IPC-7526-2007 模板和误印板清洗指南.pdf - 第11页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ of solder paste to the PCA lands. Stencil cleaning must remove all solder particles and ﬂux vehicle from stencil apertures without damaging the stencil, bonding adhesive, or elastomer frame. The degree of r…

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2 BIBLIOGRAPHY OF DOCUMENTS, SPECIFICATIONS

AND REFERENCES

The following documents are provided as possible sources

of additional information.

2.1 IPC

IPC-SC-60 Post Solder Solvent Cleaning Handbook

IPC-SA-61 Post Solder Semiaqueous Cleaning Handbook

IPC-AC-62 Post Solder Aqueous Cleaning Handbook

IPC-CH-65 Guidelines for Cleaning of Printed Boards and

Assemblies

IPC-CA-82 General Requirements for Thermally Conduc-

tive Adhesives

IPC-A-610 Acceptability of Electronic Assemblies

IPC-3406 Guidelines for Electrically Conductive Surface

Mount Adhesives

IPC-3408 General Requirements for Anisotropically Con-

ductive Adhesive Films

IPC-7525 Stencil Design Guidelines

2.2 Joint Industry Standards

J-STD-001 Requirements for Soldered Electrical and Elec-

tronic Assemblies

J-STD-005 Requirements for Soldering Pastes

2.3 American Standards for Testing Materials

2.4 Federal Laws and Standards

CAA Clean Air Act

CWA Clean Water Act

CERCLA Comprehensive Environmental Response, Com-

pensation and Liability Act

RCRA Resource Conservation and Recovery Act

SARA Superfund Amendment and Reauthorization Act

2.5 Department of Defense

MIL-C-85447 Cleaning Compounds, Electrical & Elec-

tronic Components

2.6 Occupational Safety and Health Administration

OSHA 29 CFR 1910.106 Flammable and Combustible Liq-

uids

OSHA 29 CFR 1910.134 Respiratory Protection

OSHA 29 CFR 1910.1000 Air Contaminants

2.7 Environmental Protection Agency (EPA)

EPA 40 CRF 63 National Emission Standards for Hazard-

ous Air Pollutants for Source Categories

EPA 40 CFR 82 Protection of Stratospheric Ozone

EPA-453/R-94-081 Guidance Document for the Haloge-

nated Solvent Cleaner NESHAP

EPA 40 CFR 117. Determination of Reportable Quantities

for Hazardous Substances

EPA 40 CFR 131.36 Toxic Criteria for those States not

Complying with Clean Water Act Section 303(c)(2)(B)

EPA 40 CFR 261 Identiﬁcation and Listing of Hazardous

Waste

EPA 40 CFR 302.4 Designation of Hazardous Substances

EPA 40 CFR 355.30 (b) Emergency Release Notiﬁcation

EPA 40 CFR 370 Hazardous Chemical Reporting: Com-

munity Right To Know

EPA 40 CFR 372 Toxic Chemical Release Reporting:

Community Right-to-Know

2.8 Department of Transportation

DOT 33 CFR 153.203 Procedure for the Notice of Dis-

charge

2.9 National Fire Protection Association (NFPA)

NFPA 35 Deﬁnitions of Flammable & Combustible Sub-

stances

2.9.1 Air Quality Management Standards (AQMD)

3 STENCIL CLEANING PROCESS

3.1 Objectives for the Cleaning Process

Stencil cleanli-

ness is an essential issue for delivering the proper amount

1. www.ipc.org

2. www.ipc.org

3. www.astm.org

4. ecfr.gpoaccess.gov

5. www.answers.com/topic/united-states-department-of-defense

6. www.osha.gov/

7. www.osha.gov/

8. www.dot.gov/

9. www.nfpa.org/

IPC-7526 February 2007

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of solder paste to the PCA lands. Stencil cleaning must

remove all solder particles and ﬂux vehicle from stencil

apertures without damaging the stencil, bonding adhesive,

or elastomer frame.

The degree of required cleanliness varies with complexity

of the board design. Stencils are usually cleaned to a visu-

ally clean condition. However, cleaning misprinted circuit

assemblies requires the removal of uncured solder paste

and ionic contaminants that could interfere with wetting

and bonding. For assemblies, a visually clean appearance

provides a satisfactory cosmetic condition, but this condi-

tion does not necessarily assure product performance.

