Operation BGA Piggy Back

Print

BGA Footprint Mismatch: Daughter Card to the Rescue

Figure 1: Original circuit board foot print is a 289 pin full BGA array. The BGA component needing to go in this location is a 256 pin perimeter array.

You have a new board and the BGA component doesn't fit the board footprint. Is this a nightmare of a problem? It is one we see at Circuit Technology Center every other month or so. It’s seems to be a problem faced by OEM’s early in the development cycle. In this actual example the flagship product an OEM is working on has gotten to the post assembly test phase and there's a hang-up.

The BGA that is loaded at several locations on the board is the wrong one! To further complicate matters, the footprint of the correct BGA is not the same as the original. You might ask, "How could this happen?". In the defense of the people this happens to, at this stage of the game, consider the mind-boggling variables. Marketing, engineering, customers, all pushing to fill a niche, to get to market first. People change their minds, new component s come along. Stuff happens!

Figure 2: Daughter card is .100" thick.

In any event, we have a problem. You might ask why not re-spin the board? In this case boards are already populated when this component mismatch is identified. The time and money involved in board re-spin and population; board fabrication time, component purchase time and assembly time, are prohibitive.

What can be done? Traditionally with through hole and standard SMT parts jumper wires would be employed. That is to connect all of the leads on the newly specified component to the places where they need to be on the board. Hardly an elegant looking fix (it can look like a veritable Chia Pet spewing from the component to the board location) but in some cases it’s all that’s needed to get the job done.

However, in this case we designed a daughter board to mate between the board footprint and new BGA component. Sounds simple enough doesn’t it? Like most fancy rework projects it's a bit easier to think about than to implement.

Figure 3: Daughter card soldered to main circuit board at original site.

All of the connections from the new component had to be interfaced with existing footprint on the board surface. This required using an 8 layer daughter card. In some cases this added thickness may cause a height issue, but fortunately not in this case? In addition several capacitors and resistors were also required and were added to the perimeter of the daughter card.

So what are the concerns? Will stacking a new BGA component on top of this daughter card cerate reflow problems? Will the daughter card warp or sit too low causing shorts or opens? Is the multiple reflow cycles at this location likely to cause burning or layer separation? Serious concerns and all were carefully addressed.

Figure 4: New BGA component mounted to daughter card.

The following are the details of how we fixed this BGA footprint mismatch. The original BGA component was a 289 ball full array. The new BGA component is a 256 ball perimeter array. (See Figure 1)

Due to the dense circuitry and electrical routing needed the daughter card ended up being 8 layer and .100 inches thick. (See Figure 2)

Fortunately the thickness of this board did not present a height problem for the board. However, from a rework standpoint, there was some concern about the heating of the layers during the several steps of rework. We opted to use high temperature balls for the bottom side of the daughter card. Even though eutectic balls would have supported the weight of the combined daughter card and new BGA, the high temperature balls would allow for whatever minor bumps, jumps, and hiccups might occur. Since the daughter card was to be soldered before the new BGA component the use of high temperature balls would preventing tipping or shorting of the daughter card during reflow of the new BGA component. (See figure 3)

The placement would be done in this order so that the second reflow profile could be designed to heat the new BGA component from the top and minimize the heat that would be transmitted to the circuit board. (See figure 4). Once the final BGA placement was done, we hand soldered the discrete components onto the daughter card.

In summary the steps we followed were:

1. Remove the original 289 pin BGA component.
2. Clear the site of excess solder.
3. Add solder paste to BGA pattern on the original circuit board using a Flextac BGA Rework Stencil.
4. Soldered the daughter card to the original circuit board using high temp solder balls.
5. Add solder paste to BGA pattern on the top of the daughter card using a Flextac BGA Rework Stencil.
6. Solder the new eutectic-balled BGA component onto the daughter card.
7. Hand soldered the accompanying capacitors and resistors to the perimeter of the daughter card.

This type of modification has a solid track record. It's a bit of work, but is an elegant solution. Just in case you're wondering after a modest setup fee we charged $185.00 per board for a total of 22 assemblies.

Several members of the Circuit Technology Center team contributed to this feature story.