Electronic Mfg. Services (EMS)
Automatic Profiling Complements
AOI & X-ray Inspection
By Bjorn Dahle, President, KIC
Package on packages (PoPs), μBGAs, 01005 and smaller devices are becoming more commonplace, and this cansignificantly impact SMT assembly. This technologytrend makes all assembly processes more difficult, especiallyprinting and reflow, and it makes inspection a real challenge.AOI machines are not designed to inspect leads that are hiddenbelow component bodies such as BGAs and PoP.
In addition, even for the leads that are visible to the AOIcamera, there is no way for the machineto look at the solder joint microstructureto determine whether there arequality issues related to a poor solderingprocess. A solder joint or componentcould be defective or weak as a result ofa multitude of process issues thatinclude overheating, too fast or too slowramp rates, excessive time above liquiduous,and oxidization.
Unfortunately, X-ray inspection isnot capable of handling these situations.While this type of inspection has theadvantage of seeing through the BGA and PoP component bodiesto the solder joints below, it has a similar limitation to theAOI; although it can verify things like the presence of solderballs and other solder joints, it cannot look inside the solder todetermine whether it is “healthy”. Another limitation with X-rayis that most factories use batch X-ray systems rather thaninline inspection systems. In this case, the vast majority of thePCBs do not get X-rayed and, therefore, can slip by unnoticed.However, a solution exists.
Profiling Aids Inspection
When profiling these BGA and PoP components, it can bedetermined whether they had been processed in accordancewithin both the component as well as the solder paste’s thermaltolerances. The combination of thermal profiling and AOI/X-rayinspection provides a very complete inspection process, even forthe very tiny devices. In addition, the suspect PCBs that havebeen processed outside of the thermal process window can bererouted to the batch X-ray station. This can provide for a moreeffective sampling of PCBs to be inspected compared to the randomsampling that is so often used today. Thermal profilingsimply makes the existing inspection systems in the factorywork far better than in the past.
But is not profiling suffering from the same issue as batchX-ray in that the batch x-ray does not provide 100 percentinspection? Manual profiling is a spot-check operation. Manycompanies will profile only once a day or once a week. There are,however, numerous automatic profiling systems on the marketthat profile each and every PCB on the fly without interruptingthe production flow.Automatic profiling systems can work together with AOIand X-ray systems for a total inspection solution that plugs theloopholes that currently exist, in which defects or latent defectscan slip through and end up failing later on, after it is in the enduser’s hands.
Tiny Leads
Miniaturization of componentsmeans that there are now a growingnumber of components that have suchtiny leads that the stencil printerdeposits a very small amount of solderpaste on the pads. These deposits havea much larger ratio of surface to volumethan what we have been accustomedto. This creates challenges forboth the printing and reflow process.
During the reflow process, one ofseveral challenges is that the flux may burn off prematurelyand, hence, problems with oxidization will occur. The soakphase may need to be reduced or eliminated to avoid this.
There are several other problematic aspects of the thermalprocess exacerbated by the tiny components. Generally,the acceptable thermal process window shrinks, sometimessignificantly, making it much more difficult to reflow thesenew components. Special care needs to be taken during thesetup of the reflow oven for such assemblies in order to findthat narrow “sweet spot” in the process.
During production, there always will be a certainamount of drift in the thermal process. With more narrow tolerances,the process needs to be optimized during setup whilemore frequent profiling must be implemented to verify thatall these components continue to be produced in spec.
The good news is that there is a growing, developingbody of knowledge about these issues. Some of the solderpaste manufacturers, for example, have produced excellentand helpful papers on issues such as voiding and graping. Inaddition, automatic profiling systems have become moreintelligent and affordable. They help to verify that the productionof the new, smaller components is made correctly,while complementing the existing AOI and X-ray inspection.
See at IPC/APEX Booth #1314.
Reprinted with permission from the February, 2013 issue of US Tech