IPC-7095C-2013.pdf - 第25页

thus such terms as ‘ ‘frowning’ ’ or ‘ ‘smiling’ ’ BGAs have been used to identify these conditions. The so-called ‘ ‘smil- ing’ ’ BGA interposer puts a stress on the balls at the outer rows, while the ‘ ‘frowning’ ’ BGA…

BGA issues are not insurmountable; however, they require

dedicated engineering resources to develop and implement

a consistently reliable process.

3.5.1 Visual Inspection BGA is not a package suitable

for companies that assure quality by inspection and repair.

BGA solder joints cannot be inspected unless X-ray or

optical inspection techniques are used. To reap the beneﬁts

that BGA offers, robust process control must be main-

tained. Due to limited time and training, many companies

ﬁnd implementing such tight process control to be a diffi-

cult endeavor.

There are some visual inspections of BGAs that can iden-

tify problems with solder joints. These would show good

ﬂow on an uncollapsed ball with ceramic BGAs, and also

be able to show collapsed balls on plastic BGAs on the

outer rows of the BGA. Visual inspection of the outer rows

serves as an indicator of some of these problems. Examples

are BGA alignment with the lands on the outer rows, and

how the BGA is sitting on the board, level or skewed.

3.5.2 Moisture Sensitivity The plastic BGA packages

are very moisture sensitive. This makes them susceptible to

warpage, swelling, popcorning, or cracking if the packages

are not properly baked and kept dry prior to package

assembly. Component storage and handling procedures are

critical for any moisture sensitive component including

leaded surface mounted devices, but it is critical for BGAs/

FBGAs.

Component moisture sensitivity is tested using J-STD-020

and is determined according to the package thickness. The

moisture sensitivity level must be analyzed for each BGA

package type. It is critical to know at which of the appli-

cable temperatures, 220°C, 235°C, 245°C, 250°C, or

260°C the BGA package was classiﬁed. The classiﬁcation

of the package type may drop several levels if the higher

temperature is used.

Due to improvements in molding compounds and laminate

systems, most laminate-based BGAs can be mounted and

qualiﬁed for temperatures above 220°C. The hermetic

ceramic BGAs are not moisture sensitive and therefore can

be mounted using either of the higher temperatures.

Because of the switch to lead-free solders, the industry

needs to test and verify at a higher temperature such as

260°C, which will create major issues with not only BGAs,

but also all surface mounted devices.

3.5.3 Thermally Unbalanced BGA Design The plastic

BGA package is also susceptible to warpage at which time

the package edges lift up; this could result in no connec-

tions on the outer rows. The edges may also bend down,

Table 3-3 Potential Plating or Component Termination Material Properties

Gold (Au)

Gold (Au), electroplated

Gold (Au), hard

Indium (In)

Nickel/Gold (Ni/Au) ENIG

Nickel/Gold (Ni/Au), electrolytic

Nickel/Gold (Ni/Au),

Nickel/Palladium (Ni/Pd)

Nickel/Palladium/Gold (Ni/Pd/Au)

Nickel/Palladium/Gold (Ni/Pd/Au), ENEPIG

Organic Solderability Preservative (OSP)

Organic Solderability Preservative (OSP-HT), high temp

Palladium (Pd)

Platinum/Palladium/Silver (Pt/Pd/Ag)

Silver (Ag)

Silver (Ag), electroplated

Silver (Ag), immersion

Silver (Ag), with Nickel (Ni) barrier

Silver/Palladium (Ag/Pd))

Silver/Palladium (Ag/Pd), with Nickel (Ni) barrier

Tin (Sn)

Tin (Sn), bright

Tin (Sn), bright, annealed

Tin (Sn), bright, fused

Tin (Sn), bright, reﬂowed

Tin (Sn), bright, reﬂowed over Nickel (Ni) barrier

Tin (Sn), bright, with Nickel (Ni) barrier

Tin (Sn), bright, with Silver (Ag) barrier

Tin (Sn), hot dipped

Tin (Sn), immersion

Tin (Sn), matte

Tin (Sn), matte, annealed

Tin (Sn), matte, fused

Tin (Sn), matte, reﬂowed

Tin (Sn), matte, reﬂowed over Nickel (Ni) barrier

Tin (Sn), matte, with Nickel (Ni) barrier

Tin (Sn), matte, with Silver (Ag) barrier

Tin (Sn), reﬂowed

Tin (Sn), Semi-matte (Sn)

