IPC-CM-770D-1996.pdf - 第165页

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

6-28

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

'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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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

IPC-I-

Figure 30-7 Small Hot Air Jet Lap Solder Joint

Component Removal

IPC-I-

Figure 30-8 Lap Reflow Soldering Tool Used for

Component Removal

IPC-I-

Figure 30-9 Hot Gripper

-

Hand Force Component

Remover

CAUTION: This method provides some improvement over

the Hot Gripper since it utilizes more sensitive finger tip

gripping, but still depends upon the sensitivity of the indi-

vidual. Like the hot gripper, the thermal tweezers are nor-

mally used to remove chip resistors and capacitors (or

other small two terminal components), and should be lim-

ited to removing multileaded components having less than

twenty leads.

30.6.5 Through-Hole and Surface Mounted Component

Removal

The following are the general methods for the

I

Figure 30-10 Thermal Tweezer -Finger Tip Controlled

Component Remover

removal of components or solder from solder joints on/in

printed board structures. These methods include component

removal and/or replacement of through board (standard

components, dual-inline packs, and pin-grid arrays) and

surface mounted devices.

30.6.5.1 Wicking Method

The wicking method incorpo-

rates either stranded copper wire or a braid saturated with

flux, which is then applied between the solder to be

removed and a hot soldering iron.

CAUTION: The wicking method is only recommended for

surface solder joints and is not recommended for use on

plated through hole solder joints.

30.6.5.2 Impulse Solder Sucker Method

The impulse

solder sucker method employs a single pulse vacuum gen-

erating device utilizing a squeeze bulb or a spring operated

plunger mechanism with a Teflon orifice through which the

vacuum is applied to the solder to remove it from the joint.

CAUTION: This method will not consistently remove sol-

der from plated through holes and frequently the compo-

nent lead will resweat to the plated through hole wall.

30.6.5.3 Soldering Iron SolderlSolder Method

The sol-

dering irodsolder sucker combination method is a hollow

tipped soldering iron combined with an impulse solder

sucker mechanism.

CAUTION: This method will not consistently remove sol-

der from plated through-holes and frequently the compo-

nent lead will resweat to the plated through hole wall.

30.6.5.4 Heat-and-Pull Method

The heat-and-pull meth-

ods essentially involve heating the solder joint(s) by one of

several different methods and pulling the component

lead(s) out of the solder connection.

CAUTION: The heat-and-pull methods must be used with

caution on surface mount components and are generally not

recommended for the component removal of multileaded

components with through board solder joints.

6-30

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services