IPC-7095C-2013.pdf - 第146页

may make it necessary to place large BGAs at the edges of the board. In such cases, the reﬂow soldering process win- dow will be narrowed to keep the maximum temperature the BGA components are exposed to below acceptable…

• Mechanical, electrical and thermal properties

• Reworkability

• Compatibility with lead (during transition period)

• Availability from suppliers

• Cost

• Patent concerns

Table 8-4 compares the composition selected by three con-

sortia. The compositions are very close to each other and

behave very similarly in the reﬂow soldering process. Fur-

ther, solder alloy suppliers usually state a tolerance of ±

0.2% by weight for each elemental component of the sol-

der, which is consistent with the ANSI J-STD-006 speciﬁ-

cation. When taking this into consideration, the alloy com-

positions below all overlap.

8.5.5.2 Recommendations for Alloys with Ag Content

Around 1%

• Solder joints must reach a minimum of 235°C with a TAL

of at least 60 seconds.

• Reﬂow proﬁles for some PCAs may need to be modiﬁed

when BGAs with low Ag alloys are used

• At a minimum, reﬂow proﬁles may need to be validated

using metallography, requiring additional engineering

effort

Along with careful oven proﬁling, metallography should be

performed (with statistically signiﬁcant sample sizes) to

validate that components with low Ag solder ball alloys are

forming proper joints.

The number of thermocouples used to validate a proﬁle

should be signiﬁcantly increased so that the low-Ag com-

ponent can be shown to meet the conditions given above,

and that all other components meet the existing speciﬁca-

tions (no overheating).

The 1% Ag alloys appear to be incompatible with many

current industry lead-free assembly speciﬁcations for

SAC305 that require a minimum reﬂow peak temperature/

TAL of 235°C/60 sec, while current speciﬁcations are

likely adequate for other alloys. The actual limit in Ag

content at which signiﬁcant change to the lower limit of the

reﬂow proﬁle is needed has not been precisely identiﬁed. It

appears to be between 1% and 2.3% Ag. Other variables

may impact this value, so caution is warranted for any

alloy with less than 3% Ag.

Reliability of Solder Joints – Industry studies consistently

indicate better drop performance for low Ag alloys than for

near-eutectic SAC. Results depend on the solder land ﬁn-

ishes in addition to alloy composition. The exact reason for

the improvement is still being debated. It may be due to

reductions in the stiffness and yield strength of the bulk

solder ball. It is known that changes to the intermetallic

layer composition and/or structure of the solder joint can

be expected. A reduction in under cooling is required to

solidify the BGA ball.

The published data indicates that the fatigue life of low Ag

alloys is lower than the higher Ag counterparts. However,

the existing models are very product-speciﬁc. There are

currently no agreed-to acceleration models available for

these alloys, making product life predictions difficult. The

existing study results available relate to various end-

product conditions, thus understanding how those low Ag

alloys perform relative to eutectic tin/lead is not easy.

Managing the Change to a Low Ag Ball Alloy – Due to

the potential impacts on reﬂow process window and reli-

ability, changing from a near-eutectic SAC ball alloy to one

with less than 3% Ag should be considered to be a change

in form, ﬁt, or function. In this case a change in part num-

ber for any BGA component, where the only change is ball

alloy, is considered appropriate.

8.5.5.3 Board Design Considerations Board Design for

BGA Assembly with Lead-Free Solders is generally very

similar to that for presently used tin/lead solders. The same

Design for Manufacturability (DfM) rules and guidelines

should be applied for lead-free boards as those used to tin/

lead (SnPb) boards. These include consideration for com-

ponent orientation, soldering, via holes, solder mask,

repairability and testability. Some of these are described

further below.

BGA Land Pattern Designs – As in the case of tin/lead

BGA soldering, the preferred BGA Land style for SnAgCu

soldering is the nonsoldermask-deﬁned design as opposed

to the soldermask-deﬁned land style since it allows the

maximum ﬂexibility for the PCB designer, and less stress

points are introduced to the solder joint by the solder mask.

