3D AOI Solder Joint Qualification Parameters

We have a new inline 3D AOI system. One of the options on our system allows checking if the solder joints pass the IPC Class set. We are running a class 2 product and with this solder joint inspection added, there are some parts that are no longer passing for class 2.

The area of the solder is passing but the solder wetting slope is not. Some capacitors are very thick and the PCB pad is not large enough to contain enough solder to fully go up the side to the correct slope.

Is this something that others are seeing in the industry? Would the usual fix be to correct the board layout or adjust the AOI parameters allowing this to pass since the solder joint looks good but is not truly class 2? Any insight would be appreciated.

J.S.

Expert Panel Responses

What you are seeing is very common with AOI systems. We typically start with the IPC Class set then edit the inspection parameters on a few of the solder joints. We often use the first few boards of a new production run to dial in these parameters if we don’t want the AOI operators to go through the acceptance review on every board. IF you have the ability to make design improvements to improve quality that is great, but in many cases it’s hard to justify a design change for a condition that is acceptable to the end user.

Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group

Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

Prior to a board respin - I would start by either stepping the stencil for the areas in question or increasing the entire stencil thickness. Before buying a new stencil, I would use Camalot to apply a little extra paste in those areas to see if that improves your fillet features.

That being said, I have never had great success with Class based settings in 3D AOI. Even though most of our products are Class II, I use Class III specs and individually program solder metrics (width, length, fillet height). This removes any guesswork and flags anything I need to address.

Larry Harman
SMT Engineer
ACDi

Mr. Harman is an SMT Engineer at ACDi and worked at Oven Industries as a Manufacturing Engineer. H's worked at Philips manufacturing ultrasound probes/circuitry. He holds three patents, one for an ultrasound probe design, and two for innovative rat/mouse zapper circuits. He attended Pennsylvania State University for Electrical Engineering.

The answer could be divided in two. First and foremost, I recommend looking at IPC-A-610 to verify that your suspected non-conforming areas are indeed ok. The AOI does not determine pass or fail, it's function is to point the operator's attention to suspected areas on the PCB that might have defects. Different machines have different inspection capabilities and parameters you need to configure in order to screen for possible defect areas while maintaining a minimum of false alarms. If you have too many false alarms, your operator who inspects the suspected areas can easily get tired and later on miss valid defects. So understanding and manipulating the algorithms and thresholds of the AOI machine is very important.

Secondly, to the wetting slope in question, IPC standards are more concerned with nonwetting and dewetting. Nonwetting refers to incomplete coverage of the base metal of the land or non-wetting of the terminal, and dewetting refers to irregular shapes of the solder on the pad forming hills and dips. The slope definition is added to help in distinguishing proper wetting, however, as far as I know there is no requirement for a specific angle other than the requirement for less than 90 degree angle and this has some exceptions as well. For class 3 only, you have an additional requirement for a minimum of 25% wetting of the terminal height or 0.5mm (the lower of the two is acceptable). Again this does not require a specific slope.

That being said, If you suspect your solder joints are not properly wetted, you could try elevating the temperate or time in liquidus state of your reflow profile and see if you get better wetting. The other thing would be to add in the next PCB batch a bigger land for the pads in question which would contribute to the overall wetting of the terminal.

Guy Shemesh
General Manager
ePiccolo Engineering

Mr. Shemesh has Bsc. in E.E engineering and hands-on experience with electronics (schematics & layout) since 2004. He has designed dozens of multi-layer PCBs, HDI, RF, rigid-flex, etc., and had the honor for design reviewing veteran layout engineers several times as a consultant.

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