Praveen,
For the evaluation and selection of an AOI system there are a few key points
that I suggest you focus on.
1. Solder Joint Inspection - Most AOI suppliers claim solder joint
inspection capability but this is really one of the differentiating points.
You need to decide how badly you want good solder joint inspection
capability. Truly capable systems are more complex mechanically as well as
in their programming thus requiring a higher level of expertise, as well as
investment in time, for program development and optimization. For low
volume-high mix and prototyping environments, it often takes more time to
develop robust solder joint inspection that it takes to build and ship the
product! Solder joint inspection will increase the false call rate of any
system. Finally, it is important to remember that video based AOI systems
can only operate in a line of sight fashion. Packages like SOJ's, PLCC's,
and BGA's will not support solder joint inspection. There are x-ray "combi"
machines available as well but you have to weigh the cost and throughput
implications of a combi. If you make the required commitment in capital
expenditure and engineering support, solder joint inspection can yield very
good results and help to take your process to the next level. If you don't
provide the appropriate level of commitment in support resources, then the
investment in solder joint inspection capability will be a waste and cause
frustration and confusion on the line until you ultimately turn off that
portion of the inspection. Look for systems that use convergent software
technology where the s/w keeps track of false calls and the conditions
surrounding them as a means to automatically and statistically refine
inspection algorithms as a means to reduce false calls.
2. Production Environment - As eluded to above, it is my opinion that the
state of the AOI industry today does not provide a single machine that "does
it all". If you are running in a high mix/low volume/high change
environment then machine speed/throughput is less important than ease in
program development. In this environment if you can't get a program cranked
out and reasonably optimized in 2 hours or less you will spend all of your
time programming and very little time running the machine in production.
Look for off-line programming capabilities as a means to help free the
machine up a bit more, but you still always need machine time for initial
image capture and final debug.
3. Vector Based versus Pattern Recognition -
Vector based machines will find the edges of packages and leads,
determine centroids, compare this to CAD data, and except/reject based on
numerical tolerance settings. This allows for the building of numerically
based part data files, like with pick-n-place machines. Vector based
machines can provide you with very accurate x-y offset data to validate and
tweak in your p-n-p equipment. This keeps the AOI machine from "learning" a
bad placement or condition as the "golden" model. There is also at least an
order of magnitude greater complexity, difficulty, and time investment in
the programming and optimization of vector based machines, but vector based
machines generally offer greater flexibility over the long run for odd
packages, special measurement applications, and fine pitch lead measurement.
The vector machines I am familiar with (MVT and VI) are generally faster
than the pattern rec. class of machines although some companies like CR
Technology are steadily developing faster and faster machines, thus closing
the gap.
Pattern Recognition, or correlation based vision processing compares a
bit map from the camera field of view FOV for the current board to a saved
"golden" image for the same FOV. The user defines the degree of bit map
correlation required to achieve the pass threshold. Bit map correlation
algorithms are used to break up the FOV into smaller chunks, allowing for
the detection and identification of a small defect like a skewed 0402
although comparing FOV images that may be larger (~1" x 1"). The
advantages to pattern recognition is that it is generally easier to program,
using wizard style teach functions combined with cad data, and easier to
understand. Great for a proto or high mix shop, or an environment where you
don't have the luxury of throwing a high level tech(s) or engineer(s) at the
process. The disadvantage is that often acceptable manufacturing variation,
such as a different color tantalum cap off of the approved vendor list, will
result in a reject. It then requires you to teach this as an alternate
acceptable image. Depending on who is allowed to teach alternates, you can
get in the situation where pretty soon you have dozens of alternates for
many FOV's which can start to affect the speed of inspection and
inadvertently create opportunities for false passes.
4. SPC - It amazes me how many AOI systems out there provide inadequate
supporting SPC packages. Without a system that provides good real time SPC
feedback up the line, the AOI machine is only a failure screen that helps
you increase your ICT yields but does not reduce rework (assuming you are
repairing AOI detected defects in advance of ICT). Look for systems that
allow for web based live SPC reporting so you don't have to invest in a
separate data server and set of applications/licenses requiring dedicated
data terminals. Web based output allows you to leverage your existing plant
PC infrastructure in the factory and at the desks of those who need this
data. All you need is an Ethernet card in the machine which writes data to
a network target, and a standard browser to run the AOI manufacturer's Java
scripts for displaying the live data from that target. If you want to
integrate the AOI data real time and automatically into your plant's
existing quality tracking system, evaluate the data output structures and
communications interfaces that the machine supports.
5. Company Stability/Structure - There is something like 20 players in this
market today. Considering the current electronics industry market
condition, and the natural tendency for consolidation in this type of
business, I think we'll see many of these 20 disappear over the next couple
of years either through lack of funding, lack of market share, or
acquisition by larger companies. Work to understand the AOI company's
financial health (private, public, venture capital funded, cash??, etc.),
short and long term business strategy (IPO, acquire or be acquired),
historical and current growth rate compared to competitors, and if they
posses any differentiating technology that has inherent value over their
competitors (this point often makes a small company ripe for acquisition
which can be good over the long haul). The quantum leap in PC processor
speeds, cheap RAM, and quality high resolution color CCD cameras available
today is the primary reason for this explosion in AOI products, i.e. we can
finally afford them due to affordable components! This levels the playing
field so to speak from the hardware standpoint, making the software side of
the product the key differentiating feature. Look for companies that
recognize this and possess evolved machine algorithms, program development
tools, and SPC packages. They should have a large software development
resource with many experienced software engineers and developers. They
should be able to demonstrate active software improvement projects and have
a "roadmap" of additional features and enhancements they are working on. If
they cant, and are focused on hardware and speed improvements only, beware!
There's more to think about but I feel these are the biggies.
Good luck,
Glenn Woodhouse
MCMS
Advanced Technology Development
(208) 898-2753
> -----Original Message-----
> From: Automatic digest processor [SMTP:[log in to unmask]]
> Sent: Thursday, August 30, 2001 11:00 PM
> To: Recipients of TechNet digests
> Subject: TechNet Digest - 30 Aug 2001 (#2001-510)
>
> There are 4 messages totalling 370 lines in this issue.
> ------------------------------
>
> Date: Fri, 31 Aug 2001 00:18:31 +0000
> From: praveen kumar manjeshwar <[log in to unmask]>
> Subject: AOI equipment
>
> Hi everyone,
> I just wanted to know where would it be preferable to postion an AOI
> equipment on a production line. I have known of people who prefer to use
> it
> before reflow and some who have suggested after reflow.
> What would the implications be if this equipment is placed prior to and
> after reflow?
> Also, if one had to evaluate an AOI equipment (not solder paste measuring
> equipments), then what factors would one have to consider to gage the
> capability of this equipment?
> Thanks
> Praveen
>
>
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