IPC-CM-770D-1996 - 第162页

January

1996

IPC-CM-770

D.

Thermal Parting:

Use thermal parting device with con-

trolled heating and without a cutting edge to determine

whether the coating can be thermally parted at tempera-

tures at least

50%

below the melting point of solder.

If

the

coating flows or gums up, you are too hot or the coating is

not suitable for thermal parting. Caution40 not exceed

the maximum component storage temperature or other

limitation.

E.

Stripability:

Carefully slit the coating with a sharp

blade in a noncritical area and try to peel back from the

surface to determine if this method is feasible. Due to the

adhesion required of coating materials, strippable tech-

niques without chemical aids is usually very limited.

F.

Thickness:

Determine if the coating is thick or thin by

visual means. Thin coatings show sharp component out-

lines and no fillets while thick coatings reduce sharp com-

ponent outlines and show generous fillets at points of com-

ponent or lead intersection with the printed board. Thick

coatings usually require two step removal methods to pre-

vent surface damage to the board. First reduce the thick

coating down to a thin one and then use pure abrasion

methods to reach the surface of the board. The specific

coating to be removed may have one or more of these

characteristics and consequently the removal method

selected should consider the composite characteristics.

30.4.3 Coating Removal

The surface of printed boards

and printed board assemblies needs to be prepared and

conditioned to provide a stable base for the next step of

modification and repair procedures.

Give consideration to procedures for reducing moisture

absorption and surface preparation to promote the adhesive

characteristics of the board surfaces.

Adhesive promotion procedures should be prepared by

cleaning and coating removal as mentioned in

30.5.1.

Conditioning for moisture absorption (baking) should be

performed prior to any major resoldering operation to pre-

vent blistering, measling or other laminate degradation.

The baking (drying out) procedures must be carefully

selected to insure that temperature and time cycles

employed do not degrade the product in work. Environ-

mental conditions must also be carefully considered to

insure that vapors, gases, etc. generated during the heating

process do not contaminate the product’s surfaces.

30.4.4 Legends and Markings

Modifications and repairs

may be needed on printed board structures (and assem-

blies) for legenddmarkings. Legenddmarkings can be

added, changed or replaced in any one of several ways.

The modification/repair methods include ink stamping,

handtemplate lettering, inkhpray stenciling or stick-on

labels, etc.

In all cases, though, several considerations should be given

to the new legendmarking:

The color should be selected for maximum contrast and

legibility.

The location of the new labeling/marking should be at

least spot cleaned to remove surface contaminants on the

printed board structure (and assembly) surface.

The location of the new legendmarking should be spot

abraded to improve adhesion of the legendmarking to the

printed board structure (and assembly) surface.

The labeldmarking inks, paints, or stick on label material

should be electrically nonconductive, otherwise consider-

ation should be given for electrical conductive pattern

spacing.

Stick-on labels should not be located over multiple con-

ductive patterns, unless precautions are used to eliminate

moisture traps between conductive patterns and the label.

30.5 Printed Board Structure ModificationlRepair.

There are several modification and repair procedures in

IPC-R-700 that can be used with printed board structure

base materials, conductive patterns, printed contacts and

printed boards and used with printed board assemblies.

Prior to performing these modifications or repairs, the

printed board structure (or printed board assembly) should

be prepared for these procedures by cleaning to remove

surface contaminants and removing coatings.

30.6 Printed Board Assembly ModificationlRepair Meth-

ods

The general modification and repair procedures for

printed board assembly focus on the removal and replace-

ment of through board or surface mounted components,

and the addition of jumper (hay) wires on the printed board

structure. Prior to performing these methods, printed board

assemblies may need to be cleaned, conformal coatings

may need to be removed, and bolt-on hardware or compo-

nents may need to be removed prior to component

removal/replacement or the addition of jumper wires. After

the components are removedreplaced (or jumper wires are

added), the discrete wiring or printed board assembly may

need to be cleaned, and the conformal coating should be

replaced (if required).

30.6.1 Removal and Replacement of Components

30.6.1.1 General Requirements

The following are the

general requirements for the removal and replacement

(R&R) of through-hole mount conventional (components,

dual-inline backs, and pin-grid arrays) parts and surface

mount (leadless and leaded) components. A controlled pro-

cess is essential for reworking and repairing modem-day

electronic assemblies. The process should allow for the

R&R of an individual component within the defined ther-

mal, mechanical and electrical requirements to assure sus-

taining the quality of the original assembly.