Component Placement Location on the PCB – Since

SnAgCu solders require higher reﬂow temperatures for

component soldering, the placement location of large, tem-

perature sensitive BGA components may need to be

addressed carefully. The regions near the edges of the

board are, depending on the board size, thickness and layer

count, typically 5-15°C higher in temperature than those in

the central locations. Since large packages are more prone

to moisture and thermal stress induced defects when sub-

jected to higher reﬂow temperatures, such packages, if pos-

sible, should be conﬁned to the central regions of the

board. Other factors, such as trace routability and density,

Table 8-4 Comparison of Lead-Free

Solder Alloy Compositions in The Sn-Ag-Cu

Family Selection by Various Consortia

Consortium % Sn % Ag % Cu

IDEAL 95.5 3.8 0.7

Solder Products Value Council 96.5 3.0 0.5

iNEMI 95.5 3.9 0.6

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may make it necessary to place large BGAs at the edges of

the board. In such cases, the reﬂow soldering process win-

dow will be narrowed to keep the maximum temperature

the BGA components are exposed to below acceptable lim-

its.

8.5.5.4 Reflow Soldering Considerations Reﬂow sol-

dering is commonly done in an IR convection oven with

heated air convection. Despite the higher reﬂow tempera-

tures necessary to melt the SnAgCu (SAC) solders when

compared with the tin/lead solders, new equipment may

not be necessary for lead-free reﬂow soldering. The same

ovens as used before for tin/lead solders can be used with

the obvious increase in the settings for the various heating

zones in the reﬂow oven. As the PWB mass increases the

ability to limit the maximum reﬂow temperature, delta T

across the PWB and reﬂow time becomes more difficult

without extended zone ovens.

The environment in the oven can be either air or inert, such

as nitrogen. For lead-free soldering, to minimize the oxida-

tion of the materials on the board assemblies during the

high temperature reﬂow operation, an inert atmosphere can

be helpful. A lot of the criteria depends on the solder paste

and the solder paste manufacturer as well as the thermal

mask of the electronic assembly. Some board surface ﬁn-

ishes, such as organic solderability preservatives (OSP) on

copper may require an inert atmosphere during reﬂow sol-

dering to attain acceptable solder joint yield levels.

A reﬂow solder proﬁle should be developed for all board

assemblies. Since SAC solders require higher reﬂow sol-

dering temperatures, it is important to determine the maxi-

mum temperature at various different locations on the

board. Component temperatures may vary because of mate-

rial type, surrounding components, location of part on the

board, and package densities.

To avoid moisture and thermo-mechanical stress induced

failures on plastic components, it is best to measure the

temperature of the component body and check to ensure

that it doesn’t exceed the maximum temperature it is rated

for. Hence, thermocouples, which are generally used to

measure the temperatures during reﬂow proﬁling, should

be attached at the solder joint as well as the body of the

various components during reﬂow proﬁling of board

assemblies. Large components generally have greater than

5°C difference between the leads/solder balls and the mold-

ing compounds of the component.

A typical SAC reﬂow proﬁle is compared with a tin/lead

reﬂow proﬁle for a BGA solder joint in Figure 8-21 below.

Four different regions of the reﬂow proﬁle are shown: the

Preheat region during which the low melting volatile ingre-

dient in the solder paste vehicle are evolved; the Flux Acti-

vation region which enables the temperature to start equili-

brating across the board and to start to activate the ﬂux; the

Reﬂow region where the solder melts, wets the land surface

and forms the solder joint; and ﬁnally the Cool down stage,

where the solder solidiﬁes and the board assembly exits the

oven to be cooled down by forced air blown down on the

board by fans.

The proﬁles illustrated in Figure 8-21 are termed ‘‘FAT’’

(ﬂux activation time) proﬁles since they have a soak zone

before the solder is reﬂowed. Alternatively, ‘‘ramp’’ pro-

ﬁles can also be developed which contain a continuous

ramp from the preheat zone to the reﬂow soldering zone.

These ramp proﬁles increase the throughput of the board

assemblies in the reﬂow ovens. But, care should be taken

to avoid overheating of components, particularly on the

edges of the boards.

