2.4.3 Coating Replacement, Solder Mask, BGA Locations
Printed Board Type: R/F/W/C
Skill Level: Intermediate
Conformance Level: High
Revision: A
Revision Date: May 9, 2001
Outline
This method is used to replace solder mask or coatings
on circuit boards at BGA locations. BGA locations provide
unique challenges due to their inaccessibility after
reflow, exposure to high temperatures, and requirement
that the BGA pads only are surrounded by mask. Vias
and circuits that are exposed may cause shorting or
BGA solder joint starvation.
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Acceptability
References |
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IPC-A-600 |
2.0 |
Externally
Observable Characteristics |
| IPC-A-610 |
10.0 |
Laminate Conditions |
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Soldermask Required at BGA Site |
Tools and Materials
Brush
Cleaner
Color Agents
Epoxy
Foam Swabs
Heat Lamp
Microscope
Oven
Wipes
Procedure 2.4.3 Coating Replacement, Solder
Mask, BGA Locations
Figure 1: Scrape down to the copper surface to create a
barrier to solder
flow between pad and via.
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| 1. |
Clean the area.
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| |
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Caution
Surfaces to be coated must
be thoroughly cleaned prior to coating to ensure
adequate adhesion, minimized corrosion, and
optimized electrical properties.
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| |
| 2. |
Remove all excess solder from BGA pad and the
via (top and bottom side) using solder braid
or desoldering tools. BGA pads should be flat
and the vias clear of solder.
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| |
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Note
Vias with solder trapped
inside may "volcano" underneath the reflowed
component. It is important to remove the solder
if possible.
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| |
| 3. |
Scrape solder off the "Dog Bone" section between
the BGA pad and connecting via. (See Figure
1).
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| |
Note
This is to prevent solder from flowing down the
"Dog Bone" path to the via location. |
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Figure 2: Apply the replacement
coating. A wood stick or small dispenser may be used.
Figure 3: "Dog Bone" area coated
with soldermask |
| 4. |
Mix the epoxy or replacement coating. |
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| 5. |
Apply the replacement coating to the board surface
as required. (See Figure 2). Apply a thin
coating of epoxy to prevent interference with
component placement. Adding a color agent to
the epoxy is not recommended since it may slightly
reduce the epoxy strength or adhesion.
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| |
| 6. |
Cure the epoxy per Procedure 2.7
Epoxy Mixing and Handling. Provide a full cure time
to ensure maximum epoxy strength. |
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Caution
Some components may be sensitive
to high temperature. |
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Evaluation |
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| 1. |
Visual examination for texture,
color match, adhesion and coverage. |
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| 2. |
Electrical tests to conductors
around the repaired area as applicable. |
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