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IPC-CM-770

Januaty

1996

While no known universal component R&R equipment (for

all types of printed boards exist, structures and all types of

components) the general R&R controlled process equip-

ment requirements include the following:

A. Controlled application of heat to melt the solder joints

of the component in question without causing thermal dam-

age to the base material, or to any component, or the remelt

of any adjacent solder joints.

B.

Controlled removal of the component after sensing sol-

der melt, to prevent delamination of the lands or destruc-

tion of the plated through holes on the printed board struc-

ture.

C. Preparation of the printed board lands, removal of

residual solder in plated through holes, and pre-tinning of

the component leads or lead surfaces in preparation of

component replacement and resoldering.

D. Controlled positioning of the replacement component

on the printed board structure land or through hole pattern.

E. Reflow soldering of the surface-mount solder joints or

soldering the through-hole mounted component with con-

trolled heating.

These generalized component R&R requirements are avail-

able in equipment that has been developed for a specific

component mounting technology.

30.6.1.2 Component Removal Methods

This section

discusses various methods for removing solder, through-

hole mounted components, and surface mounted devices

(from printed board assemblies). Printed board assemblies

require refined component and solder removal/- replace-

ment methods. A controlled process is essential for the

removal and replacement of both through-hole and surface

mounted components. The component/solder removal/

replacement process should operate within defined thermal,

mechanical and electrical limits. There are several factors

that need to be considered in selecting an appropriate

componentholder removal method:

A. Analyze the task to be performed.

B.

Determine any constraints on removing the compo-

nent(s).

Can you save the component and sacrifice the printed

board assembly? Or, do you need to save both the compo-

nent and the printed board assembly?

Can you sacrifice the component and save the printed

board assembly? Or, do you need to save both the compo-

nent and the printed board assembly?

C. Determine if the printed board assembly needs to have

surface contaminations and conformal coatings removed

prior to solder/component removal.

D. Examine all of the solder joints to determine if they

have the same thermal mass and surface configurations.

E. Determine the relative proximity of the solder/

component to be removed to other solder joints and com-

ponents in order to eliminate the following:

possible thermal damage or stress to the other solder

joints and components, or

solder reflow of the other solder joints.

F.

Visually determine if the component to be removed is

adhesive bonded to the printed board assemblies surface by

residual conformal coatings or staking adhesives.

The goal for successful solder joint and component

removal is to get in and get out as rapidly as possible (try

to be in and out in less than

3-5

seconds). Do not push,

apply pressure or impart any mechanical movement to the

printed board structure’s conductive patterns with any heat

source.

30.6.2 Heat Factors

The next step, after analyzing the

task and preparing the printed board structure’s surfaces for

removing the solder or the component, should be to deter-

mine the thermal characteristics of the solder joint’s heat

factors.

The removal of solder or components from a printed board

structure depends on remelting solder in solder joints. The

most critical aspects of solder or component removal from

printed board assemblies are the application and control of

the applied heat, the selection of a suitable method, skilled

and qualified personnel, and good control of the machine/

tool process.

Excessive heat can damage the component or the printed

board structure. Excessive heat can destroy the adhesive

bond of the resin system to the conductive patterns which

delamination consists of the lands, conductors and plated

through holes in the base material.

Insufficient heat will not adequately or rapidly melt the

solder, thereby affecting solder or component removal.

Conductive patterns (lands, conductors and plated-through

holes) can be pulled off the printed board structure with the

component by not adequately melting the solder in all of

the solder joint(s).

Several heat considerations affect the selection of an opti-

mum solder or component removal method. The amount of

heat needed to remelt a solder joint (in order to remove the

solder from a solder joint or to remove a component from

a printed board assembly) is critical to the solder/

component removal process. There are several heat factors

that determine the amount of heat coupled to the solder

joint:

The mass of the solder joint.

The mass and thermal characteristic of the heat source.

The thermal coupling between the solder joint and the

heat source.