8.5.5.5 Appearance of BGA Lead-Free Solder Joints The

BGA package body obscures its solder joints. However,

IPC-7095c-8-21

Figure 8-21 Comparison of a Lead-Free (SnAgCu) and Tin/Lead (SnPb) BGA Reﬂow Soldering Proﬁles

Tin-Lead Solder Profile

Reflow @ 183°C

Peak Temp. = 205 to 220°C

Lead Free Solder Profile

Reflow @ 217°C

Peak Temp = 235 to 245°C

257

227

197

167

137

107

0.0

0.6

1.2

1.8 2.4 3.0 3.6 4.2 4.8

5.4

6.0

Temperature

Time in Minutes

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with the help of special microscopes such as the endo-

scope, peripheral solder joints can be viewed. SnAgCu sol-

der joint microstructures are multiphase microstructures

and the surface of the solder joints appears rough. Figure

8-22 shows a typical SnAgCu BGA solder joint. This is

much different from typical SnPb BGA solder joints which

usually have a shiny surface. Ball

8.5.5.6 Transition Lead-Free Technologies The transi-

tion from a total tin/lead soldering system to a total lead-

free soldering system is not going to happen overnight.

There may be an interim phase where tin/lead and lead-free

solders will co-exist on board assemblies. This transition

phase entails that the impact of lead in the SAC solder be

evaluated for impact on solder joint yields and reliability.

The likely lead-free board assemblies during this transition

phase are listed in Table 8-5.

The ﬁrst possible lead-free board assembly listed above is

‘‘forward compatibility.’’ The board assembly soldering

process for forward compatibility assemblies has been con-

verted to lead-free technology with a change in the solder

paste composition and the reﬂow soldering proﬁle to match

this change. However, some components, such as BGAs

that are soldered on the board, will still have tin/lead sol-

der due to the component supplier’s lead-free roadmap

having a conversion date later than the board assembler’s

conversion date. This results in the tin/lead solder joints of

the BGA-type components being ‘‘contaminated’’ by the

lead replacement metals in the lead-free solder paste.

The second possible lead-free board assembly listed above

is ‘‘backward compatibility.’’ Backward compatibility sce-

narios arise when component suppliers introduce the lead-

free components, but not all board assemblers that use

these components have converted their board assembly

lines to lead-free technology. These assemblers will still be

soldering the lead-free components with eutectic tin/lead

solder paste using tin/lead reﬂow soldering proﬁles. A tin/

lead component would obviously be preferable in this case,

but the component suppliers may, due to economic reasons,

not want to carry two component line items, one tin/lead

and one lead free, for the same device. The solder joints

formed with this combination of tin/lead solder paste and

lead-free solder ball will have a mixed composition. When

soldering a Ball Grid Array package with SnAgCu lead-

free solder balls using tin/lead solder paste, two different

scenarios arise based on the reﬂow proﬁle used. The two

reﬂow proﬁles compared are shown in Figure 8-23 with the

total lead-free reﬂow proﬁle also shown for comparison.

The tin/lead reﬂow proﬁle, which is illustrative of the pro-

ﬁle used today for tin/lead assemblies, does not exceed the

melting point of the BGA’s SnAgCu solder ball. When a

reﬂow proﬁle is unable to make lead-free solder ball melt

or dissolve, the solder joint yield and/or solder joint fatigue

life may suffer. The tin/lead solder paste deposited on the

lands of the solder balls melts but the SnAgCu solder balls

are still not molten. In such a material combination, the

lead may likely diffuse through the grain boundaries of the

solder ball. How high the lead from the tin/lead solder dif-

fuses up the SnAgCu solder ball will depend on how high

the reﬂow temperature gets and for how long the tin/lead

solder is molten. As shown in Figure 8-24, which depicts a

micrograph of a cross-section of a SnAgCu solder ball of a

BGA package soldered to a board using a reﬂow proﬁle

which is unable to melt or dissolve SnAgCu solder ball, the

resulting solder joint microstructure is inhomogeneous.

This deleteriously impacts the solder joint reliability.

Yield impact on such solder joint is also deleterious due to

two reasons. One is due to the poor self-alignment of the

BGA during reﬂow soldering because the solder ball does

not become molten. This creates a potential for open joints

when the component is misaligned to some extent during

or after the Placement process step. Secondly, the lack of

‘‘ball collapse’’ may cause open solder joints from a lack of

contact between the solder paste deposit and the solder

ball.

Figure 8-22 Endoscope Photo of a SnAgCu BGA Solder

Ball

Table 8-5 Types of Lead-Free Assemblies Possible

Deﬁnition

Component

Termination/

BGA Ball Solder Paste

Board

Surface

Finish

Forward

compatibility

contains lead lead-free

leaded or

lead-free

Backward

compatibility

lead-free 63Sn37Pb

leaded or

lead-free

Total

lead-free

lead-free lead-free lead-free

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