IPC-7527.pdf - 第19页

Possible causes: • Board support. • PCB. • Stencil/print pressure. A.7 Issues with Cleaning Misprinted PCBs Misprinted PCBs that are rejected by a visual inspection system (manual inspection, AOI, SPI) require cleaning. …

Problem A.5.2 Solder Paste Residues Around Deposit

(Figure A-15)

Possible causes:

• Incorrect stencil cleaning.

• Printing pressure too high.

• Excessive solder mask thickness.

• Excessive silk screen thickness.

• Double printing.

A.6 Printing Problems with Solder Past Residues on Top

Side of Stencil

Figures A-16 through A-19 show how the

solder paste residues are distributed on the stencil surface.

Possible causes:

• Print pressure.

• Print speed.

• Downstop.

• Board support.

Possible causes:

• Parallelism.

• Board support.

Possible causes:

• Defect squeegee.

• Defect stencil.

Figure A-15 Paste Residues

Figure A-16 Long Smear

Figure A-17 Offset

Figure A-18 Excess Residue Along Direction of Travel

IPC-7527 May 2012

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Possible causes:

• Board support.

• PCB.

• Stencil/print pressure.

A.7 Issues with Cleaning Misprinted PCBs Misprinted

PCBs that are rejected by a visual inspection system

(manual inspection, AOI, SPI) require cleaning. Manual

cleaning will often result in solder paste being wiped over

the PCB leaving solder balls (Figures A-20 and A-21) on

the PCB surface and in via holes which can result in a short

circuit.

It is often more difficult to remove solder paste and par-

ticles than cleanable ﬂux residues. The most effective

method for removing solder paste is a combination of

mechanical and chemical cleaning.

Because solder balls are held in place by the ﬂux, an inte-

grated cleaning process will be required to ﬁrst loosen the

solder balls, followed by a rinsing process to remove them.

The reader is directed to IPC-7526, Stencil and Misprinted

Board Cleaning Handbook for more information on clean-

ing misprinted boards.

It may not be possible to clean a PCB so it will be in new

condition!

A.8 Issues with Stencil Cleaning

A.8.1 Under-Screen Cleaning

Reliable solder paste

printing for ﬁne and ultra-ﬁne pitch components places

high demands on the materials and equipment in the print-

ing process. Stencils for ﬁne pitch printing require a higher

frequency of cleaning to minimize smearing and ensure a

precise placement of the deposited solder paste. Automatic

stencil cleaning systems are normally integrated in the

screen printer that use solvents and lint free paper can be

programmed to remove solder paste residues from the bot-

tom side of the stencil.

A.8.2 Manual Cleaning of Stencil The disadvantages

and problems connected with manual cleaning are normally

greater than the advantages. Cleaning stencils is considered

as the most critical process. Manual cleaning of stencils is

normally conducted at the expense of the cleanliness of the

stencil aperture. There will often be solder balls in the

stencil apertures. Effective manual cleaning of the stencil

apertures using tools can be difficult and lead to damaging

the stencil.

Compressed air that is used to blow the apertures clean will

cause damage to fragile areas between ﬁne pitch apertures.

High pressure water cleaning may cause damage to sensi-

tive areas. Compressed air also can spread solder paste to

other surfaces or operators. Solder paste that has dried in

the stencil aperture can harden and can be very difficult to

remove.

Dried solder paste is the most common cause for blocking

of the stencil aperture affecting an insufficient paste

deposit. An automatic stencil washing system will normally

Figure A-19 Residue Across Direction of Travel

Figure A-20 Solder Balls

Figure A-21 Close Up of Solder Balls

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reduce these dangers and ensure a more consistent and pre-

dictable result. However, this is under the condition that the

cleaning process is monitored and maintained.

When using certain types of stencil cleaning systems, it

may be necessary to pre-clean the stencil. This step can be

conducted while the stencil is in the screen printer or

immediately before the stencil is placed in the cleaning

system. Normally it is possible to remove excessive solder

paste by using a squeegee or spatula.

When the material for wiping the stencil is to be chosen, it

is important that the material is designed for cleaning sten-

cils and will not release ﬁbers, lint, or glue residues onto

the stencil. These pollutants can have a negative inﬂuence

on the subsequent screen printing process.

If the cleaning process requires a pre cleaning with chemi-

cals, a solvent should be chosen with the following proper-

ties:

• Nontoxic.

• Dries without leaving residues or odors.

• Dissolve solder paste.

• Complies with the solder paste manufacturer recommen-

dations.

The recommendations in the safety data sheet should

always be adhered to when cleaning!

A.9 Recommendations

A.9.1 Test Prints

A test print is as a visual check of the

print. A non-production board can be used to avoid

repeated cleaning of a production board.

According to the complexity and volume of the product,

the determination will need to be made as to the value of

test printing.

After printing of the test board, the operator can visually

verify alignment and deposit appearance. If all parameters

are met, production can begin.

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