IPC-7095C-2013.pdf - 第130页

diode lasers and can rework both peripheral and array type packages such as BGAs, chip scale packages (CSPs) and ﬂip chips by rapidly scanning top of package surfaces. This causes BGA/CSP/ﬂip chip ball to reﬂow underneat…

then liquid ﬂux or the paste ﬂux are commonly used when

reattaching a BGA, especially BGAs with less than 208

balls. Flux is applied to the land pattern or the BGA solder

spheres and the BGA is placed over it. One drawback of

this method is the coplanarity issue. If the lands are not

perfectly ﬂat, some of the balls may not touch. Excessive

ﬂux application may also cause bridging between solder

balls. With most plastic BGAs the ball alloy will be one

which will reﬂow into the solder joint.

7.9.3.3 Paste Application Paste application is the pre-

ferred method, but it does add time to the rework process

as well as tooling cost. Paste can be applied locally with

the use of mini-stencils. When ordering mini stencils for

ceramic packages, the same aperture/thickness should be

used as used for initial attach. This will ensure the reliabil-

ity of the ceramic packages for reworked components.

These stencils can be purchased from many different sup-

pliers and are tailored to ﬁt speciﬁc land patterns. A ﬁxture

or tape can be used to hold the stencil in place during

application on the land pattern.

When using these methods, considerations have to be made

for solder paste handling and stencil cleaning. Solder paste

can also be applied to the BGA using a syringe or paste

dispensing frame with other proper tooling. The amount of

solder paste applied should be carefully controlled. When

printing solder paste for CSPs, there may not be enough

room on the bare board for mini-stencils. In this situation

it is a common process to screen the solder paste onto the

package (bottom side of the balls) and then place the mod-

ule onto the board for replacement.

7.9.3.4 Rework Issues Interpackage spacing is decreas-

ing constantly. Even if companies have some Design for

Manufacture (DfM) guidelines for interpackage spacing,

those on the front line of manufacturing know very well

that DfM guidelines are not always followed. So, using

mini-stencils to print solder paste is becoming more diffi-

cult. Also, because a mini-stencil is needed for each size

and type of part, it not only slows down the process but

also quickly adds to the cost of repair.

Using a mini-stencil is not the only issue with ever-

decreasing interpackage spacing. Using different hot air

nozzles for each size and type of part being removed also

adds to the cost and complexity of rework. Additionally,

the potential for melting solder joints of neighboring com-

ponents is a serious concern. In addition to increased inter-

metallic thickness because of unnecessary reﬂow which

weakens the solder joints, the boards must be baked before

rework, increasing cycle time.

Throughput in rework is very important. BGAs and some

of the larger components can take at least 20 minutes per

component for removal and replacement.

Another important issue in rework is PCA board warpage.

Warpage is partly due to intense local heating for a rela-

tively long time, which is necessary to remove the compo-

nent.

For BGA repair, there are two rework processes in use

today: hot air and laser. Hot air is the most common. The

new process for removal and replacement of surface mount

components, including BGAs and CSPs, is laser-based.

Multiple rework on the same BGA site can result in barrel

cracking of the printed wiring board. Consideration must

be made for printed wiring board materials and number of

heat cycles the materials can withstand.

7.9.3.5 Hot Air Systems for BGA Repair Hot air systems

are either totally manual or semi-automated. Using a

nozzle, they blow hot air on the part to be reworked. The

part is pulled away from the board when the solder on all

joints is molten. Bottom-side heating is used to uniformly

heat the entire board to a preheat setting prior to applying

topside nozzle heat, thus reducing thermal shock.The hot

air usually is directed on the BGA package by a nozzle

designed speciﬁcally for that component. The package

body is heated by the hot air impinging on the package and

conduction within the package. Initially, the package is

preheated with the nozzle some distance away (typically

25 mm or more) from the package body. Then the nozzle

is lowered to a point just above the package body and lead

temperature increases sharply until it reaches a peak. Dur-

ing this process of blowing hot air, the solder joints of

neighboring components even 12 mm away can reﬂow, an

unwanted and undesirable result. With higher velocity air,

smaller components such as CSPs are prone to movement

during rework.

After the component is removed, paste application for reat-

tachment is a most difficult and time-consuming process.

However, using ﬂux only (liquid or paste form) is only

applicable for eutectic BGAs reattachment. Some applica-

tions require solder addition to promote a robust joint.

