IPC-7095C-2013.pdf - 第94页

Reﬂowing OSP in nitrogen may also preserve the wettabil- ity of the lands for downstream processes as well as mini- mize oxidation of the test points or vias which may have an impact at in-circuit test (ICT). 7.1.4.3 Tim…

nozzle must gasket against the part, not allowing vacuum

leakage. Tactile sensing, which helps control the Z axis

(vertical) stroke of the spindle, is desirable because it pre-

vents a component from being crushed between the

vacuum nozzle and the substrate.

7.1.4 Reflow Soldering and Profiling Reﬂow soldering

is a complex process with many variables. All mass reﬂow

systems incorporate convective, conductive and radiant

means of heat transfer, to what degree depends on the

design of the reﬂow system. All designs strive to achieve

the same fundamental results. Five different phases take

place during reﬂow. The ﬁve phases are: (1) evaporate sol-

vents from the solder paste, (2) activate the ﬂux and allow

ﬂuxing action to occur, (3) preheat the components and

printed circuit board, (4) melt the solder and allow proper

wetting to occur, and (5) cool the soldered assembly.

It needs to be understood that, regardless of the basic

reﬂow heat transfer concept used, the solder balls under-

neath a BGA component are primarily heated by conduc-

tive heating through the interconnect substrate. The reason

for this is illustrated in Figure 7-3. The optimal reﬂow pro-

ﬁle (temperature vs. time) including the peak temperature

varies with the speciﬁc package and the overall assembly.

However, as a guideline, when using SAC305 solder paste

for BGA packages containing SAC305 solder balls, the

minimum peak temperature for moderately complex

PCBAs may need to be not lower than approximately

240°C. When using tin/lead eutectic solder paste for BGA

packages containing SAC305 solder balls, the minimum

peak temperature in most cases can be as low as approxi-

mately 215°C. (See Table 7-3 for recommendations for

proper reﬂow temperatures.)

Over the years, reﬂow equipment has changed frequently.

Five design concepts have been used: vapor phase, lamp

infrared (IR), panel IR, and forced gas convection and

diode laser. Vapor phase reﬂow technology evolved ﬁrst

and was popular for a few years. Eventually, IR became the

preferred approach. Today, forced gas convection (95% of

total heat source) with IR assist (5% of total heat source)

technology is the method of choice with some applications

requiring an inert oven atmosphere. Diode laser soldering

is the newcomer to the industry and used mostly in high

end high reliability applications such as space, military and

medical industries.

7.1.4.1 Forced Gas Convection Convection heat is

today’s method of choice. Heat is transferred to the printed

circuit assembly (PCA) by low velocity heated gas. Forced

gas convection is a noncontact heating method, with some

heating accomplished by radiation. The rate at which heat

is transferred to the object is directly proportional to the

difference in temperature between the heated gas and the

PCA.

Inline forced gas convection systems have three main sec-

tions: (1) preheat, (2) reﬂow and (3) cooling. Each section

contains a number of top and bottom zones. The number of

zones has a direct effect on the transport speed and the

user’s ability to ﬁne tune the proﬁle. Adding more zones

allows the user to run a faster belt speed and it permits

more ﬂexibility in accurate proﬁling. Low volume reﬂow

systems have three or four zone sections (a zone section

includes top and bottom heaters). Medium volume reﬂow

systems have ﬁve or six zone sections, while the higher

volume systems use seven or more. Typically, a six zone

system will satisfy most reﬂow requirements, including

very large PCAs and reasonably high transport speeds (up

to 60 cm per minute). Proﬁle changes are accomplished by

adjusting the transport speed and top and bottom tempera-

ture settings.

7.1.4.2 Reflow Atmosphere The atmosphere of the

reﬂow furnace will affect solder wetting. Reﬂowing in a

nitrogen environment will result in improved wetting of the

solder joint. This will sometimes allow compensation for

marginally wettable surfaces on the boards as well as oxi-

dized solder balls. In order to obtain the maximum beneﬁt

of the solder paste in a nitrogen environment, it is recom-

mended to monitor the oxygen content and control it within

the limits you have established for your process.

