Applications/Processing Guide
To minimize exposure to high-pressure water during board
wash
Rate of Change of Profile
(Indicator of Level of Stability)
• Select trimmer models with pin styles that orient the
rotor seal area away from exposure to the high-pressure
water stream
7
Unstable Temp
(# of Cycles)
SETTABILITY
°C/Min.
Settability refers to the ease with which a trimmer can be set
accurately to the position that produces the desired circuit condition.
Where the requirement is for obtaining a highly accurate set-
ting the preference is for cermet — because a small incremental
adjustment in a wirewound unit does not always produce the
expected change in output as the wiper moves off one turn of wire
and onto another.
Setting accuracy is better with a multiturn unit than with a
single-turn. This is especially true when the speed of setting is also
a requirement as on a production line (Figure 2).
Minutes
SINGLE-TURN SETTABILITY (3386)
MULTITURN SETTABILITY (3299)
Figure 1.
400
300
200
MV.
Typical temperature profile for board washing
and soldering.
400
M.V.
EXPERIENCED
TECHNICIAN
EXPERIENCED
TECHNICIAN
Critical profile parameters
1
2
3
4
5
6
7
Temperature Shock (°C)
Maximum Temperature (°C)
400
M.V.
Temperature Exposure (Minimum)
Temperature Gradient (°C)
500
Temperature Shock Decrease in Water (°C)
Temperature Shock Decrease in Water & Air Pressure (°C)
Unstable Temperature (see next page)
400
MV.
LESS EXPERIENCED
TECHNICIAN
LESS EXPERIENCED
TECHNICIAN
0
1.7 3.4 5.1 6.8 8.5 10.2
SEC.
0
1.7 3.4 5.1 6.8 8.5 10.2
SEC.
General Guidelines for Guarding Against Component Damage
To minimize temperature shock
• Pre-heat boards to maximum acceptable level
• Reduce time in solder
Figure 2.
When accurate setting is required, a multiturn trimmer
can generally be set faster than a single-turn.
To avoid heating components above their maximum rated
temperature
• Use lowest acceptable solder temperature
• Use maximum allowable conveyor speed
• Limit pre-heat temperature to maximum necessary
STABILITY
Stability refers to the ability of the trimmer to remain at the
desired setting. Environmental factors play an important role here:
stability may be affected by temperature exposure, thermal shock/
cycling, humidity, and mechanical shock or vibration.
To limit time of exposure above rated temperature
• Limit time in solder
This is not a matter of concern in most applications, since
Bourns trimmers exhibit excellent stability under all specified condi-
tions. Stability is most often a concern when cermet trimmers are
used in low current "dry" circuits (50uA amps and below). Under
these conditions the contact resistance may vary, making the wiper
appear unstable. This is most noticeable in some rheostat applica-
tions. This can be avoided by using a wirewound unit, or choosing a
cermet trimmer that has been designed for dry-circuit applications.
Bourns applications engineers can assist you on this and other
questions.
• After solder operation, cool board to wash temperature
before it enters wash
To minimize temperature difference between top and bottom of
board
• Apply pre-heat to both top and bottom
To reduce temperature shock on entering the moist environ-
ment of the wash
• Use wash/rinse temperature as near component
temperature as possible
• Extend time between solder process and wash
• Cool board after solder operation, prior to entering wash
ACCESSIBILITY
When selecting a trimmer and determining its placement on the
board, keep in mind the people who will have to use it. Bourns trim-
mers are available in a wide variety of sizes, shapes, configurations,
and placement of adjustment screws. You will usually find a unit on
which the access for adjustment will be convenient for the user.
To minimize temperature variations as component travels
through moisture
• Minimize number of wash/rinse and rinse/dry cycles
• Use heated air for air knives (to counter evaporative
cooling effect)
• Minimize difference between wash and rinse temperature
89
Specifications are subject to change without notice.
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