Ball Grid Array Joy


We’re working on a design using a couple of TFBGA parts (thin profile fine pitch ball grid array).  These packages are used in electronic assemblies these days because they allow parts with many leads to be shrunk in size.  Instead of pins the parts come with solder balls set in a grid array on the bottom of the package.

From the table above pitch is the distance between solder balls.   Anything 1.0mm, or larger, is typically BGA.  Below that some other micro-BGA package style, including TFBGA.  You can see that on the largest package shown 324 pins are nestled into an area about 2/3” square.  Pretty small, and a whole bunch of connections.

The balls on the package fit onto landings, or circular copper pads.  These pads are surrounded by solder mask, a film layer that keeps solder off of the underlying copper (and gives circuit boards their color).  The solder mask forms a tiny cup that the balls rest in.  Solder mask application needs to account for the manufacturer’s ability to accurately apply the material.  If the solder mask layer happens to overlap the copper landing you create a “step”.  This can tilt the package to one side causing the part to have poor connectivity including, shorts, opens, or the worst of all intermittent connections.

Connecting to the landing is often done with a “dogbone” shown in the image on the right.  The dogbone is a short trace and a via with solder mask (a plated, drilled hole that connects to your other circuit components).  At times people use a “via-in-pad” connection where they drill a plated hole directly through the BGA landing.  This helps reduce trace length and shortens ground or decoupling connections  Via-in-pad designs come with a lot of requirements.  The via can pull solder away from the BGA ball and push it out the other side displacing components.  Outgassing can cause parts to shift as well.  Sometimes these issues are handled by capping the via with solder mask, filling them with a conductor or epoxy, or making the via a “blind via” (one which does not extend through the entire circuit board).

As part of our service we’ll create and review the electrical schematic and complete a circuit board layout.  For most of our designs we’ll hand assemble a circuit board to complete testing and re-design phases.  With the BGA packages, there’s no way to hand assemble the parts, so this design’s prototype will go to a contract assembly house .  These parts are extremely difficult to inspect.  There’s no way to tell if the parts have been correctly soldered, unless you use a 3D X-ray machine.  Intermittent failures due to the assembly process and/or board design are the engineer’s worst nightmare.  They can exhibit themselves in random failures, and not show up until the sample group of parts built is quite large.  In other words during production.

But people like their electronics tiny, and complex, and so that’s the direction things are going.

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