Tuesday, April 24, 2012 | Zulki Khan, NexLogic Technologies, Inc.
Eidotor's Note: This article originally appeared in the March 2012 issue of The SMT Magazine.
The industry is turning the page on coplanarity and finding a host of new issues. Some are brought on by the fact that BGA technology is now at the 0.3 mm and 0.25 mm ultra-fine-pitch stage and requires a renewed and critical look at surface finishes. Other coplanarity issues deal with BGA placement, stencil design, and PCB design.
It’s no great secret that the BGA world is constantly evolving. However, as it accelerates, it brings the industry new PCB assembly challenges with 0.3 mm and 0.25 mm ultra-fine-pitch devices. When the technology gets to this stage, the balls on a micro BGA are extremely tiny.
What this says is you’ve got to have a remarkably perfect board surface to effectively assemble BGA and micro-BGA devices on a PCB. Beginning with BGA placement, of utmost importance is the need for the most accurate pick-and-place system on your SMT line.
If the PCB surface is not 100% uniform or non-planar, a perfect BGA ball collapse won’t occur, opening the door to such placement errors as open solder joints or cold solders.
Some of those tiny BGA balls will have more solder than others, as shown in Figure 1. Those with more solder will probably not collapse properly, thereby creating dullness of the joint and/or a cold solder at that particular joint. This can cause elusive BGA intermittent connections and can cause latent field defects and failures, which can be extremely costly.
Figure 1: Some BGA balls will have more solder than others.
The industry may already be facing coplanarity issues like this in many instances, largely because we’ve gotten comfortable using hot air solder leveling (HASL) as the surface finish of choice. HASL brings a number of important benefits both to the contract manufacturer/EMS provider and to the OEM. Those benefits played especially well at the BGA’s 1.0- and 0.8-mm pitch stage.
But now at 0.3 mm and 0.25 mm ultra-fine-pitch, it’s a different story. Keep in mind that at 1.0- and 0.8-mm pitch there’s a bit more real estate to work with and HASL PCB surface finish worked perfectly fine. This process involves immersing the PCB in a tin/lead alloy and then forcing hot air across the board.
Solder is deposited on the surface mount pads, but there’s no uniform process. Some of the pads, as well as some of the balls on the BGA, have a bit more solder on them compared to others, hence no uniformity on pad surfaces. But, since BGA pads are bigger at the 1.0- and 0.8-mm pitch, there is sufficient room to get a good ball collapse and BGA assembly. Even if conditions are not 100% perfect, BGA balls still align properly because of surface tension and they get soldered accurately.