may be applied by dipping the component in a universal transfer plate.  Implementing tacky rework flux, however, is more common.  As displayed in Figure 5, the BGA is dipped into a Flux Transfer Plate (Part: FTP-ATGDP) that contains a pool of tacky flux 300µm deep.  Now the tip of each of the 421 solder spheres contains a small amount of flux that is necessary for proper wetting and joint formation.

The component is now ready to be optically aligned over the matching pads on a circuit board.  As the Split Vision Optics arm is moved into position (Figure 8).  It enables viewing solder spheres of a BGA and pads of a board simultaneously on the same screen.  Magnification and LED lighting intensity may be adjusted to create optimum solder sphere size and contrast between the two images.  

Figure 6 shows a camera view screenshot where solder spheres and pads are misaligned.  By adjusting the X-Y micrometers of a vacuum lockable board holder and theta rotation adjustment, BGA’s spheres were aligned over the board’s pads (Figure 7).  The component is now ready to be placed and soldered.

Upon moving the optics arm to its original position, the device may now be placed on a board and solder reflowed.  As in the removal process, the same pre-established profile is selected from the software’s library and a Start icon is selected.  The sequence in an Install mode instructs the rework station to automatically place the BGA on a board using placement force feedback control, lifts up the vacuum tip away from the component’s surface, and begins heating.  Figure 9 shows a nozzle covering the BGA during reflow. 

Heating is precisely controlled by way of closed-loop software control.   It mimics an original reflow profile with heat applied from both the top and bottom sides of a circuit board.  A Uniform stream of cool air or nitrogen is directed during the cooling phase through the nozzle to form strong, high-quality solder joints.  Upon completion of a cooling stage, the machine moves the nozzle up along the Z-axis to its starting point.  The installation process is now completed.

Conclusion

In order to properly perform BGA or SMD rework, the tool used (rework station) must contain key features like split vision optics, software controlled sequencing and thermal management, and automation.  These features not only simplify the process but also enable achieving high-quality results on a consistent basis.

Before going into the details of how to create a reflow profile, it is important to establish what a profile is and some key functions behind it.  A reflow profile is essentially a heating cycle or recipe that follows specific temperature ramp up rates, soaking and peak temperature setpoints, set times at temperature, and cooling rates in a soldering machine (reflow oven or reflow furnace) .  These settings allow the product that is being soldered to be subjected to a heating state that will melt the solder paste or solder preforms and thereby create a metallurgical joint between the SMD components and a substrate. 

In terms of Printed Circuit Board (PCB) assembly or RF and microwave housing assembly, a reflow profile refers to a heating cycle as measured on the product, not the air temperature settings for a reflow oven.  This is by far the most common mistake most technicians make in starting the process of profile development.

Solder Paste & Component Manufacturers

There are several sources for how a reflow profile graph should look like.  The most common are solder paste and component manufacturers.  The solder paste manufacturer will base the profile on the specific alloy and type of flux.  Below in Figure 1 is an example of an Alpha OM-353 / SAC 305 Lead Free profile graph.  A component manufacturer will typically base their recommended profile on JEDEC J-STD-020 testing which tests the part for survivability when subjected to reflow temperatures.  Example reflow profile is shown in Figure 2. 

The go to profile to use should be the one from the solder paste / preform manufacturer.  It is important to note that the profile is only a recommendation and a general guideline.  The manufacturers do not know specific nuances of an end user PCB design and if there are heat sensitive components with temperature restrictions.  Additionally, a smaller low thermal mass PCB will be easier to profile and conform to the setpoints and limits on the profile graph when compared to a large multilayer panel with a heavy ground plane and much heavier overall thermal mass.  So a lighter board may be profiled to peak at the higher temperature (i.e 245 Celsius) while a larger and heavier board will typically peak at the low end of the temperature scale (i.e. 235 Celsius). 

The component manufacturer may have their own profile but generally speaking it is not a good reference for how the overall temperature curve should look like.  There may be dozens of fine pitch ICs on the assembly, all from different sources, so unless a part has temperature limitations and is rated to peak temperature below the standard JEDEC J-STD-020 guidelines, it should not be the driving factor when creating a profile for the whole PCB.  Think of part manufacturer profiles as maximum temperature extremes that the component can withstand, not necessarily ideal for best solder joint wetting and quality.  

Wiring with External Thermocouples

Once parameters for a profile are established, the circuit board used for profile development needs to be wired with external thermocouples to measure temperature as the assembly is subjected to the heating cycle inside a reflow oven.  At the very least, one thermocouple should be mounted to the pad of a light mass part like a resistor or capacitor, another thermocouple to the location of a heavy part like an IC, and another to a part that would be connected to the ground plane (i.e. QFN).  There are several industry accepted methods for thermocouple attachment and include using epoxy, high temperature solder, and Kapton tape.  We will not discuss the details in this article and will address the specifics in another blog.            

Regardless of the type of reflow oven or furnace used, air or Nitrogen convection, inline conveyor or batch system, settings that are programmed in the machine should yield a desired temperature curve as measured with the external thermocouples mounted directly to the circuit board.  It may take several trials depending on the capabilities and necessary adjustments to the soldering machine. Ultimately, once a reflow profile has been optimized and settings in the system established, they can be saved as a specific profile for a particular PCB project and recalled from the software for future reflow assembly builds.   

You may view our applications page for more information on BGA Rework, Reflow Soldering, Die Bonding and Stress Testing of Microvias or give us a call today!