|Ask the Experts|
September 26, 2018
Reworked BGA Component Bridging at the Corners
We recently reworked a BGA component and the corners all bridged. This was a new BGA component that had never been installed on a circuit board before. Can these components be removed and reused if we can remove the warp? Would you consider the component to be scrap? What caused the corners of the BGA component to warp and how can we prevent it in the future?
|Expert Panel Responses|
From your line of questioning, you are pointing toward the warp being purely in the BGA. The board can be contributing to this as well. This condition is usually seen around a fairly large BGA. In our experience, this situation can be solved via the profile and not by a complicated manual use of spacer or shim to fight the effect. However, this will depend on the thermal management capability of your rework system. Only in the case of large ceramic, or heat spreader versions, the solder may not be able to support the weight of the package when all balls are in a liquid state. In this scenario, high temperature solder balls are used for support. Based on your description, I suspect that the profile is "aggressive" from the top side heating perspective. If this is the situation, you will likely get a non-uniform heating effect where warping of the BGA will happen and perimeter balls will reflow before the center balls, creating this bridge effect. More bottom heat (but a very controlled one) could help to minimize the work needed from the nozzle side and create a more thermally stable situation. Yes, the component can be saved. I would recommend to site dress this BGA and re-ball the BGA array.
The warping you describe can be caused by moisture in the component or more likely too high a reflow temperature during assembly. You can remove the component, desolder the old solder balls, reball it and bake it. Then you can attempt to reflow the component. Things you will want to do during your second attempt:
Regional Sales Manager
OK International Inc.
Bowing or warping is typically caused by thermal gradients across components, boards or both. Components are comprised mostly of fiberglass, ceramic, copper, silicon, and plastic, they expand and contract at different rates causing dissimilar forces to occur, also resulting in bowing. Reducing heating and cooling rates while applying uniform heat across the entire board and component may relieve bowing, especially with components with thin interposers. Once thermally optimized, persistent bowing may continue by the inherent design of the assembly. The use of temporary or permanent mechanical corner spacers has known to minimize pancaking of corner BGA spheres. If your specific BGA is reusable, depends on its total thermal exposure and whether complies with its manufacturer's specifications.
Regional Sales Manager
The short answers is bga's can be reballed and replaced. We do it all the time for clients and we control the squat or fall of the bga device to insure they do not short at the corners. The long answer requires more data because one must know if the pcb board is moving and what the degree the bga is moving -- i.e. warping or curling. The size of bga and the pcb board and the profile used for reflowing bga all play an important variable in the success of replacing bga's as well as the board prep surface wise as well as the type of flux and or paste being used. Again there are techniques used to help compensate for any of these moving variables but we need to know more information about size of bga and board and reflow profile and if you are using localized heat or a reflow oven and wether the whole board and bga are being equalized during the reflow process But again one can rework if one understands and controls the variables.
VP Engineering Services
STI Electronics Inc.
Moisture is often the culprit in cases like this. Absorbed moisture and resultant hydrothermal stresses during reflow can cause excessive BGA warping; enough to induce solder ball shorting. This is why tracking a BGA's cumulative exposure to ambient humidity is so critical. Was this BGA stored in a sealed moisture barrier bag or 5% RH dry cabinet prior to use? Is it possible that the allowable exposure time (aka Floor Life) had been exceeded? When in doubt the safest practice is to bake the BGA to dry it out before any reflow operation. This applies to initial assembly, as well as any subsequent rework operation. If you have a batch of BGAs that are warping, try baking the remaining ones. If baking makes the problem disappear then you know the original culprit was moisture. The industry standard for use and handling of moisture sensitive devices (IPC/JEDEC J-STD-033B.1) lists allowable floor lives (Table 7-1) and provides guidelines for baking components when necessary (Table 4-1).
