7.1.2 Preparation For Soldering And Component RemovalPrinted Board Type: R/F/C
Skill Level: Intermediate
Conformance Level: N/A
Revision: D
Revision Date: Jul 7, 2000
Outline
This procedure covers the basic concepts for preparing circuit
boards and soldering tools for soldering and component removal.
Note
Refer to 7.1.1 Soldering Basics before proceeding.
Tools and Materials
Cleaner
Flux
Microscope
Soldering Braid
Soldering Iron with Tips
Solder Removal Tool
Wipes
Caution
Silicon based lubricants or hand creams should not be used on
or near surfaces to be soldered.
Soldering Iron Tip Selection
The size and shape of the soldering iron tip will have an effect
on the rate of heat transfer. Larger tips with more surface
area will transfer heat faster than smaller tips.
Tip size is based on the size of the component. While there
is no exact rule about how the size of a soldering iron tip
should compare to the size of the termination, if the tip extends
too far beyond the edges of the joint, it could come in contact
with another component or the surface of the board. Where possible,
the width of the soldering iron tip should be slightly smaller
than the width of the pad.
Component Removal Tip Selection
The size and shape of the component removal tip will have an
effect on the rate of heat transfer. Larger tips with more surface
area will transfer heat faster than smaller tips
| 1. |
Vacuum Desoldering Tip Selection
The smallest tip should be selected providing that the
tip fits over the component lead and allows room for
molten solder and air to pass through it. The outside
diameter of the tip should not cover the pad completely
or touch the circuit board base material or solder mask.
If the tip extends too far beyond the edges of the joint,
it could come in contact with another component or the
surface of the circuit board.
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| 2. |
Hot Air Tool Tip Selection
The smallest tip should be selected providing that the
proper air flow is delivered to the leads and solder
joints. If the tip is too large it may extend beyond
the edges of the component and cause reflow to adjacent
components or burn the surface of the circuit board.
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| 3. |
Conductive Tool Tip Selection
The smallest tip should be selected providing that the
tip fits over the entire component and contacts all
the leads evenly. If the tip is too large it may extend
beyond the edges of the component and contact another
component or the surface of the circuit board.
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General Preparation
| 1. |
If needed, circuit boards should be cleaned prior to
soldering and component removal operations. Oxidation
and contamination should be removed by methods that
do not damage leads or parts, and do not cause contamination
or hinder solder wetting.
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| 2. |
If required, circuit boards should be baked in a suitable
oven to remove any absorbed moisture. Time between bake
and soldering should not exceed 5 days, depending on
atmosphere humidity levels. Temperature and time of
baking is to be determined on individual basis.
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| 3. |
If needed, tin component leads prior to soldering.
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| 4. |
If needed, reform component leads or replace the component
if the leads do not meet the specification required.
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Preparation – Excess Solder
Before inserting a component into a plated through hole for
through hole soldering, or onto pads for surface mount soldering,
it may be necessary to remove any excess solder. This is recommended
for 2 main reasons:
| 1. |
If a component was previously soldered at the rework
location, some of the original solder will remain attached
to the pad. That solder has already been heated twice.
If it becomes part of the new solder joint, it will
have been heated at least three or even four times.
Reheating solder three or four times - even with the
addition of flux - may affect the physical composition
of the metals. Every time that solder is reheated, the
molecular structure tends to become increasingly brittle.
Solder needs to remain ductile in order to absorb the
stresses of expansion and contraction caused by heating
and cooling. Old solder should be removed and replaced
with new solder whenever a component is reattached.
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| 2. |
Excess solder in a plated through hole or excess solder
on surface mount pads may interfere with the proper
placement of the new component.
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Figure 1: Remove excess solder
from plated holes using a vacuum desoldering tool. |
PROCEDURE – Plated
Through Hole Preparation, Vacuum Desolder Tool Method
Solder removal by wicking is not recommended for removal
of solder from a plated hole. A powered vacuum desoldering
tool is recommended.
The powered vacuum desoldering tool has a heated tip
with a hole in the center to vacuum melted solder away.
