IPC-7095C-2013.pdf - 第121页

result in higher levels of voids in these solder balls after the SMT reﬂow soldering process. Reﬂow time/temperature proﬁle, solder paste formulation, solder paste amount, component and PCB contamination/ oxidation and m…

any occurrence of exceeding the characteristics for void

size shown in Table 7-7, requires 100% evaluation of the

total lot.

The appropriate corrective action may vary based on class

of product and customer requirements. The ultimate action

is to remove and replace the affected component; however,

those solutions must be carefully evaluated as the product

should have been designed to permit repair procedures that

include reevaluation.

Once a void protocol has been established as to the percent

attachment permitted, random samples should be taken

from the production units and evaluated as to compliance

with the accepted conditions identiﬁed in the protocol. The

decision as to how many samples to take depends on the

number of units being produced. A consideration should

also be given to the number of BGAs that are part of the

particular assembly.

The index value of 2.5 is a good representation that can

provide an insight into the capabilities of the process

regarding BGA assembly and the occurrence of voids. The

Level A 2.5 index can be used for evaluating commercial

product used in consumer goods and computer applica-

tions. It is also appropriate for telecommunication equip-

ment. For a Class 2 equipment where high performance

and extended life is required, and for which uninterrupted

service is desired but is not critical, and certain cosmetic

imperfections are allowed an index indicator of 1.5 should

be used.

Class 3 is for High Reliability Electronic Products and

includes equipment for commercial and military products

where continued performance or performance on demand is

critical. Equipment downtime cannot be tolerated, and

must function when required such as for life support items,

or critical weapons systems. Printed board assemblies that

contain BGAs in this class are suitable for applications

where high levels of assurance are required and service is

essential; thus, the evaluation requires a sampling of 1.0

index value.

The purpose of the index value is to deﬁne the possibility

that, although the samples reviewed all meet the deﬁned

void protocol, there is still a statistical possibility of the

percentage shown by the index that some of the product

may not meet the conditions established. A 1.0 index for

level C requires that 13 samples be examined from a pro-

duction lot of 125 assemblies. Even if the samples all meet

the protocol, there is still a possibility that 1.0% of the lot

size do not. This relates to less than 2 assemblies and

would be a satisfactory risk in many applications.

7.7 Process Control for Void Reduction

7.7.1 Process Parameter Impact on Void Formation

order to establish process control for BGA assembly it is

important to understand what parameters can impact void-

ing frequency and size in board assembly process. The

voids found in solder joints after assembly are usually

called process voids, which are also known as macrovoids.

The process voids are normally caused by the evolution of

volatile ingredients of ﬂuxes and solder pastes. However, if

big voids in solder joints are found after the SMT process,

they may not be generated during the SMT process only.

The ball attach process for attaching solder balls to the

BGA package substrates can also generate voids in the sol-

der balls. It is therefore necessary to investigate the amount

of voids within the solder balls of BGA components. This

voiding level within the balls of BGA components can be

very high. The JEDEC guideline, JESD 217, suggests the

Pre-SMT BGA voids percentage should be less than 15%.

High levels of voids within the BGA solder balls may

Table 7-8 C=0 Sampling Plan (Sample Size for Speciﬁc Index Value*)

Lot Size

Class 1 Class 2 Class 3

2.5* 4.0* 6.5* 1.5* 2.5* 4.0* 0.10* 1.0* 2.5* 4.0*

1-8 5 3 2 ** 5 3 ** ** 5 3

9-15 532853**1353

16-25 533853**1353

26-50 555855**1355

51-90 765876**1376

91-150 11 7 6 12 11 7 125 13 11 7

151-280 13 10 7 19 13 10 125 20 13 10

281-500 16 11 9 21 16 11 125 29 16 11

501-1200 19 15 11 27 19 15 125 34 19 15

1201-3200 23 18 13 35 23 18 125 42 23 18

3201-10,000 29 22 15 38 29 22 192 50 29 22

10,001-35,000 35 29 15 46 35 29 294 60 35 29

*Index Value is associated to the A.Q.L. value. If a particular product is determined to be ‘‘critical’’ by the user and a smaller index value is required, the user

shall designate the requirement in the procurement document and should state the ‘‘critical’’ requirement on the master drawing.

**Denotes inspect entire lot.

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result in higher levels of voids in these solder balls after the

SMT reﬂow soldering process.

