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I read the following sentence in this Maxim appnote. (WLP = Wafer Level Packaging, CSP = Chip-Scale Package)

WLP technology differs from other ball grid array, leaded, and laminate-based CSPs because no bond wires or interposer connections are required.

No bond wires? Then how is the die connected to the ball grid? Can anybody explain the difference between WLP and BGA in more detail? They look very similar.

MAX97200 WLP

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    \$\begingroup\$ WLP came from "flip chips" and I believe are a little more fragile when talking about thermal cycles than BGA. I haven't run across WLPs much, as most things I see believe BGAs to be better all around. I know WLPs have been around a long time. IBM used them for their mainframe chips in the early 70s. \$\endgroup\$ – Joe Jun 8 '11 at 16:59
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This picture can help to see the difference between BGA and WLP This picture can help to see the difference between BGA and WLP

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  • \$\begingroup\$ Thanks RC. Doesn't this use a lot of die real estate? Classical bonding pads can be a small as 35 µm x 35 µm (Mosis), while the balls that connect to the PCB on the MAX87200 WLP are 0.27 mm in diameter. \$\endgroup\$ – stevenvh Jun 9 '11 at 15:17
  • \$\begingroup\$ The real estate is probably non-issue, as the inner balls are reflowed on top of extra layer of oxyde, metal, metal-wetter, metal needle crystals barriers etc sandwich. But I dont know really. I only looked few times with microscope at Fab with reflow ready pads, when worked in company making solder printers for this chips \$\endgroup\$ – user924 Jun 9 '11 at 16:23
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For all practical purposes, it is a BGA. And you can treat it like any other BGA. The differences are mainly internal to the part and of no real concern to the normal user of the parts.

There are a lot of packages that the different companies call different things-- but are essentially the same. The only things that most users would care about is size, solderability, handling requirements, and heatsinking issues. In other words, what's inside isn't all that important to most people and can be safely ignored.

I suspect that, in this case, the WLP is almost the bare die with balls on it. As in, the balls connect directly to the pads on the die without a bond wire in between. The die isn't completely bare, of course, as there would be a protective coating on the sensitive bits. This type of package is not at all unique to Maxim. TI has some opamps in that package, and I've used some ESD diodes in a 4-ball version.

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Formally, to be qualified as a CSP the package must be not greater than 120% of the die area. BGAs are usually greater than 120% of the die area and thus usually do not qualify as CSP.

Appendix

1) Flip chip is an example of CSP. However, not every CSP is a flip chip (e.g. lead-frame based CSP).

2) To the best of my knowledge, wire bonding is used extensively in BGAs: most of pins are connected with wire bonds. In CSP majority of pins are directly connected to the board with solder bumps or lead-frame.

enter image description here
Pic. 1: Internal structure of a BGA chip showing wire bonding

enter image description here
Pic. 2: Typical BGA, flip chip and CSP structures. Source URL

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