|Variety of typical BGAs. |
Reballing — it's a simple idea — just put those little spheres back on component pads. But the reality is that it can be very simple process or as annoying as keeping your Buffalo, NY driveway snow-free throughout December — an impossible task.
There are several unavoidable steps required to prepare used components for reballing, no matter which method is used. First, the components must be pre-baked to ensure they're moisture-free before actually working on them. You can go with what the component manufacturer's spec says — if it can be found — or simply use what one BGA reballing equipment supplier recommends: 125°F (51.66°C) for 8 hours. When experimenting with reflowing reballed, un-baked components on a hot plate some years ago, we had spheres flying everywhere as the component popcorned. "Popcorning" is a cute way to say your BGA will be quickly and dramatically destroyed as moisture inside becomes gaseous and delaminates the chip. Not a desirable scenario.
|Result of "popcorning" from trapped moisture. |
Next, the original solder from the pads must be removed. This can be done using solder wicking or a hot-air de-soldering tool. The solder wick method can work well, but the technician needs to avoid scratching or lifting the pads and needs to avoid over-heating the component. Hot air de-soldering systems (or solder suckers) are available on many BGA rework stations and typically give good results. However, it's still important to avoid scratching up the pads. No matter the method used, the cleaner the pads, the better the result obtained with the reballing process.
While there are some fully-automated BGA reballing systems, the cost is typically justified only for very high volumes, not a part of this discussion.
|Inserting the preform. |
We're also going to omit the surprisingly popular alternative — where spheres are individually placed onto pads using tweezers and a microscope. The level of success for this technique is directly related to the operator's patience and intake of coffee/tea/soda, and possibly palsied hands. This leaves three primary methods — Tilt and Roll; Preforms; and Vacuum Pick-Up.
Tilt and Roll. With most tilt and roll devices, flux is screen-printed onto the pads. Sphere migration may be limited by printing on the pads only, but some systems recommend covering the entire BGA pad side with flux rather than a screen print method. The BGA is then placed under a screen matching the component's sphere pattern. Some devices have the spheres go through the screen and onto the pad and flux immediately. Other devices have a stop plate of sorts to keep spheres from dropping through. The operator would pour spheres onto the screen and tilt things around until spheres drop into each aperture. It is very reminiscent of those little games from childhood where you try to get all the BBs to rest in the proper holes all at the same time.
|Applying paste. |
Once the deposition process is finished, the screen tilts to an exit area and lets the excess spheres go either back into the sphere container or trash. If reused, there is a danger of flux being on a sphere in the proper hole location but coming back out to get into the sphere reservoir, where flux contamination can cause a myriad of problems. With all spheres in position, either the screen is lifted off and the spheres remain on the fluxed pads or the restraining device is removed to let the spheres drop onto the waiting component. A few, smaller tilt and roll mechanisms will leave the stencil in place to try and hold the spheres during reflow. Tilt and roll requires either one or two stencils — one for flux if applying to pads only, and one for the sphere clearance. Each component pattern may have its own stencil to limit labor and setup time.
|Inserting BGA. |
Some manufacturers try to use a generic pitch and sphere size stencil and have the operator try to cover any unused apertures — usually with Kapton tape. Problems with tape residue have been observed when used around small BGA spheres, so this can get rather messy in a hurry.
While this method seems simple, there are increased labor costs when compared to some other methods. There are tooling costs involved for the stencils and for some level of mechanism. Also, time is needed to coerce the spheres into their proper places.
Preforms. At least two sources are available to provide spheres in their proper array. These use either water-soluble paper or polymer. A large number of arrays are available, and the cost is low for small quantities since there is minimal tooling cost. Both methods require additional component cleaning with water and baking to dry the BGA again before use.
With paper preforms, the preform is placed into a template that matches the outside dimensions of both the preform and component. The BGA surface is then covered with water-soluble paste flux and placed down onto the preform. The two are reflowed together. After reflow, the charred paper is peeled off and the BGA is scrubbed with a brush and DI water to remove remaining paper remnants and flux. The polymer preform process has the operator put water-soluble flux onto the component pad side and lay it flux-down on top of the preform. The outside edges of the BGA and preform are aligned to line up the spheres and pads. The pair then go through reflow and the polymer is peeled off after cooling. A polymer preform vendor's website states: mentions "It's not unusual with any reball process to occasionally have one or two balls not adhered to BGA after processing. That's why we've included our repair stencil. It is used when there is a need to replace only a few balls." A translation is that if spheres come off as the polymer is peeled back, the operator would replace the individual spheres and reflow the part again.
Vacuum Pick-Up. The third method uses a vacuum head with the sphere pattern, coupled with a flux stencil. There is an initial cost for a vacuum and control system. After that, tooling is needed for each unique BGA pattern. However, the process is extremely fast and typically eliminates the need for final cleaning and re-baking. A new, patented process for supporting the spheres reportedly slashes vacuum head cleaning issues and enhances sphere pick-up and release. The vacuum pick-up process tries to emulate the original bumping process.
|Moisten and remove paper. |
A vacuum work plate has two pins that extend to center the device throughout the operation. The technician then places a template over the pins — the template matching the BGA's outside dimension and slightly thinner than the BGA. The tech then places the component in the template where the vacuum port holds the component in place. A stencil is placed over the tooling pins and no-clean tacky flux is screen-printed onto the pads. The stencil is then lifted off. The operator then picks up solder spheres from a reservoir using the vacuum head (wand). The stencil face of the wand is dipped into the spheres and lifted, where the operator can see if the pattern has been filled and if excess spheres need to be brushed off.
|Hepco 9400-1 solder sphere placement system. |
Once a full pattern is achieved, the wand is lowered over the tooling pins until the spheres contact the component. The stencil is designed so 1/2 the sphere diameter extends past the wand stencil face so flux doesn't touch the wand itself. The operator then depresses the foot pedal to cut off vacuum and actuate a vibrating hammer which assists sphere release. The wand is lifted off the part and the wand and component are quickly inspected to ensure the spheres are in position. At this point the BGA may be removed and run through reflow, after which it is ready to be used.
While reflow is fairly straightforward, reflow of the spheres does need to be addressed. With a typical profile, the spheres would go liquidus for around 30-45 seconds — the temperature depending on the solder alloy being used. The ramp rate should be kept to around 0.7°C per second ideally, but definitely less than 2°C per second. With hot air, be careful to keep flow down. Although it seems obvious, a lot of spheres have been blown around during reflow. Also, nitrogen atmospheres are always going to provide a better finished product if such an atmosphere is feasible. Component preparation and reflow are fairly consistent regardless of the method used to place the spheres. The three primary methods include tilt and roll systems, solder preforms, and vacuum pick-up. Preforms offer a good solution where very low volume and high mix are the rule. Additional cleaning labor and baking time need to be factored in, along with a possible additional heat cycle if spheres don't adhere. Vacuum pick-up offers value at mid- to high-volume and provides a finished product quickly with virtually no cleaning needed after bumping. Each method has a price point and volume where it may be the proper choice.
Contact: HEPCO, Inc., 150 San Lazaro Ave., Sunnyvale, CA 94086 408-738-1880 fax: 408-732-4456 E-mail: firstname.lastname@example.org Web: http://www.hepcoblue.com