Semiquantitative and qualitative ionic contamination test-

ing provides cleaning veriﬁcation that may be important for

long term ﬁeld reliability. While cleaning to deﬁned

requirements is the primary objective, other objectives

must also be set and achieved. The cleaning process must

not damage the parts being cleaned and cleaning must be

accomplished in a practical and cost effective way. The

process employed must be safe to operators and environ-

mentally compatible.

State of the art stencil cleaning processes integrate

mechanical and chemical cleaning forces. Cleaning chem-

istry suppliers often work closely with cleaning equipment

manufacturers to provide an integrated process. Stencil

cleaning faces the challenge of removing solder paste and

SMT adhesive from tiny apertures while not damaging the

thin stencil foil. To achieve reproducibility in cleaning, the

process requires a mild chemistry integrated with stencil

cleaning equipment that provides mechanical scrubbing

action and exceptional residue removal. Further, there must

be a way to clarify the cleaning chemicals by extracting

and isolating the adhesive or solder solids and ﬂux materi-

als.

In summary, the stencil cleaning objectives are the removal

of contaminants in the form of nonreﬂowed solder paste,

ﬂux residues, uncured adhesives, and other process resi-

dues. Process engineers are seeking robust processes that

provide practical, cost effective, safe, and environmentally

friendly methods.

3.2 Substrate IPC-7525 Stencil Design Guidelines docu-

ments the design and fabrication of stencils for printing

solder paste and surface-mount adhesive. The fabrication

of stencils combines various metal alloys (stainless steel,

copper, aluminum, nickel) as well as plastics in their con-

struction. Frames are typically aluminum with the mesh

border permanently mounted using an adhesive. The mesh

material holding the stencil to the frame is typically poly-

ester ﬁber. Frames may be tubular or cast aluminum with

the border permanently mounted using adhesive.

Cleaning chemistry and temperature, used in the process,

may affect stencil compatibility. Cleaning chemistry

designs must consider stencil construction materials and

address compatibility constraints. Alkaline cleaning agents

may chemically react with metals such as aluminum, caus-

ing the surface to tarnish or darken over time. Elevated

cleaning temperatures, in excess of 110°F, may result in the

interaction of the cleaning chemistry with the adhesive

bond to the frame, causing the stencil to loosen or break

from the frame. Compatibility concerns must be considered

when selecting a cleaning chemistry.

3.3 Tools Squeegees, spatulas and solder paste pots are

some of the many tools that require cleaning. Similar to

compatibility issues discussed for stencils, the cleaning

chemistry must be compatible with tools being cleaned.

3.4 Pallets Some assemblers also use their stencil clean-

ing process for removing ﬂux residue build up on pallets.

Pallets passed over a solder wave see many process cycles

before cleaning. This requirement calls for a cleaning

chemistry that exhibits enhanced effectiveness for remov-

ing baked-on ﬂux residues.

4 MISPRINTED CIRCUIT BOARD CLEANING

To reduce problems such as those shown in Figures 1-1

through 1-10, misprinted solder paste or adhesive and

related contaminans must be totally removed from the

board surface.

4.1 Solder Paste PCBs that are misprinted and rejected

by vision inspection systems require cleaning of nonre-

ﬂowed solder paste or adhesive. Hand wiping is not effec-

tive due to solder ball smearing over the surface of the cir-

cuit assembly. Tiny solder balls may end up in vias or other

small spaces and lead to shorts.

Solder paste and particulate removal is often more difficult

than dissolving uncured ﬂux residue. The most reliable

removal method for removing solder paste is a process that

integrates mechanical and chemical driving forces. Solder

balls are held in place by the ﬂux composition. Developing

an integrated cleaning process, releases the solder balls and

allows removal during the wash and rinse cycles.

Solder balls are collected in the cleaning chamber over

time. Most cleaning machines use ﬁltration methods to pre-

vent solder balls from being picked up by the pump and

resprayed onto the board. Typically, the equipment-

operating manuals provide maintenance procedures to

remove uncured paste from the wash holding tank and ﬁl-

ters.