Tin/Bismuth (SnBi), <5% Bi

Tin/Bismuth (SnBi), =>5% Bi

Tin/Bismuth/Gold (Sn/Bi/Au)

Tin/Copper (Sn/Cu)

Tin/Copper (Sn/Cu), annealed

Tin/Copper (Sn/Cu), HASL

Tin/Copper (Sn/Cu), hot dipped

Tin/Copper (Sn/Cu) matte

Tin/Lead (Sn63Pb37)

Tin/Lead (Sn90Pb05)

Tin/Lead/Silver (Sn/Pb/Ag)

Tin/Silver (Sn/Ag)

Tin/Silver (Sn/Ag), hot dipped

Tin/Silver (Sn/Ag), plated

Tin/Silver/Bismuth (Sn/Ag/Bi)

Tin/Silver/Bismuth/Copper (Sn/Ag/Bi/Cu)

Tin/Silver/Copper (Sn/Ag/Cu)

Tin/Silver/Copper (Sn/Ag/Cu), hot dipped

Tin/Zinc (Sn/Zn)

Tin/Zinc/Aluminum (Sn/Zn/Al)

Tin/Zinc/Nickel (Sn/Zn/Ni)

NAC* (Not applicable - Must provide Comment)

IPC-7095C January 2013

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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thus such terms as ‘‘frowning’’ or ‘‘smiling’’ BGAs have

been used to identify these conditions. The so-called ‘‘smil-

ing’’ BGA interposer puts a stress on the balls at the outer

rows, while the ‘‘frowning’’ BGA interposer puts the stress

on the connection of the inner row ball locations. Package

warpage is of real concern in ﬂux-only applications during

rework. Large die sizes can cause CTE mismatch between

the PCB and the package laminate material, which can cre-

ate package warpage (see Figure 3-8).

Thermally unbalanced package designs, particularly those

with heat spreaders on the top, will warp according to the

classic bi-metal effect.

3.5.4 Rework Although BGAs do not require nearly as

much rework as ﬁne pitch lead-frame devices, many

assemblers are apprehensive about using a component

package that is difficult to be reworked. While BGA rework

is difficult, it is by no means impossible. Tools and tech-

niques for rework are currently available that range from

manual to automated techniques to reball the BGA or

redress the land pattern. Several factors must be addressed

during the rework operation. These are:

• Number of heat cycles

• Ball collapse during re-balling

• No damage to pads on BGA interposer

• Proper land redressing and no damage to lands on the

product board

• Appropriate reﬂow temperature for re-attachment based

on alloy used

• Proper cleaning to remove ﬂux residue unless no-clean

ﬂux is used

3.5.5 Cost The BGA still has a slight cost differential

compared to ﬁne pitch peripheral packages that it replaces.

However, competitive pressures keep bringing costs lower

to meet new targets. Further costs accrue with the increased

board layer counts that BGAs require; however, there are

many advantages to the interconnecting concepts and the

performance characteristics resulting from BGA implemen-

tation.

Following are some of the key reasons for higher BGA

package cost:

• Higher cost substrate (ﬁne line/space)

• High T

BT (bismaleimide-triazine) resin

• Thermal enhancements

• Electrical enhancements

• Very ﬁne external pitches

• High temperature reﬂow requirements

• Thin proﬁle heights

IPC-7095c-3-8

Figure 3-8 BGA Warpage

January 2013 IPC-7095C

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All of these have been addressed in the last few years and

great progress has been made.

In general, it has been difficult to create standard pin count

BGA designs because every die has different requirements.

Each package/die combination is unique; therefore, econo-

mies of scale that manufacturers can achieve with

perimeter-leaded packages are not necessarily seen with the

area array devices.

Table 3-4 shows the expectations of the semiconductor

industry as to what they expect to pay on a cost per pin

relationship for the different technologies over the next

several years. The shaded sections indicate a challenge and

degree of difficulty in achieving the predicted goals. Table

3-4 is taken from the ITRS 2010 Roadmap and the costs

are very aggressive. The lower range of costs reﬂects

peripheral leaded packages and the higher ranges reﬂect

array style packages like BGAs/FBGAs.