Reﬂow time/temperature proﬁle, solder paste formulation,

solder paste amount, component and PCB contamination/

oxidation and multiple reﬂow are the parameters believed

to impact the voiding level in solder joints after assembly.

However, these parameters may impact BGA voids by dif-

ferent levels. The details of each parameter’s impact to sol-

der joint voiding are discussed as follows.

7.7.1.1 Solder Paste Formulation Flux used to assemble

BGA components tends to generate lower voiding in solder

joints than solder paste. The reason is the oxide of the sol-

der powder in solder paste reacts with the solder-paste-ﬂux

and generates outgassing in reﬂow process, which leads to

more voiding in BGA solder joints.

BGA solder joints assembled with water soluble solder

paste tend to produce higher voiding than with no-clean

solder paste. This is because the ﬂux formula in water

soluble and that in no-clean paste is different. The water

soluble solder-paste-ﬂux is more aggressive than the

no-clean solder-paste-ﬂux, and reacts faster and more com-

pletely with the oxides on the solder powder. This reaction

leads to more outgassing in the reﬂow soldering process.

For no-clean lead-free paste, the voiding level in solder

joints with different types/manufacturers of no-clean paste

does not make a big difference. Even though the pastes

vary in ﬂux solvent and metal content, the voiding level is

not signiﬁcant in solder joints (<1%).

Solder powder type in solder paste also impacts the void-

ing level after board assembly. The ﬁner the solder powder,

the more the surface area to volume ratio of the solder

powder particles and hence the higher their oxide content.

There is more outgassing which is caused by more oxide

reacting with ﬂux in the reﬂow soldering process.

7.7.1.2 Solder Paste Volume Addition of solder paste

applied to lands on PCBs may result in more voiding in the

solder joints. This is because the more paste applied, the

more ﬂux solvent is present, which leads to more outgas-

sing from reacting with solder powder oxide and solder

ball oxide. Solder paste volume will become more essential

to voiding generation since ball size is getting smaller, and

the ratio of paste volume to solder ball volume is getting

larger.

7.7.1.3 Component/PCB/Solder Paste Contamination/

Oxidation

Solder ball and land surface contamination

may lead to potential process voids generation in the SMT

process. The contamination may present a nonsolderable

surface on the PCB lands which may keep macrovoids

attached to the lands and may generate volatile gases at

soldering temperatures if the contamination decomposes.

Solder powder in the solder paste may oxidize if the paste

is exposed to air or humidity conditions. In the reﬂow pro-

cess, ﬂux reacts with the oxidation layer of solder powder

and generates volatile gases, which form voids in solder

joints. Finer sized powder has more surface area and leads

to a potential of more outgassing in the SMT process.

BGA solder balls exposed to elevated temperature, humid-

ity condition or ambient temperature long enough develop

a layer of oxide crust formed outside of the solder balls.

This oxide crust can prevent voids from escaping from the

molten ball during reﬂow soldering.

Compared to solder ball oxidation, the oxide of the PCB

land surface does not impact voiding level solder joints

signiﬁcantly. This may be because compared to solder

powder surface area, the land surface is much smaller and

ﬂux may not react with oxide on the land surface com-

pletely.

7.7.1.4 Reflow Profile Reﬂow peak temperature and

time above liquidus (TAL) are two key parameters consid-

ered to impact voiding in reﬂow proﬁle. Some manufactur-

ers believe that increasing peak temperature and prolonging

time above liquidus will let the voids in solder joints

expand and grow. However, the test results show increas-

ing peak temperature and prolonging time above liquidus

only bring limited impact to voiding level in solder joints.

For example, increasing peak temperature from 235°C to

250°C and TAL from 35 sec to 85 sec, the mean value of

voiding level only increased less than 1%.

There are two types of conventional oven proﬁle, ramp

soak spike (RSS) and ramp to spike (RTS) proﬁles. The

RSS proﬁle is recommended to control voids in solder

joints by some paste suppliers, because the soak stage may

allow the ﬂux to function well to get rid of voiding. How-

ever, the RSS proﬁle and RTS proﬁle do not have a big

difference in voiding level. For example, if the RSS and

RTS with the same peak temperature and the same time

above liquidus (TAL) are used to assemble the BGAs,

using the same solder paste, the voiding level will be very

similar. The mean value of %voiding in solder joints is

(<1%).