Sales Manager (Americas)
What you describe is certainly not an unheard of phenomenon during BGA rework. Assuming proper moisture handling protocols were followed for the device and board prior to rework, I would suggest a careful examination of the reflow profile that was used to attach the BGA. It sounds like the profile may be too aggressive with either the ramp rate or overall top side heat.
Attaching an external thermocouple to the top of the device and one or two others adjacent to and below the device on the bottom side of the board can help paint a picture of what is happening when you run the recipe on your rework station. You should be able to re-ball the device provided it can be safely removed from the assembly and properly cleaned prior to the ball attach process.
Circuit Technology Center
The component package should be considered when reballing/reworking BGA. Is the BGA a ceramic, FR4 laminate system or some other package type? What was your root cause for corners to bridge? Sounds like "Oil Canning" of the PCB during spot reflow. Due to this, you may have scrapped assembly, not BGA. If the preheater size and preheat the PCB prior to spot reflow, will improve your Z axis movement and probably reduce/remove the bridging result.
Capital Equipment Operations Manager
Specialty Coating Systems
The bridged corners are likely caused by the CTE mismatch within the BGA body. Your rework profile must be too aggressive. To solve the issue you should try to reduce your peak top heater temperature. This can be done by reducing your ramp rate and/or increasing the bottom heater temperature. If your system has the capability to inject cooling air through the pick up tube (Active Component Cooling, or Positive Flow) this may also help. It is likely that the component will return to flat once it has been removed. If so, reballing with Winslow Automation or BEST reballing performs should do the trick. For rework, we recommend attaching the solder balls to the board rather than the bare BGA. This will reduce one thermal cycle for the BGA. Once the array of balls is attached to the rework site (perform just like you would attach a BGA) you can place and reflow the bare BGA. Hopefully your less aggressive profile will eliminate the warp and corner bridging problem.
VJ Technologies, Inc.
Alpha has introduced aproduct to mitigate corner ball shorting on BGA devices. It is a TrueHeightTM Spacer, which isbasically a non-collapse disc made of copper and plated for solderability thatis placed in solder paste deposit at the four corners of the BGA prior tocomponent placement. The spacer preventsthe BGA corners from coming too close to the PCB during reflow, preventingshorting of the corner BGA signals.
Global Product Manager
You may find the following industry standards helpful if trying to quantify the effect of warpage in your bridging: JEDEC JESD22-B112A for BGA component warpage and IPC-9641 for the local area PCB warpage. I often direct my customers to these standards when dealing with these types of defects.Neil Hubble, Akrometrix, USA
The solder bridging on large ceramic BGA'swith very large dies are due to PCB warping that canbe minimized by baking the PCB prior to reflow, less aggressive profile thatresults into uniform heating. what type of heating and cooling rates arerecommended when one is dealing with nm process nodes blow 28nmor20nm with verythin dielectric layers between the metals and extremely small micro bumps.Sharma Nirmal, GE Sensing
In most cases, solder bridges at BGA corners aregenerated do to PCB warping, you can optimize your thermal profile, but youshould use a PCB support on the bottom side of the PCB in order to prevent PCB warping.Sergio Ilescas, Arris Group de Tijuana
We had a similar problem with our hot-air rework station. When reflowing some BGAs we got shorts in random corners. The most notable was a large BGA, about 1.5" square with a metal and epoxy body. We matched the nozzle to the body size and preheated the PWA to 125C. This didn't help to eliminate the shorting. We did some investigation and as I was watching under the scope, I could actually see corners bend down with resultant shorts. Our thought was the part was heating too fast and acting like a thermostat. Our solution was to select a nozzle approx 2mm larger all around than the part to direct some hot air to the board. We also reduced the heating ramp rate of the hot air. This combination of heat transfer to the PWB AND the part allowed a more uniform heating profile. The result is we have not had any more shorts from our hot air rework station. This is some statement considering we have reworked many 1000's of BGAs.Jerry Wiatrowski, General Dynamics, USA
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