There are different tip sizes depending on the size
of the job. The diameter of the tip should match the
width of the pad. A larger tip will extend over the
edge of the pad and could potentially burn the board.
| 1. |
If needed, clean the area.
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| 2. |
Inspect the hole. If there is not sufficient
solder covering the pad to provide for proper
heat transfer, the hole should be filled with
solder. Filling the hole improves the thermal
linkage between the desolder tool tip and the
solder in the hole. This ensures a rapid melt
and reduces potential for pad or hole damage.
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| 3. |
Place the heated desoldering tip onto the pad
until you feel the solder melt. Do not apply
any downward or sideways pressure on the pad.
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| 4. |
After the solder melts, activate the vacuum
and suck the solder through the hole in the
tip into the solder storage chamber. (See Figure
1).
It should only take a few seconds for all of
the solder to be removed. After the solder is
vacuumed from the hole, lift the tool. Continue
the vacuum for an additional few seconds to
make sure that the solder has had enough time
to travel through the tip into the storage chamber.
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| 5. |
If needed, remove solder from the remaining
holes so that the component can be inserted
without force.
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| 6. |
Clean the area.
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Figure 2: Remove excess solder
from surface mount pads using a
solder iron and solder braid. |
PROCEDURE – Surface
Mount Pad Preparation, Solder Braid Method
Solder Braid is made from stranded copper with a powdered
flux inside the copper strands. Solder braid will absorb
the solder when heat is applied to the braid and solder
surface.
| 1. |
Solder braid comes in different widths. Select
a size that matches the width of the pad, or
just slightly smaller, where possible.
Note
Most solder removal braid comes with a powdered
flux inside the copper strands. Adding additional
flux will help to transfer the heat faster and
helps to improve the wicking or capillary action
of the copper braid.
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| 2. |
Select a soldering iron tip to match the width
of the pad. It the tip is too large for the
braid, it will hang over the edges and could
burn the board or the solder mask. If the tip
is too small, it will take much longer to heat
up the braid.
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| 3. |
Add a small amount of liquid flux to the braid.
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| 4. |
Place braid over the pad and rest the iron tip
on the braid. (See Figure 2). As you apply the
heat, it's important to avoid putting any downward
or sideways pressure on the pad, since the adhesive
resin underneath the pad is being heated at
the same time. Adhesion between the circuit
board and the pad is at its weakest when heated.
Sideways pressure against the pad can lift the
pad off the circuit board surface.
The weight of the soldering iron should apply
sufficient contact to quickly heat the solder
braid. The heat that passes through the braid
should melt any solder that remains on the pad
within a few seconds.
The wicking action of the copper will draw the
solder away from the pad. This wicking action
should be visible. When the wicking action stops,
remove the braid and the iron. The used portion
of the braid should be clipped off and any other
pads should be prepared exactly like the first.
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Figure 3: Remove excess solder
from surface mount pads using a vacuum desoldering tool. |
PROCEDURE – Surface
Mount Pad Preparation, Vacuum Desolder Tool Method
The powered vacuum desoldering tool has a heated tip
with a hole in the center to vacuum melted solder away.
There are different tip sizes depending on the size
of the job. The diameter of the tip should match the
width of the pad. A larger tip will extend over the
edge of the pad and could potentially burn the board.
| 1. |
Apply a small amount of liquid flux to the pad.
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| 2. |
Place the heated tip onto the pad until you
feel the solder melt. Do not apply any downward
or sideways pressure on the pad, the weight
of the handpiece tip is sufficient.
Caution
Avoid exerting any pressure on the pad.
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| 3. |
After the solder melts, activate the vacuum
and suck the solder through the hole in the
tip into the solder storage chamber. (See Figure
3).
It should only take a few seconds for all of
the solder to be removed. After the solder is
vacuumed from the pad, lift the tool. Continue
the vacuum for an additional few seconds to
make sure that the solder has had enough time
to travel through the tip into the storage chamber.
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| 4. |
Any other pads should be prepared exactly like
the first.
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| 5. |
Clean the area.
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