IPC-7095C-2013.pdf - 第24页

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…

3.1.5 Inspection Requirements As with any surface

mount part, BGAs should not be moved after component

placement because this may smear the paste and cause sol-

der bridges. Even if some misalignment is visually

observed, many parts will self-align during reﬂow if their

terminations are no more than 50% off the land. If a BGA

has a gross misalignment problem, it should be removed

before reﬂow and reworked later. Though it may not be

practical for high volume production, using X-ray or opti-

cal inspection (endoscope) to inspect failures before

removing the part may be desirable.

3.1.6 Test Test strategies need to be developed before

using BGAs. The solder joints cannot be probed and test

points are required. It may be difficult to incorporate

enough test points to adequately test all solder joints. Some

alternative test strategies may be needed such as designing

BGA components with boundary scan capability designed

into them reducing the number of I/O fanout probe points.

In an effort to improve test capability, some BGA compo-

nents had test points designed right on the top of the pack-

age. This was not a good solution since it put pressure on

the BGA components and the joints.

BGAs into products, it is important to address not just the

technical issues, but also the possible business implica-

tions. It is very likely that time-to-market will be adversely

impacted if both products and the technology are devel-

oped simultaneously. It is a good idea to ﬁrst develop and

validate the technology before implementing it on a real

product. Otherwise, if any problem develops in the product

or technology, the deadline for product shipment will be

missed. To assure time-to-market readiness, BGA imple-

mentation methodology and process steps should be ana-

lyzed.

3.3 Methodology Several factors must be considered

when making design decisions in an attempt to balance

size, cost, and functionality. In addition to these consider-

ations, the product must also be able to perform reliably

over the expected lifespan of the product in its intended

environment. Package selection may be inﬂuenced by these

reliability factors and environmental conditions such as

temperature vibration, shock, and humidity.

3.4 Process Step Analysis There are several available

paths to utilizing BGAs effectively. The length of each path

depends on what design and assembly facilities a company

presently has, and how quickly they can be made ready for

production. The following is an example of one approach:

1. Select a list of candidate products for BGAs.

2. Develop an equipment list based on the projected vol-

ume needs. If sufficient in-house expertise does not

exist, it may be desirable to use a reputable training

center or consultant to save cost and time.

3. Organize a team representing design, production, test,

quality, and purchasing. This team is responsible for

component and equipment selections and review.

4. Develop a comprehensive BGA design guide that

stresses manufacturability. Use existing standards where

possible.

5. Design the candidate products starting with the conver-

sions of existing products using ﬁne pitch components.

6. Determine the need for lead-free products including the

alloy used on the part as well as the surface ﬁnish

needed on the mounting substrate.

7. Conduct rigorous assembly and test reviews. Carefully

monitor component purchasing to assure that compo-

nents have the speciﬁed package, shipping method, met-

allization, solderability, and orientation in the shipping

containers.

8. Develop comprehensive workmanship standards and a

process control system that is statistically sound.

9. Design the remaining candidate products.

With the major emphasis on using parts that meet both

customer requirements and conform to new environmental

regulations, many customers are requiring reports listing all

materials used at the component level and also for com-

pleted assemblies. To help facilitate this, IPC has devel-

oped IPC-1751 and IPC-1752 on Materials Declaration,

and have encouraged software providers to make tools

available that meet the requirements of those standards.

The requirement to establish a formal declaration system

has been in place since the automotive industry was chal-

lenged by the ‘‘End-of-Life’’ European directives.

To show an example of the breadth of the variation in

material properties that may occur in products, Table 3-3

shows a list of materials that might be used as a surface

ﬁnish or a material that was added to the assembly as the

second level interconnection.

3.5 BGA Limitations and Issues Even though BGA tech-

nology has moved into the mainstream, there are still some

decisions that need to be considered. These are business

and technical issues that must be resolved. The areas of

special concern are:

• Visual inspection

• Moisture sensitivity

• Rework

• Cost

• Availability

• Voids in BGA

• Open joint (BGA or PoP BGA)

• Head-on-pillow phenomenon

• Standards and their adoption

• Reliability concerns

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---

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

--`,,```,,,,````-`-`,,`,,`,`,,`---

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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---