IPC-7095C-2013.pdf - 第95页

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 (S…

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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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

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

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

7.1.4.5 Thermocouple Attachment Figure 7-8 shows

recommended locations of thermocouples on a board. It is

important that thermocouples be attached onto small and

large components at the solder joints. For BGAs, it is also

important to attach a thermocouple at the top of the pack-

age.

In developing any proﬁle, it is very important to use the

right thermocouple. Type k thermocouples with wire gauge

of 36 AWG should be used. Thicker thermocouple wires

add too much heat sink. Thermocouple wire length should

not exceed three feet for good accuracy. To ensure accu-

racy, thermocouple junctions must be welded. No twisting,

crimping or soldering should be used.

Care should be exercised when using high temperature

tapes such as polyimide (kapton®) or aluminum tapes.

Tapes tend to come loose during reﬂow and the system

measures the temperature of the air in the oven and not the

temperature of the solder joints. It is important to make

sure that there is good contact of the tape, otherwise, a high

temperature solder or thermally conductive adhesive

should be used to attach thermocouples to the solder joints.

One beneﬁt with using tape is that the thermocouples can

be reused repeatedly without damaging them.

In the case of BGAs, drill holes in the center and corner

balls of the BGA from the bottom of the board and push

the thermocouples to the top to correctly measure the tem-

perature of BGA balls. It is important that the difference in

temperature of the center and corner balls of the same

BGA are within 2°C of each other. There are also some

thermocouples that can be inserted under the BGA, elimi-

nating the hole drilling process; however, in this instance,

the thermocouples may only be measuring the temperature

under the device.

Four to six thermocouples should be attached at various

component locations to represent the lowest to highest ther-

mal mass areas including at least two thermocouples for

BGAs. Figure 7-9 shows locations of thermocouples on a

BGA.

7.1.4.6 Soak Zone The soak zone is intended to bring

the temperature of the entire board up to a uniform

temperature. The ramp rate in this zone is very slow,

almost ﬂat when raising the temperature from 75-220°C.

The soak zone also acts as the ﬂux activation zone for sol-

der paste. The consequences of having too high a tempera-

ture in the soak zone are solder balls, solder splatter due to

excessive oxidation of paste, and spent ﬂux activation

capability. The purpose of long soak zone is to minimize

voids, especially in BGAs. It is also common practice not

to use soak zone but to steadily ramp the temperature from

preheat zone to peak reﬂow. However, the likelihood of

voids may be increased when ramping steadily to peak

reﬂow temperature.

7.1.4.7 Reflow Zone The peak temperature in the reﬂow

zone should be high enough to obtain good wetting, and

create a strong metallurgical bond. However, it should not

be so high as to cause component or board damage or dis-

coloration or, in worst case, charring of the board. If the

temperature is too low, cold and grainy solder joints, non

melted solder, or poor inter-metallic bonding may result.

As shown in Table 7-3, the peak temperature in this zone

Figure 7-7 Examples of Lead-Free Proﬁles with Soak (Top)

and Ramp to Peak (Bottom) with Multiple Thermocouples.

The Proﬁles with Soak Tend to Reduce Voids in BGAs.

Figure 7-8 Locations of Thermocouples on a Board with

Large and Small Components

IPC-7095c-7-9

Figure 7-9 Recommended Locations of Thermocouples

on a BGA

IPC-7095C January 2013

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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