IPC7527-2012RequirementsforSolderPastePrinting.pdf - 第16页

Figure A-5 illustrates the ef fect of solder mask. Due to sol- der mask coverage on the tracks, the stencil will be lifted. Result: Increased height of solder paste deposit and solder paste smearing. Figure A-6 illustrat…

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Appendix A
Guideline for Operator Troubleshooting in the
Solder Paste Screen Printing Process
This section can be used as a guideline, so that the
operator/user can identify error types and problem areas
with relevant solutions that can be used in connection with
correcting/optimizing of the solder paste deposits.
The following requirements should be met before the
screen printer process optimization can be started:
• The solder paste should meet the requirements of J-STD-
005 or equivalent.
• The stencil should be in good condition and have the cor-
rect tension.
• The squeegee blades/printing head should be in good con-
dition and mounted correctly.
• There should be sufficient board support for the PCB dur-
ing the printing process.
• The stencil should be clean on both sides and in the aper-
tures (inspect with microscope).
A.1 Visual Check for Solder Paste Printing Optimization
• Does the solder paste roll when pushed by the squeegee
blades?
• Are all apertures filled 100% with solder paste?
• Is the top side of the stencil free of solder paste residues?
• Is the solder paste released from the stencil apertures?
• Is there solder paste residues on the bottom side of the
stencil?
A.2 Tolerances The nominal solder paste deposit height
equals the stencil thickness. However, this will not always
be true because of other factors that influence the paste
deposit height such as solder mask, silk screen, labels,
masking, and contaminates on the PCB surface (Figures
A-3 and A-4).
Note: There may be differences in the solder deposit
depending on the type of squeegee blade used; e.g., metal,
rubber or plastic. The customer and manufacturer should
agree on the acceptable conditions when other than metal
blades are used.
IPC-7527-A-1
Figure A-1 Solder Paste Printing with Squeegee Blades
1. Squeegee Pressure
2. Blade Height
3. Squeegee Speed
4. Paste
5. Squeegee
kg
1
2
3
4
PCB
Stencil
5
IPC-7527-A-2
Figure A-2 Solder Paste Printing with Enclosed Head
System
1. Printing Speed
2. Enclosed Head
3. Paste Pressure
1
3
2
Figure A-3 Plastic from Packaging
May 2012 IPC-7527
9
Copyright IPC-Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
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Figure A-5 illustrates the effect of solder mask. Due to sol-
der mask coverage on the tracks, the stencil will be lifted.
Result: Increased height of solder paste deposit and solder
paste smearing.
Figure A-6 illustrates the effect when solder mask is com-
bined with silk screen. This gives a further increase in the
paste deposit height and smearing.
Figure A-7 illustrates the effect of solder paste smearing
resulting in solder balls after the reflow process.
When the solder paste smears, a small amount of paste will
be left on the bottom side of the stencil (Figure A-8), which
may then be deposited on the next PCB. This problem can
be reduced by cleaning the bottom side of the stencil.
There are automatic cleaning systems that can be fixed into
the screen printer; these can be programmed so that the
stencil is cleaned with fixed intervals or as required. See
Section A.8, Issues with Stencil Cleaning.
A.3 Varying Paste Deposit Shape This section is a
guideline to suggest possible causes of the listed problems.
It is not intended to be a corrective action checklist.
Problem A.3.1 Saddle Shape Paste Deposit
(Figure A-9)
Possible causes:
• Printing pressure too high.
• Squeegee blades too soft.
• Aperture area too large compared to squeegee type and
pressure.
Problem A.3.2 Roof Top Shape Paste Deposit
(Figure A-10)
Possible causes:
• Solder paste is not rolling during printing.
• Print speed too high.
• Insufficient solder paste on stencil.
Figure A-4 Glass Fiber from PCB
IPC-7527-A-5
Figure A-5 Effect of Solder Mask
PCB
Stencil
IPC-7527-A-6
Figure A-6 Effect of Silk Screen
PCB
Stencil
Silk Screen
IPC-7527-A-7
Figure A-7 Effect of Solder Paste
PCB
Stencil
IPC-7527-A-8
Figure A-8 Result of Silk Screen Interference
PCB
Stencil
Figure A-9 Saddle Shape Deposit
Figure A-10 Roof Top Shape Paste Deposit
IPC-7527 May 2012
10
Copyright IPC-Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
--`,,```,,,,````-`-`,,`,,`,`,,`---
Problem A.3.3 Spikes on Paste Deposit
(Figure A-11)
Possible causes:
• Incorrect solder paste viscosity.
• Poor stencil quality.
• Dried out solder paste residues in stencil aperture.
• Incorrect separation speed.
• Squeegee pressure too low.
A.4 Varying solder paste deposits quantities
Problem A.4.1 Increased Quantity Compared to Stencil
Thickness (Figure A-12)
Possible causes:
• Incorrect down stop.
• Print pressure too low.
• Incorrect parallelism.
• Print speed too high.
• Incorrect snap off.
• Excessive solder mask thickness.
• Excessive silk screen thickness.
• Incorrect adjusted PCB thickness.
• Incorrect/insufficient board support.
Problem A.4.2 Decreased Quantity Compared to Stencil
Thickness (Figure A-13)
Possible causes:
• Dried solder paste.
• Print speed too high.
• Insufficient solder paste on stencil.
• Dried solder paste in stencil aperture.
A.5 Solder Paste Residues Outside Designated Print
Area
Problem A.5.1 Slumping/Smearing
(Figure A-14)
Possible causes:
• Temperature too high in printing area.
• Incorrect solder paste viscosity.
• Printing pressure too high.
• Excessive solder mask thickness.
• Excessive silk screen thickness.
• Uneven surface on footprint, e.g., Hot Air Solder Level
(HASL).
• Incorrect snap off.
• Misalignment of solder paste printing.
• Insufficient stencil tension.
• Excessive paste pressure in closed head printing system.
Figure A-11 Spikes on Paste Deposit
Figure A-12 Increased Deposit Quantity
Figure A-13 Decreased Quantity
Figure A-14 Slumping/Smearing
May 2012 IPC-7527
11
Copyright IPC-Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
--`,,```,,,,````-`-`,,`,,`,`,,`---