IPC-7095C-2013.pdf - 第149页

process avoids paste alloy , PCB surface ﬁnish, and compo- nent ﬁnish compatibility issues that can be very complex to balance for successful production results. Reballing of BGA components involves removal of the origin…

Hence, for better solder joint yields and solder joint reli-

ability, the backward compatibility reﬂow soldering proﬁle

depicted in Figure 8-23 should be used. During this reﬂow

proﬁle, the SnAgCu solder ball also melts, and lead from

the molten tin/lead solder paste mixes thoroughly with

molten SnAgCu solder ball and generates a homogeneous

structure of a lead-rich phase in the tin matrix. Such a

microstructure is shown in Figure 8-25.

Moreover, since the SnAgCu solder ball melts and col-

lapses, the self-aligning process and coplanarity reduction

also occur, thereby enhancing solder joint yields of the

BGA.

8.5.5.7 Ball Replacement Due to limited availability of

components with SnPb balls, some functional designs may

require ball replacement of the BGA component to allow

common processing with tin lead production assemblies.

This practice, while commonly employed, has drawbacks

due to the additional thermal cycles required for the ball

removal and subsequent replacement. Additionally, the

resulting interface alloy at the component to ball attach-

ment site may not achieve the desired or expected eutectic

properties. The process, however, does allow the use of

limited component types to comply with tin lead use strat-

egies for performance, contract mandates, and/or compat-

ibility with legacy or production designs that are suscep-

tible to the additional temperature excursion required for

lead free production processes. The sphere replacement

IPC-7095c-8-23

Figure 8-23 Comparison of Reﬂow Soldering Proﬁles for Tin/Lead, Backward Compatibility and Total Lead-Free Board

Assemblies

300

250

200

150

100

150 200 250

300

350 400

Tmelting for SnAgCu

Tmelting for SnPb

SnAgCu Balls do NOT Melt

Not good for backward compatibility

SnAgCu BAlls Melt

Will be OK for backward compatibility

Typical Lead Free Solder (SnAgCu) Profile

Temperature, Deg C

Time, Seconds

Figure 8-24 Micrograph of a cross-section of a BGA

SnAgCu solder ball, assembled onto a board with tin/lead

solder paste using the standard tin/lead reﬂow soldering

proﬁle. The SnAgCu solder ball does not melt; black/grey

interconnecting ﬁngers are lead-rich grain boundaries; rod

shape particles are Ag3Sn IMCs; grey particles are Cu6Sn5

IMCs.

Figure 8-25 Micrograph of a cross-section of a BGA

SnAgCu solder ball, assembled onto a board with tin/lead

solder paste using a backward compatibility reﬂows sol-

dering proﬁle. The SnAgCu solder ball has melted.

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process avoids paste alloy, PCB surface ﬁnish, and compo-

nent ﬁnish compatibility issues that can be very complex to

balance for successful production results.

Reballing of BGA components involves removal of the

original balls with a localized heat source such as a

vacuum desoldering tool, blade tip or solder wick. The

component surface is then cleaned and prepared for attach-

ment of new spheres that are located with a ﬁxture that

aligns to the original pattern of the component. The spheres

are then attached to the component using a typical rework

heat source such as a convection reﬂow oven, or the ﬁxture

with component and replacement spheres can be placed in

a reﬂow oven. As an alternate when many parts are to be

reballed, one can use automated laser systems that will pick

and place balls from a tray and then reﬂow them in place

on the BGA substrate using the laser beam.

In either case, the heat source must be controlled using a

thermal proﬁle appropriate for the component and new ball

alloy. Once the balls are attached, the BGA package is

removed from the ﬁxture and the balls can be examined

and cleaned in preparation for installation and soldering

with the remainder of the assembly components. As with

all BGA component processes, moisture and ESD sensitiv-

ity must be considered. BGA balls are commonly available

in a wide range of solder alloys and dimension but they

must be selected for compatibility with other process ele-

ments such as the board surface plating and solder paste. It

is also critical to maintain the component design spacing

and pitch through proper ball size and placement.

Detailed procedures for BGA Reballing are available in

IPC-7711/21B, ‘‘Rework, Modiﬁcation and Repair of Elec-

tronic Assemblies.’’

8.6 Design for Reliability (DfR) Process As a general

recommendation, the Design for Reliability procedures

detailed in IPC-D-279 should be followed.

Appropriate DfR measures to improve reliability for grid

array components take one of two forms, which are best

employed in combination for improved reliability margins.

These measures are:

1. CTE-tailoring to reduce the global expansion mismatch.

2. Increasing attachment compliancy to accommodate the

global expansion mismatch by increasing the solder

joint height (stand-off).

3. Further, a DfR procedure aiming at high-reliability

could also include:

4. Eliminate the effect of the global expansion mismatch

by mechanically coupling the component and the sub-

strate with an appropriate underﬁll.

5. Choosing a soft die attach to reduce the impact of the

low CTE of the die (2.7 to 2.8 ppm/°C) on both the

global and local thermal expansion mismatch.