Typically, a mini-stencil or dispenser is used to apply the

paste. Both hot air nozzles and ministencils are needed for

each type and size of part being reworked. Both these items

require sufficient interpackage spacing for rework. The

forced convection heating of the bottom side of the board

will minimize the temperature of the nozzle required to

achieve an acceptable lead-free proﬁle. Typically tin/lead

bottom side preheating is approximately 100°C. This tem-

perature should be increased to 130°C minimum for lead-

free products.

7.9.3.6 Laser Systems for BGA Repair The laser sys-

tems use from one to four diode lasers. Some of the laser

systems are limited to reworking only peripheral compo-

nents, in which the leads are in the laser’s line of sight.

However, there are other laser systems that use multiple

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diode lasers and can rework both peripheral and array type

packages such as BGAs, chip scale packages (CSPs) and

ﬂip chips by rapidly scanning top of package surfaces. This

causes BGA/CSP/ﬂip chip ball to reﬂow underneath by

conduction through the package, as is the case in hot air

rework. Some of these laser systems also have a built-in

automated thermal management capability to monitor and

control package temperatures within the speciﬁed limits to

prevent overheating. There are laser systems with or with-

out dispensing and pick and place capabilities.

Because the laser beam is very narrow, components even

1 mm away do not experience any heat. Laser systems heat

the package without melting the solder joints of neighbor-

ing components.

7.9.3.7 Profile Requirements Whether using laser or hot

air, the reﬂow proﬁle for a BGA during rework is the same

as the proﬁle for a convection oven. Preheating the board

to 100°C before initiating the removal or replacement cycle

should be sufficient in keeping board warpage to a mini-

mum. These requirements are summarized in Table 7-9 for

tin lead and 7-10 for lead free. Be careful not to push the

preheat towards 120°C since this is where some ﬂuxes

typically activate. If this happens the ﬂux could be acti-

vated before it is needed and cause poor solderability dur-

ing reﬂow. For lead-free solder pastes, this preheat tem-

perature should be 120°C to 130°C minimum.

Sufficient time should be allowed for the ﬂux to clean the

ball and the land during the reﬂow proﬁle. Flux should stay

within 120-150°C for 30 to 120 seconds. After the ﬂux has

cleaned the site, a ramp rate of 2° to 4°C can be used. The

standard 2°C for SMT proﬁling can be amended since there

should not be any heat sensitive components such as

capacitors or resistors within the reﬂow nozzle when using

hot air. The components that are adjacent to the nozzle

should be shielded with polyimide tape or water soluble

mask to protect components from thermal damage when

using hot air. The reﬂow dwell time should be in the range

of 30 to 90 seconds with the solder joint peaking between

200-220°C (for tin/lead) and 235-245°C for lead free. The

balls at the center of the BGA package may exceed the 90

second recommendation due to entrapped heat from the

reﬂow process. The board temperature should not be kept

above 150°C for any longer than four minutes. This

requirement is due to the glass transition temperature for

FR-4 (see J-STD-020).

Table 7-9 Repair Process Temperature Proﬁles for Tin Lead Assembly

Proﬁle Topic Temperature Range Time Range

Preheat 100-150°C; not to exceed 150°C N/A

Soak or preheat activation 100-180°C* 60-120 seconds*

Component ramp rate 2-4°C per second

Reﬂow dwell Above 183°C 60 to 90 seconds

Solder joint peak 210-220°C Not to exceed 10 seconds

A moisture sensitive component maximum

temperature

225°C Not to exceed 20 seconds

Component maximum temperature 230°C 60 seconds

Maximum adjacent component temperature** 170°C 0 seconds

Board temperature Above 150°C Not to exceed four minutes

* Verify with supplier

** Adjacent component equals 5 mm away

Table 7-10 Repair Process Temperature Proﬁles for Lead-Free Assemblies

Proﬁle Topic Temperature Range Time Range

Preheat 100°C to 190°C; not to exceed 190°C N/A

Soak or preheat activation 140 -220°C* 60 to 150 seconds*

Component ramp rate 2°C to 4°C per second

Reﬂow dwell Above 220°C 60 to 90 seconds

Solder joint peak 230-245°C Not to exceed 20 seconds

A moisture sensitive component maximum

temperature

245°C Not to exceed 20 seconds

Component maximum temperature 245°C 60 seconds

Maximum adjacent component temperature** 210°C 0 seconds

Board temperature Above 190°C Not to exceed four minutes

* Verify with supplier

** Adjacent component equals 5 mm away

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8 RELIABILITY

Reliability is the ability of a product to deliver designated

functions under given conditions, for a speciﬁed period of

time within an acceptable level of conﬁdence. The reliabil-

ity of electronic assemblies requires a deﬁnitive design

effort to be carried out during the developmental phase of

the product to meet requirements. Reliability is deﬁned in

IPC-SM-785.

