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By Zulki Khan, NexLogic Technologies Inc.
BGA reballing takes on increased significance as OEM designers continue to populate PCBs with greater numbers of BGA-packaged devices such as FPGAs and other highly integrated components. Extremely high speed and expensive BGA devices like FPGAs cost in the range of $2,500 per part, sometimes even more. Therefore, when a BGA's balls are inadvertently damaged during assembly, it is more prudent and economical to perform re-balling than to discard the entire BGA. This is especially true for expensive components like FPGAs and microprocessors.
Due to their growing popularity, BGAs are splintered into various types and configurations. Currently, there are plastic, metallic, aluminum, glass, or epoxy-based frames and packages. They also come in different sizes with different pin counts or number of leads, ranging from 156 to 1,278 balls and more. Regardless of size, a BGA comprises a substrate or package body and the balls, which come in different sizes. The balls can be leaded or eutectic, lead-free or high temperature.
The most challenging and cumbersome aspect of dealing with BGAs is installing them during PCB assembly. An EMS provider must be well-equipped with essential installation and inspection tools to satisfactorily perform BGA assembly. Plus, to ensure the installation is performed properly, it is critical to have a modern high-powered X-ray machine. This is required to be able to see through the package to make sure all wire bonds are intact and all the balls are properly collapsed.
Rounding Up SuspectsDuring BGA installation at assembly, there may be a device or two that is suspected of not having a good ball collapse, which is visible with X-ray in place. They may be showing voids within the collapsed balls, opens, and/or shorts between different balls. Also, there can be various issues associated with an imperfect BGA assembly. For example, Figure 1 show excessive solder paste being applied, creating bridging between BGA balls, also called ball shorts.
When deficiencies and defects like these occur and manufacturing engineers are not satisfied, appropriate remedies must be applied. Initial steps to try to correct them can include reflow and re-placement. But if those fail, the ultimate step is to reball the BGA, especially if the BGA balls get damaged during the initial assembly process.
Steps associated with BGA reballing are straightforward, yet highly complex. Consequently, it's imperative to have an experienced EMS provider do the job properly. Correct re-balling techniques represent one side of the story; the other is correctly managing moisture sensitivity levels (MLS) at storage and throughout the assembly process.
Re-balling a BGA is a highly controlled process, meaning caution must be exercised throughout because it involves numerous and potentially damaging mechanical, thermal, and stress-related issues. This means the right tools must be used: flux, solder wick, isopropyl alcohol wipes, and a fume extractor system, among a host of others.
Flux is initially applied to all the balls. Care must be taken to ensure the flux is not evaporated, which makes ball removal more difficult. Balls are then removed from the pads, using a soldering braid or iron placed on top of the flux, after which the solder iron is traced on top of the braid. The solder iron heats the braid and melts the solder ball so that they are de-soldered from the package.
Once the balls are removed, the package is cleaned with an isopropyl alcohol wipe. It's important to make sure flux residues are removed by rubbing the complete surface of the package and making them entirely clean. Caution must be used here to make sure too much pressure isn't applied. If strong mechanical stress is applied to the board, there's a chance of damaging or cracking the substrate within the package itself.
Also, it's worth noting that flux can be corrosive. Hence, extra cleaning is sometimes required if the package is left idle for a period of time before reballing.
The package is then closely inspected under a microscope to see that: the pads are clean; there is no damage; and all the un-removed balls have no flux or paste residue.
The next step is to print the solder paste over the BGA ball locations of the cleaned substrate and then install the balls by using a fixture. Once all the balls are installed, a microscope re-inspection is conducted to assure the balls are properly populated and then, reflow is applied based on the designated temperature stated in the spec sheet of the BGA manufacturer, keeping in mind the accurate thermal profile of the BGA package.
Moisture Sensitivity and Other ConsiderationsAside from those steps, emphasis must be placed on BGA substrate MSL. There are two aspects involved here relative humidity (RH) and ambient temperature. These two factors must be kept in mind during reballing. Relative humidity should be less than 50% in the atmosphere; ambient temperature less than 80°F.
If these levels aren't maintained, then certain airborne chemicals such as sulphur, ammonia, or acetic chloride will reduce chances of proper BGA balling. The reason is those airborne chemicals reduce the probability of balls wetting process during the reflow process.
A veteran, experienced EMS provider measures the temperature and humidity at different parts of the floor and keeps a log of both to assure tight manufacturing controls.
Other major considerations include flux types used during reballing process, the types of balls involved, stencil design, and avoiding costly shortcuts. The flux type is important because solder alloys are exposed to the oxygen in the atmosphere, which is at an elevated temperature, for example, 125°C or above evaporates, and flux rapidly oxidizes. The main function of any flux is to protect the alloy and reduce surface tension to solder the joint properly.
Special considerations should go toward ball size and type, whether it is eutectic or lead-free. Sphere or ball size can vary from 0.3 to 0.76 mm and require specific solder paste. Careful evaluations are demanded here to match alloys and ball sizes with the right solder paste and reflow temperatures, along with the appropriate flux types.
As for plastic BGAs, moisture absorbency must be taken into account. JEDEC calls for a standard atmosphere of 30°C at 60% RH to develop the time limit exposure. If the plastic BGA is exposed to more humidity or a higher temperature, then less-than-optimal results are achieved, creating less-than-optimal solder joints at BGA sites.
However, exposing the BGA substrate to more humidity or temperature doesn't mean it isn't acceptable for re-balling. The main requirement is to bake it according to the JEDEC standard for 24 hours at 125°C. Baking takes the moisture out of the package before soldering. Once baked, the substrate needs to be reballed within a few hours. On the other hand, if the substrate is baked and not used, it must be immediately vacuum sealed using a drying agent to remove additional moisture.
Stencil quality plays an important part as well in reballing, helping ensure solder properly adheres to the site. Figure 2 shows an example of an effective solder joint showing good ball collapse. When the right stencil design is used and an adequate amount of solder paste is dispensed, the result is a good solder joint.
Stencil design includes the aperture opening and sizes in relation to the SMT pads along with the thickness of the stencil foils. Correct stencil design confirms that enough paste will be dispensed on the SMT pads, especially if these are fine-pitch devices. If too much paste is dispensed on the SMT pads, the chance of shorting between the BGA balls increases; if not enough paste is dispensed, voids and opens become more probable.
Lastly, when BGA reballing adversely affects OEM PCB shipment schedules, shortcuts are occasionally taken to make up for lost time. Also, in some cases, inexperienced technicians may inadvertently skip certain reballing steps, posing subsequent assembly issues, further extending shipment time. The most damaging shortcut is failing to subject final reballed BGAs to a high-powered X-ray and completely overlooking voids within the balls. Here again, a well-equipped and experienced EMS provider can eliminate potential problems that can result from inexperience and shortcuts.