7.7.1.5 Reflow Atmosphere (Nitrogen or Oxygen)

Atmospheres with lower oxygen content produce less

amount of oxidation of the solder ball and PCB surfaces

during the reﬂow soldering process. Less oxides will result

in less process voids since reduction of oxides creates

water vapor which is one source of macrovoids and lack of

an oxide crust on the other surface of the balls when mol-

ten; this oxide crust can prevent voids from escaping from

the molten ball during reﬂow soldering.

7.7.1.6 Multiple Reflow Cycles Sometimes, the same

BGAs will go through multiple reﬂow cycles due to being

located on double sided boards or due to rework. Voiding

level of BGA solder joints after multiple reﬂow cycles is

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much higher than that of solder joints after 1X reﬂow. For

example, the mean value of %voiding in solder joints after

3X reﬂow is about two times the %voiding level in solder

joints after 1X reﬂow. This is because the multiple reﬂow

cycles give extra time for existing voids to grow and

expand, or for small voids to coalesce together to form

bigger voids, when the solder joints are molten. This

growth and coalescence mechanisms have been conﬁrmed

through analysis of 3D CT X-ray images before and after

each SMT reﬂow cycle.

7.7.1.7 Land Surface Finish The impact of land ﬁnish

type on process voiding is still not clear. But co-deposited

volatile organic compounds from surface ﬁnish plating

operations is believed to generate more process voids in

BGA solder joints. Although there has been much debate

on the impact of different land ﬁnishes, the voiding level of

these different ﬁnishes does not vary too much.

BGA voiding has become a focus in electronic industry due

to quality concern. For quality issue, some SMT process

parameters are essential to the voiding level in the BGA

solder joints. For example, if the component is exposed to

high humidity conditions or elevated temperatures, the sol-

der balls will have a layer of oxide crust, which could lead

to more voiding trapped in solder joints in reﬂow soldering

process. The other SMT process parameters which can sig-

niﬁcantly impact voiding levels include multiple reﬂow

and solder paste formulation. Inspection of voiding levels

of BGA components is very important in order to under-

stand the percent, size and location of voids caused by the

SMT process.

There is no data showing a relationship between the BGA

void and reliability. Size of BGA voids is insufficient to

deﬁne acceptance criterion for fatigue life of the solder

joints. Macro process voids observed in X-ray images may

not cause any signiﬁcant reliability impact, but microvoids

which cannot be seen in X-ray images may deleteriously

impact reliability. Large voids away from the solder joint to

component land interface do not impair the solder joint

fatigue life, but small voids at the interfaces can be

‘‘zipped’’ through once a crack starts. Location of voids

within the solder joint is as much, if not more, important

than void size.

7.7.2 Process Control Criteria for Voids in Solder

Balls

There is a continuing need for process development

and control to accommodate changing technologies. As

BGA land sizes, solder ball sizes and land pitch continue to

decrease, the dimensional parameters used on the produc-

tion ﬂoor need to change. New materials and processes

may be required to meet quality and reliability goals.

Voids will likely be encountered during various stages of

product life from development through manufacturing.

Maintaining a minimum acceptable standard is necessary to

assure that the product meets customer expectations,

product-life and reliability requirements. Manufacturers

need to use process control and continuous product

improvement techniques for void control. Readily available

statistical process control and process improvement tools

may be used.

A change in frequency and size of voids should indicate a

need for process control as well as improving the process

and materials. A baseline can be used to determine the need

for process adjustments to control the frequency and size of

voids. In addition, a void size limit could be established.

Size is determined in relationship to the ball. Thus a void

size larger than 25% of the solder ball’s cross-sectional

image diameter is approximately 6% of the total projected

area variation (see Figure 7-46). Any such process control

limits should be set with customer agreed-to contractual

commitment.

When there is more than one void per solder ball, the

dimensions of the voids will be added to calculate the total

voiding in that solder ball.

The equations for determining percentages are as follows

and all relate to the diameter of the ball or the contact sur-

face.

Void diameter percentage

Dvoid

Dball

x 100 = void percentage

Void Area

Contact or Ball Area

x 100 = void percentage

* Area is determined using the diameter and Pi (πr

Once the relationship of the void diameter to the ball image

diameter is known, the actual numbers and size relation-

ships become important. A 25% difference in diameters is

IPC-7095c-7-46

Figure 7-46 Example of Voided Area at Land and Board

Interface

Solder Outline

Void Outline

0.25 d

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