With the migration of products into lead-free technology,

the solderability of the lead-free solder pastes is not as

good as that of tin lead. It is also possible to use up the

activators in the solder paste to clean the oxides off the

paste, land, and balls prior to reﬂow if paste selection is not

made correctly. This would result in a nonuniform shaped

solder joint and possibly nonwetting of the land. Some sol-

der paste formulations may also be affected by reﬂow

atmosphere in the formation of voids. When using lead-free

processes with OSP surface ﬁnishes, it may be desirable to

reﬂow the product using nitrogen, especially for hole ﬁll

problems in mixed assemblies. Hole ﬁll problems can be

encountered in lead-free process especially when using no

clean ﬂux and 1

or 2

generation OSPs. Use of water

soluble aggressive ﬂuxes and 3

generation OSP should

help minimize hole ﬁll problems.

IPC-7095c-7-3

Figure 7-3 Example of Peak Reﬂow Temperatures at Vari-

ous Locations at or Near a BGA

220°C

240°C

245°C

for LF-solders

230°C

235°C

260°C

225°C

for Sn/Pb

205°C 217°C

230°C

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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Reﬂowing OSP in nitrogen may also preserve the wettabil-

ity of the lands for downstream processes as well as mini-

mize oxidation of the test points or vias which may have

an impact at in-circuit test (ICT).

7.1.4.3 Time/Temperature Profiles The solder proﬁle,

also known as thermal proﬁle, is one of the key variables

in the manufacturing process that signiﬁcantly impacts

product yield. Conveyor speed and panel temperatures are

two variables in solder proﬁle development. The solder

proﬁle is not only product speciﬁc, it is also ﬂux depen-

dent. Different pastes require different proﬁles for optimum

performance, so it is important to consult the paste manu-

facturer before developing the solder proﬁle.

For developing the proﬁle, the loaded board is needed for

which the proﬁle is being developed. Start with a given belt

speed and monitor the top-side board temperature using

thermocouples. Most new reﬂow ovens have built-in

thermocouples and software packages to record the thermal

proﬁle. Also, commercial hardware and software packages,

such as MOLE, data pack and many others are available to

make thermal proﬁle development an easy task. Use of

such proﬁlers has been important in tin/lead assemblies.

Now it is not only important, but critical, that such proﬁl-

ers be used on each product to achieve good yield without

exceeding the temperature constraints imposed by different

types of components. Table 7-3 provides key reﬂow pro-

ﬁles for both tin/lead and lead-free assemblies and also for

mixed assemblies (backward and forward compatibility

proﬁles). Note that proﬁles for Lead-free and forward-

compatibility are the same.

With tin/lead, there has been general consensus in the

industry about the composition of solder to be used: eutec-

tic solder with 63% tin and 37% lead composition with

melting point of 183°C. With this composition, there was a

big difference between the melting point (183°C) and peak

temperature (220°C). Even though it has been the recom-

mended practice to maintain the temperature all across the

Table 7-3 Proﬁle Comparison Between SnPb and SAC Alloys

Proﬁle Topic SnPb Alloy Proﬁle

Mixed/Backward

Compatibility Proﬁle

Pb-Free Alloy (SAC 305)/

Forward Compatibility Proﬁle

Alloy Solidus

temperature

183°C 183°C/220°C 217-220°C

Target alloy peak temp

range

210-220°C 228-232°C 235-245°C

Absolute minimum peak

reﬂow temperature**

205°C 228°C 230°C

Component ramp

up rate

2-4°C/second* 2-4°C/second* 2-4°C/second*

Component ramp down

rate

2-6°C/second* 2-6°C/second* 2-6°C/second*

Soak or preheat

activation temperature

100-180°C* 100-180°C* 140-220°C*

Soak or preheat

activation time

60-120 seconds* 60-120 seconds* 60-150 seconds*

Dwell time above

liquidus

60-90 seconds 60-90 seconds 60-90 seconds

Dwell time at peak

temperature

20 seconds max 20 seconds min 20 seconds max

Solder Paste used Tin/Lead Paste Tin/Lead Paste Lead Free (SAC 305) Paste

SMT component types

All SMT Type Tin/Lead

and Lead Free BUT NOT

Lead Free BGA Balls

All SMT Type Tin/Lead

and Lead Free INCLUDING

SAC Lead Free BGA Balls

ALL COMPONENTS INCLUDING

BGAs are Lead Free including

BGAs with SAC 305

Lead Free BGA balls

Reason for peak

temperatures

Lead Free surface ﬁnishes on

BGA Parts have no problem

melting at 205°C. Also all

tin/lead surface ﬁnishes have

90% tin any way. Lead Free

ﬁnishes have close to 100% tin

with some other lead free

elements like Bismuth

A compromise temperature is

needed so that tin lead parts do

not get overheated and lead free

SAC BGAs with melting point of

220°C can melt, collapse and

fully mix with tin/lead paste.

