Application Notes—Analog Optical Isolators
APPLICATION NOTE #2
Handling and Soldering AOIs
All opto components must be handled and soldered with care,
especially those that use a cast or molded plastic and lead frame
construction like the LEDs used in AOIs.
In LED lead frame construction, the emitter chip is mounted directly to
one lead and a wire bond is made from the chip to the other lead. The
encapsulating plastic is the only support for the lead frame. Care must
be taken when forming the leads of plastic opto packages. Excessive
mechanical force can cause the leads to move inside the plastic
package and damage the wire bonds. Weakened bonds can then
“open up” under further mechanical or thermal stressing, producing
open circuits.
When hand soldering, it is important to limit the maximum temperature
of the iron by controlling the power. It is best if a 15W or 25W iron is
used. The maximum recommended lead soldering temperature (1/16"
from the case for 5 seconds) is 260°C. An RMA rosin core solder is
recommended.
In order to form leads safely, it is necessary to firmly lamp the leads
near the base of the package in order not to transfer any force
(particularly tension forces) to the plastic body. This can be
accomplished either through use of properly designed tooling or by
firmly gripping the leads below the base of the package with a pair of
needle nose pliers while the leads are being bent.
Sn60 (60% tin / 40% lead) solder is recommended for wave soldering
opto components into printed circuit boards. Other alternatives are
Sn62 and Sn63. The maximum recommended soldering temperature
is 260°C with a maximum duration of 5 seconds.
The amount of tarnish on the leads determines the type of flux to use
when soldering devices with silver plated leads.
Condition of Leads
Recommended Flux
Clear Bright Finish
(Tarnish Free)
RMA - Mildly Activated
Dull Finish
(Minimal Tarnish)
RMA - Mildly Activated
RA - Activated
Examples of Tooling Fixtures Used to Form Leads
Light Yellow Tint
(Mild Tarnish)
For highest reliability, avoid flush mounting the AOI body on the printed
circuit board. This minimizes mechanical stress set up between the
circuit board and the LED and photocell packages. It also reduces
solder head damage to the packages.
Light Yellow / Tan Color
(Moderate Tarnish)
AC - Water Soluble,
Organic Acid Flux
Dark Tan / Black Color
(Heavy Tarnish)
Leads Need to be Cleaned
Prior to Soldering
Cleaners designed for the removal of tarnish from the leads of
electronic components are acidic and it is best to keep the immersion
time as short as possible (less than 2 seconds) and to immediately
wash all devices thoroughly in ten rinses of deionized water.
Good printed circuit board layout avoids putting any spreading (plastic
under tension) force on the leads of the LED and photocell.
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PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
