IPC-CM-770D-1996 - 第163页

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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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

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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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

molten and if the component can be removed without

damage to the component, lands, conductor patterns or

plated-through holes.

Adhesive bonding of the component to the printed board

structure’s surface, can significantly increase the risk of

damaging the printed board structure using component

removal methods.

After the component has been removed the replacement

component can be inserted while the solder joints are still

molten, or the remaining solder must be removed from

the component mounting holes by a second operation.

With some equipment, the printed board structure is

maintained at soldering temperature for a longer period of

time in order to remove the solder from the holes using

an auxiliary part of the component removal equipment.

With other equipment, the printed board structure is

allowed to cool and the solder is removed using a sepa-

rate process.

The replacement component is then placed in position

and soldered in place.

Pin grid array component removal presents serious prob-

lems. This is due to the cooling sink action of the compo-

nent itself, the number of component leads, and the possi-

bility of blind hole solder joints. Pin grid arrays can be

removed, with various degrees of success, from printed

board assemblies using solder extraction with vacuum fol-

lowed by pressure component removal methods.

30.6.4 Surface Mounted Devices

A controlled process

for the Removal and Replacement (R&R) of surface

mounted devices (i.e., leadless, short leaded and long

leaded) is essential in repairing modern day electronic

assemblies. The process should allow for the R&R of an

individual component within the defined thermal, mechani-

cal and electrical requirements to assure sustaining the

quality of the original assembly.

A controlled surface mount removal and replacement pro-

cess requirements include:

Controlled application of heat to melt solder joints of the

SMD in question without causing the overheat of the base

material, or of any component, or the remelt of any adja-

cent solder joints.

Controlled lift

off

of the component after sensing solder

melt and to prevent delamination of the lands from the

printed board structure.

Preparation of the printed board lands and pre-tinning of

the SMD prior to replacement soldering.

Controlled positioning of the component on the P&I

structure land pattern.

Reflow soldering of the SMD solder joints with the con-

trolled heating.

30.6.4.1 Heating Methods

There are various methods

and devices available for removing and/or replacing

surface-mounted components. However, many of them

have specific limitations and must be used with appropriate

caution. Some of the heating methods that have been uti-

lized, attempted or proposed include Hot Air (or gas),

Vapor Phase, Infra-Red (IR), Hot Gripper, Hot Plate, Ther-

mal Tweezers, and a few other heat transfer methods.

While each of these methods can be made to work under

certain conditions, other equipment or methods may be

more suitable for the R&R task at hand. The following is a

description of some of the component removal and/or

replace methods and some of the cautions or other consid-

erations for each of the selected methods:

A. Combined Removal

&

Replacement

Surface mount

R&R systems should have a controlled process of concen-

trated selective heating, controlled lift

off,

and accurate

positioning.

Equipment has been developed that meets these general

requirements and provides various degrees of control,

capability and sophistication for the R&R of these compo-

nents (see Figure

30-2

and

30-3).

Commercially available

surface-mount R&R equipment provides varying degrees

of control of the flow of the hot aidgas around the compo-

nent to be removed and replaced, and to minimize (elimi-

nate) the application of heat to other components in the

local area.

Figure 30-2

R&R

System Having a Very High Degree of

Airlgas Flow Control

B.

Component Removal.

1.

Hot Air

-

Heat guns with controlled heating, air flow

and shaped orifices are applied to one or both sides of

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services