CTE-tailoring involves choosing the materials or material

combinations of the multilayer board and/or the compo-

nents to achieve an optimum CTE. An optimum CTE for

active components dissipating power is 1-3 ppm/°C

(depending on the power dissipated) with the multilayer

board having the larger CTE, and 0 ppm/°C for passive

components. Of course, since an assembly has a multitude

of components, full CTE optimization cannot be achieved

for all components—it needs to be for the components with

the largest threat to reliability. For military applications

with the requirement of hermetic and thus ceramic compo-

nents, CTE-tailoring has meant the CTE-constraining of

the multilayer boards with such materials as Kevlar™ and

graphite ﬁbers, or copper-Invar-copper and copper-

molybdenum-copper planes. Such solutions are too expen-

sive for most commercial applications for which glass-

epoxy or glass-polyimide are the materials of choice for the

multilayer boards. Thus, CTE-tailoring has to take the form

of avoiding larger size components that are either ceramic

(CGAs, MCMs), plastic with Alloy 42 lead frames

(TSOPs, SOTs, or plastic with rigid bonded silicon die

(PBGAs).

Increasing attachment compliancy for leadless solder

attachments means increasing the solder joint height (C4,

C5, shimming, gluing, 10Sn90Pb balls, 10Sn90Pb col-

umns) or switching to a leaded attachment technology. For

leaded attachments, increasing lead compliancy can mean

changing component suppliers to those having lead geom-

etries promoting higher lead compliancy or switching to

ﬁne-pitch technology.

The DfR process needs to emphasize a physics-of-failure

perspective without neglecting the statistical distribution of

failures. The process might involve the following steps:

• Identify Reliability Requirements – expected design life

and acceptable cumulative failure probability at the end

of this design life.

• Identify Loading Conditions – use environments (e.g.,

IPC-SM-785) and thermal gradients due to power dissipa-

tion, which may vary and produce large numbers of mini-

cycles (Energy Star).

• Identify/Select Assembly Architecture – part and substrate

selections, material properties (e.g., CTE), and attachment

geometry.

• Assess Reliability – determine reliability potential of the

designed assembly and compare to the reliability require-

ments using the approach shown here, a ‘‘Figure of

Merit’’ approach, or some other suitable technique; this

process may be iterative.

• Balance Performance, Cost and Reliability Requirements.

8.7 Validation and Qualification Tests Performance test

methods and qualiﬁcation requirements are speciﬁed in

IPC-9701; IPC-9701A includes guidelines for lead-free

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solder joint reliability testing. There are multiple models

available and people use this to estimate the Pb-free solder

joint reliability, for their products and applications. There is

no universal agreement since these model are product-

speciﬁc.

The validation and qualiﬁcation tests should follow the

guidelines given in IPC-SM-785, Guidelines for Acceler-

ated Reliability Testing of Surface Mount Solder Attach-

ments. However, for large components with signiﬁcant heat

dissipation, for components of asymmetric construction,

and for small global CTE-mismatches, temperature cycling

tests are inadequate to provide the required information;

full functional cycling including external temperature and

internal power cycling is necessary.

8.8 Screening Procedures

8.8.1 Solder Joint Defects

The solder joint defects of

greatest reliability concern are those involving inadequate

wetting for whatever reason. Properly wetted solder joints

have adequate strength even for severe mechanical loading

conditions, without diminishing thermal fatigue resistance.

However, solder joints not properly wetted, can prema-

turely fail both as the result of mechanical and thermal

cyclic loading.

Voids in the solder joints are generally regarded as not

constituting a reliability threat. Possible exceptions are

large voids reducing the solder joint cross-section enough

to reduce a required thermal heat transfer function, and

voids in high-frequency applications where the voids can

cause signal deterioration.

BGA components having noncollapsible balls (high tem-

perature solder 90% Pb 10% Sn, with a melting point of

302°C) typically will have few or no induced voids

because the ball solder never melts during the reﬂow pro-

ﬁle.

8.8.2 Screening Recommendations Effective screening

procedures need to be capable of causing the failure of

latent solder joint defects, i.e., weak inadequately wetted

solder joints, without causing signiﬁcant damage to high

quality solder joints. The best recommendation is random

vibration (6-10 grams for 10-20 minutes), preferably at low

temperature, e.g., -40°C. This loading does not damage

good solder joints, but overstresses weakly bonded ones.

Thermal shock can also be successfully used; however,

some damage to good solder joints can be expected, par-

ticularly for larger components.

8.9 Accelerated Reliability Testing The validation and

qualiﬁcation tests should follow the guidelines given in

IPC-SM-785, Guidelines for Accelerated Reliability Test-

ing of Surface Mount Solder Attachments and/or IPC-9701,

Performance Test Methods and Qualiﬁcation Requirements

for Surface Mount Solder Attachments. Although acceler-

ated temperature cycling (ATC) is often conducted, for

some products, ATC needs to be combined with mechani-

cal shock and/or vibration testing. From a single set of ATC

test conditions or inadequate ATC test conditions may not

provide valid conclusions.

Accelerated reliability testing is carried out on design pro-

totypes, typically to failure or until a predetermined reli-

ability goal is achieved. The appropriate reliability goal can

be determined with an appropriate acceleration model (see

IPC-D-279 Design Guidelines for Reliable Surface Mount

Technology Printed Board Assemblies).

Once failure occurs, the resulting failure modes are ana-

lyzed as to the underlying failure mechanism(s). If it fails

expectations then corrective action is necessary. Either the

assembly process needs to be improved or the product

needs to be redesigned. In either case retesting may be

necessary after the corrective action has been implemented.

Recognizing that a matrix was needed to determine the

exact requirements and the testing necessary for perfor-

mance under various conditions, the IPC Product Reliabil-

ity Committee developed the following table, ‘‘Product

Categories and Use Environments.’’ Table 8-6 attempts to

relate seven product categories by typical application to the

thermal, mechanical, atmospheric, and electrical perfor-

mance requirements that they must meet during typical

manufacturing processes, storage, and during operation.

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