IPC-CM-770D-1996.pdf - 第164页

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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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

January

1996

IPC-CM-770

'igure

30-3

Commercially Available Surface Mount

Component

R&R

System

the printed board structure to melt the solder joints, fol-

lowed by manual lift

off

of the component after the sol-

der has melted. Heat guns with a simple fixture mount-

ing (see Figure 30-4 and 30-5) were first used for the

removal of surface-mount components. Later, commer-

cial systems (see Figure 30-6) provide much the same

function but with better air flow controlled by nozzles,

and better control of the printed board structure assem-

bly with board positioning holders.

CAUTION: The hot air is not highly concentrated nor

selectively applied and can cause undesirable overheat-

ing of the components conductive patterns. The printed

board substrate itself can be readily damaged since con-

trol of manual lift

off

depends on human judgment and

sensitivity.

2.

Hot Air Jet

-

Heat is transferred from a hot gas pencil

or a solder extractor by reversing the flow of air through

the tool to locally heat and desolder individual leaded

lap solder joints on components. Caution must be exer-

cised to insure that all residual solder has been removed

from the solder extractor (see Figure 30-7).

3. Lap Reflow Desoldering

-

A lap reflow soldering tool

can locally melt the solder in individual short leaded lap

solder joints (see Figure 30-8).

4. Thermal TweezerdHot Gripper

-

Heat is transferred

from the formed solder joints along the perimeter of the

leadless and short-leaded device. The operator grips the

component to be removed with the hot gripper tool by

hand force, then after the operator senses the solder

joints are molten the hot gripper tool is used to lift

off

Figure

30-4

Single-sided Hot Air Component Remover

IPC-I-

Figure

30-5

Double-sided Hot Component Remover

IPC-I-

Figure

30-6

Commercial Hot Air Component Remover

the component from the printed board surface (see

figures 30-9 and 30-10).

CAUTION: Due to human sensitivity limitation, uncon-

trolled clamping and shearing forces can be applied prior to

solder melt, all resulting in damage to the substrate conduc-

tive pattern.

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