The opinion of many resource experts is that the prices in

the future years may not be able to be achieved with prof-

itability because of the low costs forecasted.

3.5.6 Availability The 1.27 through 0.8 mm are available

in high volumes in many locations in the world. The

0.50 mm and below pitch packages are also becoming

available, and are used in many advanced portable elec-

tronic applications. Some of the component manufacturers

are developing their own version of the BGA package. Part

of this has to do with making it more difficult to copy the

design; another part has to do with maintaining market

share. The concept is one that prevents second source entry

and locks the designer that chooses a particular nonstan-

dard component product into a single component supplier.

3.5.7 Voids in BGA Many companies use X-ray,

In-circuit Test (ICT) and Automatic Optical Inspection

(AOI) in combination to improve their process control for

BGA solder joints. Some look for voids through X-ray to

determine accept/reject criteria. Some level of voiding in

any kind of solder joint is inevitable, but there is still

debate as to what is acceptable or an excessive void. The

proponents of voids argue that it is not the void that is bad,

but its location. The review of voiding has many consider-

ations, and in order to assist in process improvement

criteria, several tables in Appendix A are available to assist

in establishing process improvement goals. The informa-

tion on voiding has been analyzed in many controlled

experiments with no correlation being established that

relates the amount of voiding to reliability performance

under thermal or mechanical stress.

As the pitch of the BGAs become smaller and the ball size

is reduced, the number of voids in an individual ball

becomes more of a concern. One suggestion has been to

correlate void acceptance to the environment in which the

ﬁnal product must perform. It is recommended that each

product establish a criterion for a Void Protocol which

would establish the goals for a process or a product.

3.5.8 Pad Cratering A new concern for BGA implemen-

tation is the phenomena of pad cratering. Pad cratering is

deﬁned as a separation of the pad from the PCB resin/

weave composite or within the composite immediately

adjacent to the pad. It is also known as a ‘‘laminate crack.’’

Examples of pad cratering are shown in cross-section pic-

tures of BGA solder joints in Figure 3-9. Much of the rea-

son for this condition rests with the new formulations for

laminate resin systems that have been formulated to meet

the higher temperature requirements of the lead-free solder.

Some of the new materials are stiffer and may be more

brittle than those used in the past.

There are several possible failure modes for a BGA solder

joint. The different conditions are depicted in Figure 3-10

which is intended to highlight the location of pad cratering

in relation to other forms of solder joint failures. The Pad

Crater failure mode is in location 5. It occurs between the

land pad and the PCB laminate. The failure between the

component substrate and the component pad at location 1

is also very similar to pad cratering, but this type of failure

is usually attributed to the component packaging process

rather than the printed board assembly. The occurrence of

failure in location 1 is usually discovered during compo-

nent reliability evaluations or during the classiﬁcation con-

ditioning described in paragraph 3.5.2.

3.5.9 Standardization Issues Many of the BGAs are

using conventional printed board (interposer) materials but

are being tested to the standard component reliability tests.

Table 3-4 Example of Semiconductor Cost Predictions

Year Roadmap Input 2008 2009 2010 2011 2012 2015 2018 2020

Cost per Pin Minimum for Contract Assembly [1,2] (Cents/Pin)

Low-cost, hand held and memory 0.24-0.47 0.23-0.46 0.22-0.45 0.21-0.43 0.20-0.42 0.19-0.38 0.18-0.35 0.17-0.34

Cost-Performance 0.63-1.00 0.62-0.96 0.61-0.94 0.60-0.92 0.58-0.90 0.55-0.85 0.52-0.80 0.50-0.79

High-Performance 1.68 1.64 1.61 1.58 1.55 1.45 1.37 1.32

Harsh 0.23-2.00 0.22-1.90 0.22-1.54 0.21-1.46 0.20-1.38 0.19-1.17 0.18-1.00 0.17-0.89

White - Solutions exist Yellow - Solutions being pursued Red - No known solutions

IPC-7095C January 2013

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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