Some of this was copped from Fred.
There are numerous package types that now fall under the rubric of land grid array [LGA]. Land grid devices [ie, Bumped Chip Carrier� [BCC], LGA, Quad Flat-pack No-lead [QFN], MicroLeadFrame�, etc] are essentially BGA devices without balls or solder on the pads. [Dinosaurs: Think PGA w/o pins.] Generally, any device that has bare pads under them that attach to corresponding pads on the board qualifies as a land grid array.
General comments: * Apply a sufficient amount of solder to ensure proper connection to the board, since the component has no solder on the pads. A 4-6 thou thick stencil should do the job. Try to end-up with about 1-2 thou of solder between the pad and the component. * In most cases, the pads are huge [eg, 450 thou^2]. So, this gives room to tweak apertures to obtain a good solder joint without any stencil aspect issues. A LGA with flat pads on the bottom drops to a thou or two of standoff height. * Determine the proper size and shape of the brick that produces a reliable soldered connection through a little experimentation. * Profile the solder paste reflow by placing a thermal couple under the component pad, similar to a BGA. * Make sure that you know where your solder is after reflow. See BCC note below. * Treat LGA like any other plastic SMT IC device (ie, ESD, moisture, handling, etc).
If a board sees any kind of flexing, keeping the component on the board could become an issue.
With such a low standoff [eg, 1-2 thou of solder], under-fill materials take quite a bit of time to flow to the underside of a LGA and most assembly throughput cannot take that kind of time hit. So in real life, LGA are "edge" filled. But, this lack of standoff usually means that the residue from no-clean fluxes needs to be selected, so not to work against the under-fill.
Watch out for LGA with a thermal "slug" under them, like a QFN. The slug really limits the options if overprinting. If the opening for the thermal slug is too great when using blades (not a pump), the blades can "scoop" paste from the center of the deposit for the slug. Since the pads do not get scooped, the component can end-up sitting higher than the paste deposits under the heat sink. Support the board under the component if the paste opening is larger than about 100-125 thou^2. Print the heat sinks as a grid apertures that are the size of pads.
This leads to the obvious extension. Excessive pick and place pressure can schmush the paste between pads and cause bridging. This often shows-up with BCC. BCC have rectangular pads that have a radius to them. The pads are like little canoes that are only plated plastic and have no solder on them from the supplier. Too much placement force can squish the solder paste out to the sides of the print, and, if you are not careful, 0.4mm spaced pads short together very easily, becoming 0mm spaced pads. ;-)
Also, the location of microvias in the LGA pads can cause opportunities for rework. If the out-gassing of microvias is violent enough, solder can bridge between pads when the microvias on adjacent pads are positioned in the same location on pads. Staggering of the location of microvias reduces, but does not eliminate, the problem.
As mentioned by another poster on this thread, the repair process is the BIG drawback to the LGA family of devices. Removing the device and dressing the pads on the board are the easy parts. Attaching the component to the board is the tough part, since the LGA has no solder, you need to: * Print solder paste on the bottom of the component, similar to a BGA. This is not a lot of fun on the small pitch devices. OR * Have the LGA supplier provide parts with solder already applied. Not a $$$$ saver. OR * Talk someone, like Winslow, into developing a �SolderQuik� solution, like for BGA.
Some LGA have ENIG finish on them. So, soldering them to an ENIG finished board can cause problems with gold embrittlement to kill your solder connections. Search the fine SMTnet Archives for background on acceptable gold levels.
One kinda neat thing about these packages is that in a pinch you can normally eliminate solder bridging without removing the component. Just squirt some tack flux that is compatible with your normal solder paste all around the edges of the LGA. Run the board through your reflow oven again and 90% of the time the solder bridges disappear.
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