Lower peak temperatures will

cause SAC BGA balls to either

not melt or partially melt and

increase the incidence of HoP,

Opens and poor reliability

All components are lead free and

can take higher heat. However,

too high a peak temperature may

cause BGA ball drops, opens,

dewetting and board warpage

and Large BGAs are tested

for maximum of 245°C

for MSL level rating

* Verify with the supplier

**Coolest temperature on the board

IPC-7095C January 2013

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board between 210-220°C, one could easily get away by

maintaining temperature between 190-225°C, a variation of

almost 35°C and still achieve good reﬂow soldering results.

In lead-free assembly, the commonly used SAC (Sn, Ag,

and Cu) solders contain 3 to 4% silver, 0.5 to 0.7% copper

and the rest tin. These alloys have the melting point around

220°C. A few components, such as some aluminum elec-

trolytic capacitors, put restrictions on maximum tempera-

ture and duration above 230°C to which they can be sub-

jected. Additional constraints will be dictated by low cost

laminates, plastic connectors and moisture sensitive com-

ponents if used.

To accommodate such constraints, the peak temperature in

lead-free assemblies should be maintained between 230-

245°C, a variation of only 15°C—a tight process window

indeed. This is about a 60% drop from 35°C variation

within tin/lead assemblies as mentioned earlier. The diffi-

culty of achieving a reﬂow proﬁle to meet the deﬁned pro-

cess window is further increased if large components with

high thermal mass are used on the same board with smaller,

temperature sensitive components. The reasons are simple.

The large components with high thermal mass require a

larger heat input to meet the process window requirements

for Peak Temperature and time above liquidus. However,

this large heat input may result in the smaller, temperature

sensitive components falling outside the process window

requirements. To resolve this issue, very tight process con-

trol and narrow temperature bandwidth across the board is

necessary. Many assembly houses may have a hard time

meeting such requirements, especially on complex boards

without concerted time and effort in developing reﬂow pro-

ﬁles.

The problem can be further compounded by backward

compatibility issues where some lead-free components are

used on a primarily tin/lead board. In such cases, the pro-

ﬁle must accommodate both tin/lead and lead-free package

requirements.

Figures 7-4 to 7-7 show schematic and actual proﬁles for

tin/lead, lead free, and assemblies with tin/lead and lead-

free components.

7.1.4.4 Preheat Zone The temperature in the preheat

zone can range from 30-175°C and many component sup-

pliers generally recommend 2-4°C/second ramp rate to

avoid thermal shock to sensitive components. Such guide-

lines are considered conservative since some capacitors are

wave soldered where they go from preheat temperature of

about 120°C to wave pot temperature of 260°C. The fast

ramp rate does increase the potential for solder balls and

hence should be kept as low as feasible; however, consid-

eration should be given to the acceptable ramp rate of the

most sensitive component on the assembly.

IPC-7095c-7-4

Figure 7-4 Schematic of Reﬂow Proﬁle for Tin/Lead

Assemblies

PREHEAT

SOAK

REFLOW

COOLING

210 to 220°C

183°C

Ramp to Peak Profile

with no Soak

100-180°C

Cool Down

4-8°C/Second

60-90 SEC

30-60 SEC

for cooling

to room temp

MAX SLOPE

OF 5°C/SEC

90-120 SEC

Figure 7-5 An Example of Tin/Lead Proﬁle with Multiple

Thermocouples

IPC-7095c-7-6

Figure 7-6 Schematic of Reﬂow Proﬁle for Lead-Free

Assemblies

PREHEAT

SOAK

REFLOW

COOLING

235 to 245°C

217°C

Ramp to Peak Profile

with no Soak

140-220°C

Cool Down

4-8°C/Second

60-90 SEC

30-60 SEC

for cooling

to room temp

MAX SLOPE

OF 5°C/SEC

90-120 SEC

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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