Double-sided surface mount assemblies are printed, popu-

lated and reﬂowed on one side; then ﬂipped, printed, popu-

lated and reﬂowed on the other-side. From a cleaning per-

spective, the scenario that is commonly overlooked is

reﬂowed ﬂux residue on the second-side of the misprinted

board. In such a scenario, the cleaning process must be

capabile of removing both cured and uncured ﬂux residues.

February 2007 IPC-7526

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This issue changes the scope of the cleaning process and

cleaning chemistry selection. In manufacturing operations

that have this requirement, it may be necessary to develop

a new cleaning process that optimizes the static and

dynamic cleaning forces.

4.2 Adhesives Cleaning uncured SMT adhesive from

stencils and boards can also be accomplished in an auto-

mated cleaning process. Many aqueous cleaning chemis-

tries are not designed for removing SMT adhesives. When

removal of SMT adhesives is required, selection of a clean-

ing chemistry will be necessary to meet this challenge.

4.3 PCB Cleaning Process Considerations Misprint

Board cleaning requires a number of process consider-

ations. The following list comprises factors for consider-

ation when engineering the process for cleaning misprinted

boards.

• When using a spray-in air cleaning system, both sides of

the board need to be cleaned simultaneously.

• Spray-in-air systems should be equipped with a ﬁlter in

the recirculation wash to remove dislodged particles from

the cleaning ﬂuid.

• A clean water rinse prevents wash contamination from

redepositing on the surface of the board.

• Rinse pressure and duration are important process consid-

erations when removing the cleaning chemistry from

under components.

• The drying cycle when cleaning a misprint from the sec-

ond side of a double-sided populated assembly may need

to be lengthened.

• Positioning of the PCB is important when cleaning a mis-

print board with high impingement air spray. An adjust-

able board holder that secures the board in place and

maintains the correct impingement angle for particle

removal facilitates this process.

• Cleaning systems that use ultrasonic technology should

have the PCBs oriented with unreﬂowed solder paste side

down to allow gravity to carry the solder paste away.

• A separate ultrasonic DI water rinse may be necessary

when cleaning boards.

• Studies by the Electronics Manufacturing Productivity

Facility (www.empf.org) suggest an ultrasonic frequency

of 40 kHz or higher is effective and should alternate or

‘‘sweep’’ ± 3 kHz.

• Sweep technology eliminates hot spots or focused ultra-

sonic energy in the wash bath.

• The lower the ultrasonic frequency, the more aggressive

the scrubbing action (cavitation).

• The power density is the amount of electrical energy

delivered to the bath via the ultrasonic generators. This

energy is measured in watts per liter of wash solution.

For example, if the wash bath contained 100 liters and

the generator is rated at 1000 watts, the power density

would be 10 watts/liter. The same studies indicate that

the power density should be around 10 watts per liter or

less.

• Table 4-1 lists process recommendations when cleaning

different types of contaminants from misprinted boards.

Table 4-1 Process Recommendations for Misprinted PCBs with Different Types of Contaminants

Contaminant Source Nature of Contaminants Cleaning Process Steps

Adhesive Dispense Misprinted, Uncured Adhesive Do not scrape the adhesive - Remove all compo-

nents carefully if populated and not reﬂowed

Make sure no adhesive residues are present on

lands or in the via holes

Send the board through the cleaning system with

appropriate cleaning chemistry

Adhesive Curing Populated with components and cured The board is not cleanable

Paste Printing Wet or dry misprinted solder paste - no compo-

nents placed

Wet or dry misprinted solder paste - components

placed

Wet or dry solder paste + uncured adhesive with

populated or unpopulated boards

Do not scrape wet solder paste or placed compo-

nents from the boards. If parts have been placed,

pick off carefully.

Brush with appropriate cleaning chemistry to dis-

solve the adhesive and/or solder paste and send

the board through the cleaning system

Clean and Inspect per IPC J-STD-001

Printing, Placement

and Reﬂow Soldering

Excessive SMT no clean solder paste ﬂux residue

>20 solder balls per panel or all over the laminate

(>5 mils diameter or 5 solder balls/square inch)

<20 Solder balls at a ﬁne pitch SMT component

Micro solder balls (<6 solder balls to a side of a

land) or >20 solder balls in via holes

Clean in the Aqueous/Semiaqueous Wash System

with appropriate Wash Chemistry.

IPC-7526 February 2007

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