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What you need to know before buying your AOI solution
What you need to know
before buying your AOI
Paul Groome, Machine Vision Products, Carlsbad, CA, USA
1. Why are you purchasing an aOi
When defining an
inspection strategy for your
The first question you should ask yourself
manufacturing processes, there
is why are you purchasing AOI? Hopefully
are many factors that need
the answer is to detect defects. Some users,
to be considered. Likewise
in our experience, will leave this out of a
system evaluation.
there are many products that
When you look at defect coverage, do
seem to have solutions to your
you know your process and the defects that
inspection requirements. How
can arise during SMT manufacturing—
do you make the decision of solder defects (tombstones, opens,
which system to use? How do solder amounts etc), presence/absence,
you determine the viability
component measurement data, wrong
of manual inspection, of
components, etc? This knowledge, and
template/comparative vs.
how you use this data, is important to
measurement-based AOI,
making an inspection decision. There are
what resolution to use, which
many industry bodies that can help you
cameras, what data outputs
to determine yields and possible defects.
As a guide, based on internal studies and
are required, do you need
industry standards, DMPO (defects per
Figure 1. Jlead defect on a single-camera system using
measurement data, attribute
million opportunities) rates defined in tri-color lighting.
data or both?
Tables 1 and 2 can be used as a base line for
The goal of this paper is defect detection requirements.
to detail which variables are
presence or absence of the component
When considering defect inspection
and compare the joint to a known good
important versus those that
capabilities there are a number of items to
joint, which can lead to a higher volume of
can confuse the AOI selection
false calls. When, as in most manufactur-
process. For this purpose, this
ing scenarios, defects follow the DPMO
paper will discuss the top five
Inspection techniques, capabilities of
rates shown in Table 2 and you have
variables that can ensure a
template matching vs. measurement based
solder defects, tombstones and opens, it is
successful AOI deployment in
AOI platforms.
important to select a system that has the
There are two basic techniques used by
your manufacturing processes.
capabilities to inspect for solder.
AOI systems: template-based comparison
Another question that is normally
systems and rules-based measurements
discussed is do we need to use angled
systems. The main difference is measure-
cameras to locate defects. There are many
ments. For example, a rules based system
Keywords: AOI, Defect
arguments to this, but generally angled
will measure the width, length and position
Detection, Template Matching,
cameras are complex to calibrate and can
of a component then from this position
Measurement-Based, False Calls
be significantly affected by board warp. If
measure the actual amount of solder
you understand the reflow process, a single
on each pin (referenced to component
telecentric camera system can provide the
position), then perform OCV or OVR de-
same defect coverage as an angled camera
pendant on component marking. The com-
without calibration or warp issues. Figure
ponent body measurements provide the
1 shows a real life example where the left
capability to find the correct component,
center pin (pin 25) is open. On a single,
and the solder measurements will provide
telecentric camera system using tri-color
the exact percentage of solder between the
lighting (tri-color technologies use angled
component and the pad. A template-based
lighting vs. angled cameras) the defect is
system, on the other hand, will look for
10 – Global SMT & Packaging – October 2009
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