Solving a BGA footprint mismatch: daughter card to the rescue.
Discovering a Mismatch
A footprint mismatch is not rare; original equipment manufacturers (OEMs) often encounter the problem early in the development cycle. In one classic example, an OEM's flagship product had reached the post-assembly test phase before discovering that they had loaded the wrong BGA onto the board. To further complicate matters, the footprint for the correct BGA was not the same as the original. The company had to fix the problem, so why not re-spin the board? As is often the case, the boards were populated by the time the mismatch was identified. The time and money involved in board re-spin and population including board fabrication, component purchase and assembly time--were prohibitive.
Solution: The Daughter Card
With standard through-hole and surface-mount parts, the solution to a board/component mismatch generally involves jumper wires. Although modifications can look messy, they generally work well. However, given component design and circuit board layout, using jumper wires for BGA modifications will not work.
The only viable solution is to build a daughter board to mate the existing footprint and the new BGA component. All connections from the new component must be interfaced with the existing footprint on the board surface. In the OEM's case, this required an eight-layer daughter card with several capacitors and resistors around the card's perimeter.
Naturally, process concerns are associated with this fix. Stacking a new BGA component on top of the daughter card may create reflow problems. Additionally, the daughter card might warp or sit too low, causing shorts or opens. The added thickness of the daughter card may sometimes present a height issue. Multiple reflow cycles at this location may also damage the board surface or solder mask. Each of these issues must be carefully addressed and resolved.
In the OEM's case, the original BGA was a 289 ball full array, and the new component was a 256 ball perimeter array. Due to the dense circuitry and electrical routing of the board, the daughter card was eight layers and 2.54 mm thick. Fortunately, the thickness of the daughter card did not present a height problem for the assembly. Although eutectic balls would support the combined weight of the daughter card and the new BGA, the company used high-temperature balls to allow for minor bumps, jumps and hiccups. Since the daughter card was soldered on prior to the BGA, the high-temperature balls also prevented any tipping or shorting of the card during BGA reflow.
The new component only had to go through one reflow cycle. Once the final BGA placement was done, the discrete components were hand soldered to the perimeter of the daughter card.
Remove the original BGA component and clear the site of excess solder. Add solder paste to the BGA pattern on the original board using adhesive-backed, flexible rework stencils. Solder the daughter card to the circuit board using high-temperature solder balls and a standard BGA rework process. Add solder paste to the BGA pattern on top of the daughter card. Rework the high-temperature-balled BGA component onto the daughter card. Hand solder the accompanying capacitors and resistors to the perimeter of the daughter card.
Daughter-card modification has a solid track record. Although it requires a bit of work, it is an elegant solution. In addition to the design and fabrication costs of the daughter cards and components, each modification took less than four labor hours and cost $185 per board.
Jeff Ferry is president of Circuit Technology Center, Haverhill, MA; (978) 374-5000; www.circuittechctr.com.
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|Title Annotation:||Rework and Repair Depot|
|Date:||Feb 1, 2003|
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