Thermal mass and other thermal characteristics of the sol-

der joint determine the amount of heat needed to remelt the

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Licensed by Information Handling Services

January

1996

IPC-CM-770

solder joint. The mass of the solder joint and the cooling

sink characteristics of the base material, coupled with inter-

nally connected thermally conductive loads, can signifi-

cantly affect the amount of heat required to remelt the sol-

der joint. Each solder joint can have a different thermal

mass due to the size of the lands, size/location/number of

interconnection conductor patterns, number of layers, com-

ponent lead configuration (leaded or leadless), power,

ground planes, etc. Based on industry experience, the sol-

der should be removed from a solder joint or most compo-

nent should be removed from the printed board assembly in

less than

3-5

seconds.

The thermal coupling between the heat source and the sol-

der joint depends on the surface conditions of the area of

contact between the heat source and the solder joint. Any

conformal coatings (or residues) or surface contaminants

(oxidized surfaces, old flux residues, dust, dirt, etc.) can bar

the flow of heat from the heat source to the solder joint.

Thermal coupling between the heat source and the solder

joint can be optimized by cleaning the solder joint surfaces

by a light abrasive brushing (oxide removal), applying a

thin coating of a liquid flux, then clean the heat source, and

perform the solder component removal operation.

A thermal link between the heat source and the solder

joint(s) can significantly increase the transfer of heat to the

solder joint(s). The effective variations in solder joint ther-

mal mass can be significantly reduced by having good ther-

mal linkage between the solder joint(s) and an adequate

heat source. A sufficient solder fillet on the solder joints

(normally termed excess solder) combined with tinning the

heat source with fresh solder just prior to solder or compo-

nent removal will improve heat transfer by optimizing the

thermal linkage between the solder joint(s) and the heat

source. Frequently it is desirable to add solder to the solder

joints prior to solder or component removal in order to

maximize thermal linkage.

30.6.3 Through-Hole Mounted Components

There are

two primary methodologies for removing through-hole

mounted components from printed board assemblies, each

of which has its own advantages and limitations.

30.6.3.1 Solder Removal

This methodology is based on

the ability to remove all of the solder from each solder joint

and simultaneously prevent the component lead and plated

through-hole wall (or land) from forming a sweat solder

joint while the residual surface solder is in the molten state.

After all the solder is removed from all solder joints and all

the component leads are free in the through-holes, then the

component can be readily removed from the printed board

assembly. Next the area is generally cleaned of residual

contamination then the replacement component is inserted

and soldered into place.

Note: This solder removal methodology has a built-in capa-

bility to alert the operator and prevent damage to the

printed board assembly during component removal. When

all the solder has been removed from all of the solder joints

and all of the component leads are “free” in the holes, if

the component cannot freely be removed from the printed

board assembly, then the repair person should check to

make sure that the component is not adhesive bonded to

the surface of the printed board structure.

Comment: The preferred solder removal method is for the

operator to remove the solder from each solder joint, one

at a time. This method has several advantages:

It allows a skilled and trained operator to compensate the

solder removal procedure for variations in thermal mass

and thermal lineages for each solder joint.

It allows the operator to skip around, removing solder on

non-adjacent solder joints, thereby reducing thermal

build-up in the printed board structure.

It allows the operator to use auxiliary heat sources or

combined solder removal methods to optimize the solder

removal process.

It allows the operator to stop the solder removal in mid

process if solder removal is not being removed as

planned.

30.6.3.2 Component Removal

The component removal

methodology relies on the ability to simultaneously melt all

the solder joints and then remove (pull) the component out

of the printed board assembly’s component mounting

holes. After the component has been removed from the

printed board structure there are several different subse-

quent processes depending on the capabilities of the com-

ponent removal equipment being used to remove the com-

ponent.

With some equipment, the replacement component is

immediately inserted into the printed board structure while

the solder in the holes is still molten. The heat source is

removed from the printed board structure and the solder

joints are allowed to cool.

With other equipment, the solder is removed from the holes

while the solder is still molten and with other equipment,

the solder is allowed to cool and is subsequently removed

at a later time prior to inserting the replacement

component.

Comment: Several factors need to be determined prior to

using through-hole component removal methods. The

printed board structure can be significantly damaged if the

through-hole component removal process limitations are

not defined and the process is not under control.

The major concern for through-hole component removal

methods is the variation of solder thermal mass and ther-

mal linkage.

Another major concern is the ability of the operator or the

equipment to determine when all of the solder joints are

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Licensed by Information Handling Services