IPC-7526-2007 模板和误印板清洗指南.pdf - 第12页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ 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 opti…

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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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5 CONTAMINANT TYPES AND REMOVAL CHARACTER-

ISTICS

5.1 Polar Residues

Materials that form ions when dis-

solved in water are termed ionizable or polar. For example,

when a typical polar residue, ‘‘salt’’ (sodium chloride) in a

ﬁngerprint deposit, dissolves in water, the sodium chloride

molecule dissociates in water into positive sodium ions and

negative chloride ions: (NaCl

=Na

+Cl

). In its ionized

form, sodium chloride will increase the electrical conduc-

tivity of water. The more ions formed the higher the elec-

trical conductivity of the water. Ionic residues cause signal

changes in electrical circuitry and may initiate electromi-

gration and corrosion. Highly ionic contamination over the

board surface may reduce coating adhesion.

As polar residues are soluble in water, they can be removed

by a plain water wash. However, polar residues are often

embedded in water-insoluble deposits. A good example is a

ﬁngerprint residue, where salt and perhaps some water-

soluble amino acids are covered by skin oil. As water can-

not wet the composite residue and solubilize the oil ﬁlm,

additives must be used to remove the oil ﬁlm and allow

dissolution of the embedded polar materials.

Typical Polar Residues:

• Plating and etching materials

• Chemicals from the substrate or component fabrication

process

• Water soluble soldermask constituents

• Deposits from manual handling

• Water soluble solder ﬂux constituents

• Activators from rosin or SA type solder ﬂux

5.2 Nonpolar Water Soluble Residues Organic materi-

als that are water soluble but do not ionize in water are

capable of interfering with wetting and bonding of confor-

mal coatings when cleaning misprinted circuit boards. If

the nonpolar materials are hygroscopic, formation of sur-

face water ﬁlms can be expected with a resulting decrease

in surface resistivity and, under favorable conditions, elec-

tromigration may occur.

Polyglycols are water-soluble but nonionizable. They are

widely used in water-soluble ﬂux formulations and in wave

oils. The degree of solubility of polyglycols varies with

compound type used.

5.3 Nonpolar Water Insoluble Residues Rosin, no-clean

resins, and SMT adhesives are common examples of water

insoluble residues. The presence of these residues can

interfere with wetting, bonding and coating operations, as

both wetting of surfaces and bond development will be

adversely affected when cleaning misprinted circuit boards.

Typical Nonionic Water Insoluble Residues

• Rosin

• Synthetic resin

• Organic compounds from low residue/no-clean ﬂux

formulations

• Plasticizers from core ﬂux

• Greases and oils

• Finger print oils

• Release agents on components

• Insoluble inorganic compounds (oxidation products)

• Rheological additives to solder pastes

• Improperly cleaned ﬂux residue

5.4 Nonreflowed Solder Paste Fluxes used for solder

paste comprise resins, activators, solvents, and rheological

additives. For special systems, additives such as tackiﬁers,

surfactants, and corrosion inhibitors may also be used. Res-

ins are organic materials compounded with medium and

high molecular weight rosin, synthetic materials, and poly-

mers. Activators, being acidic, boost ﬂuxing activity and

are easily removed by the cleaning agent. Oxygenated sol-

vents are used to give the solder paste a maneuverable

homogeneous ﬂuid form. Rheological additives increase

wetting, spreading and tack life of the solder paste. Fluxes

are categorized as water-soluble, rosin, no-clean and syn-

thetic. The cleaning chemistry must dissolve the ﬂux com-

position to allow the solder balls to break up and remove

from the stencil aperture.

5.5 Reflowed Flux Residue Reﬂowed ﬂux residues are

more difficult to clean than nonreﬂowed solder paste.

Water-soluble ﬂux residue is the easiest residue to clean

and is easily removed with most cleaning chemistries used

in the stencil cleaning process. Rosin and no-clean

reﬂowed ﬂux residues require engineered cleaning chemis-

tries speciﬁcally designed to remove baked-on ﬂux residue.

SMT assemblers who have this requirement must select a

cleaning chemistry with this application in mind.

5.6 Uncured (Wet) SMT Adhesive SMT adhesives are

engineered with polymers, a thermosetting component to

adhere surface mount components onto SMT boards. SMT

adhesives cure during the reﬂow process. Stencil printed

SMT adhesives must be cleaned from the stencil before the

adhesive cures. This requires a cleaning process that dis-

solves or displaces the adhesive. Since SMT adhesives are

insoluble in water; cleaning agents must be engineered

with ingredients that couple the adhesive with the water-

based cleaning bath. When diluting the cleaning chemistry,

the solvent phase will dissolve or disperse SMT adhesive

and remove the uncured adhesive from the stencil aperture.

The adhesive will separate from the aqueous cleaning solu-

tion and ﬂoat to the surface. Filtration systems are needed

to remove the adhesive from the wash bath.

5.7 Insoluble Residues Removal of residues that are

insoluble in water and organic solvents, and cannot be

February 2007 IPC-7526