Short term reliability is threatened by early life failures

generally attributed to insufficient production quality.

These ‘‘infant mortalities’’ can be reduced by appropriate

screening techniques prior to shipping, but not eliminated.

Longer term failures are the result of premature wear-out

damage caused by inadequate design of the assembly. The

design guidelines in IPC-D-279 are a good reference.

Reliability qualiﬁcation requirements are given in IPC-

9701 together with well-deﬁned test methodologies. For

lead-free solder attachments, IPC-9701A contains guide-

lines for accelerated solder joint reliability testing. In the

absence of an agreed-to acceleration model for the various

lead-free solders, reliability requirements based on acceler-

ated thermal cycling is difficult to establish. Most of the

existing models that have been developed are very product-

speciﬁc.

8.1 Reliability Drivers for BGAs When the soft solder

(tin/lead or lead-free) is used as BGA solder joints to con-

nect BGA packages onto the PCB substrate, solder joints

do ‘‘wear-out’’ during their service life. This degradation

process is unavoidable. The objective of solder joint reli-

ability is to make sure that solder joints do not degrade to

the level that they lose their required functionality, be it

electrical, thermal, or mechanical, under a designated ser-

vice life and anticipated service environments.

Solder joint failure, absent other extraneous causes, is often

attributed to thermo-mechanical reliability which occurs

due to fatigue and creep interaction. The solder fatigue/

creep damage is caused by the temperature excursion and

ﬂuctuation as the result of in-circuit functions, power

on-and-off, and/or external environmental temperature

exposure. These temperature changes inevitably generate

cyclic thermal stress, thus cyclic strain on solder joints, in

a thermal expansion coefficient mismatched system during

the service life of electronic packages and assemblies,

causing thermal fatigue (as opposed to mechanical fatigue).

Due to the recent advent of handheld consumer products,

mechanical reliability of the solder joints has become an

added requirement. Shock, transient and cyclic bending of

the board as well as vibration, are the major stress genera-

tion factors that can impair the solder joint functionality.

All these factors are described below.

8.1.1 Cyclic strain Heat is generated from electronic

devices and circuits during the power-on of an electronic

system, following the well-known Joule’s law. The heat

will then be gradually dissipated to external environment

while power is off through the basic heat transfer mecha-

nisms of conduction, convection, and radiation, making the

system reach ambient temperature again. Many such tem-

perature cycles during the service of an electronic system

are expected and inevitable.

Barring other extraneous causes, the solder joint intrinsic

degradation process engages two scientiﬁc phenomena—

fatigue and creep. Other extraneous causes include the

improperly formed solder joints due to wetting problem,

inadequate process issue, surface ﬁnish-induced problem at

or near the interface, such as Au-embrittlement and other

intermetallic issues, etc.

8.1.2 Fatigue Fatigue is a progressive and localized

structural damage (atomic and larger scale levels) which

occurs when the solder joint is subject to cyclic stresses

(loads and unloads). When the stress exceeds a certain

threshold, microscopic cracks will begin to form. This

localized damage mechanism separates fatigue from creep

behavior.

As the cyclic stress continues, a crack will propagate and

eventually reach a critical size, and the solder joint will

fracture. Generally the fatigue process undergoes three

stages: crack initiation, crack propagation and fracture. For

solder joints functioning as the electrical, thermal and

mechanical interconnections in a circuitry, a mechanical

fracture is generally not a practical criterion of failure. The

electrical performance, as measured by resistance increase

due to cracks, is used as a failure criterion. The electrical

failure often proceeds mechanical fracture. When in the

presence of corrosive elements, a corrosion-enhanced

fatigue could also occur.

Fatigue strength, correlating to fatigue life, is deﬁned as the

stress value at which a failure occurs after a given number

of cycles. The fatigue strength depends, not only on the

speciﬁc solder alloy material, the extreme high tempera-

ture, and the extreme low temperature, but also other fac-

tors: surface defects, notches, holes, residual stress, voids,

gas porosity, and inclusions. All of these ‘‘imperfections or

defects’’ could act as the stress concentration sites to initi-

ate the localized damage under the fatigue environment.

This is the reason why defects such as voids or surface

cracks should not be categorically dismissed even when

some test results do not reveal the performance difference

between the presence and absence of such defects.

In the atomic level, the solder joint fatigue mechanism

starts with dislocation movement and then forms a slip

band that nucleate short cracks. Microstructurally, grain

coarsening is often a result as observed under the SEM

examination. Grain size is important to fatigue—the

smaller, the better. Yet when other defects, such as a sur-

face defect exist, a defect rules.

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