Supporting niche service needs at a regional level—a case study
other two are for actual electrical
contact.
PCB substrate selection is important.
Ultra bright LEDs require additional heat
sinking. This is typically done using an alu-
minum backed PCB. This is usually a very
thin FR4 material bonded to a thin sheet
of aluminum. Another option is an FR4
substrate with a specific via-hole pattern
that distributes heat more efficiently. There
is usually a cluster of 8-13 vias around the
heat sink pad near the LED. This layout
is more common in high-powered silicon
devices.
There is a 1:1 aperture ratio for the
spacing of the paste placement on the
board. The 1:1 land pattern allows an
ample amount of paste to be deposited. It
is important to make sure that via holes
aren’t too close to the solder pads for the
LEDs and that an ample amount of solder
mask is present to minimize the solder
wicking into the via holes.
Actual volume component placement
is identical to other SMT components.
Most LEDs are packaged on tape and reel.
Prototypes are typically hand placed.
From a reflow standpoint, forced air
Volume component placement for LED assemblies is identical to other SMT components.
convection ovens with eight zones are ideal
because the heat and subsequent outgas-
ment of robust thermal profiles in tighter In some cases, this is unacceptable in the
sing can be controlled better with a larger
process windows. final application, but in others the slight
number of zones. Ovens with three to five
SMT LEDs are much more robust with variance is not an issue. Where variance
zones do not offer as much control. When
a tightly controlled temperature profile. may be a problem, production lots are
heat is not properly controlled the outgas-
Even small variations can create issues. For sequenced to component lots.
sing of solder can cause massive voids. This
example, in one production lot, a blower
leads to problems with high frequency
was failing intermittently in the oven. The conclusion
lines because the void acts a conductor. Ad-
oven wasn’t signaling the failure, but some LCD manufacturing uses many traditional
ditionally, a large void reduces the current
boards were not completely soldered. The EMS production capabilities. At the same
carrying capability of that joint.
KIC monitoring system made it easy to time, it does require a stronger focus on
This contractor uses a Heller eight-
identify the zone with the temperature process control, thermal profiling, the min-
zone forced air convection oven with KIC
fluctuation and identify the blower that imization of handling, and a clean room
process optimization software. The process
was failing. environment. Highly skilled operators at
window is initially defined by determining
Cleaning can be an issue. An aggressive final assembly and specialized engineering
the intersection among solder specifica-
flux is used in the reflow process, requir- support are also required. With those ele-
tions, the substrate specifications and
ing the use of an aquatic type of wash in ments in place, a wide range of specialized
component tolerances. Thermocouples
the cleaning process. The water must be LCD applications can be supported.
are attached to a sample product and a
high enough in temperature and pressure
manual profile is run which can be com-
to wash the flux out between the LED and Barry Reese is in program management at
pared against the process window. In the
board. Clover Electronics. He has more than 20+ years
case of ultra bright LEDs on PCB strips,
The final assembly process of the modi- experience in the commercial, industrial, and
additional thermocouple probes are used
fied LCD, defined by an optics engineer, government manufacturing sectors of avionics
to better monitor the heat distribution
requires highly skilled labor and is per- and aerospace, and as has served as general
pattern across the board as it goes through
formed in a class 100 cleanroom to prevent manager—production at Sypris Electronics. He
the oven.
contamination within the optical stack of can be contacted at breece@cloverelectronics.
The software generates a Process
the assembly.
Window Index (PWI), a single number
Functional test equipment is usually Ken Russell is Clover Electronics’ engineering
that indicates how the process fits the
supplied by the customer so that testing is manager. During his 24 years in electronics
defined process window. The lower the
performed under ‘field-test’ conditions. In manufacturing technical and management
number, the closer the process is towards
addition to visual inspections, luminance, positions, he has also been associated with
the center of the process window. The size
color and uniformity measurements are Optima Computer Systems, Authorized Com-
of the process window and capabilities of
often collected. A challenge is that there puter Repair and Quadram Corporation. He
the reflow oven determine how low the
may be variance in color and/or brightness can be contacted at krussell@cloverelectronics.com.
number can be and facilitates the develop-
among production lots of the same LED.
34 – Global SMT & Packaging – January 2009 www.globalsmt.net
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