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reflow process. The objective of negative pressure reflow is to generate a continuous suction from the start of heating to the end of the cooling process within the heated chamber. A slow cooling rate profile is adopted …

he miniaturization of electronic

components has led to the use of

copper pillars, smaller bump sizes

and narrower pitches. This has resulted

in lower flexibility of joints and more

sensitivity to the influence of coefficient

of thermal expansion (CTE) mismatch.

Component tilt, open solder joints and

cracks with the use of extreme low-K

(ELK) dielectric materials are defects

that increase with this trend.

BTU’s solution, TrueFlat tech-

nology, which uses negative pressure

reflow is designed to alleviate yield

losses due to CTE mismatch during

reflow soldering. Advanced thermal

control (ATC) is introduced to control

the rates of heat transfer for processes

sensitive to abrupt expansion and con-

traction.

ATC is an enhancement of exist-

ing control for heating and cooling

rates to achieve continuous, uniform

heat transfer. This provides the abili-

ty to control the abrupt expansion and

contraction stress produced by micro

spikes.

Challenges of Miniaturization

Silicon flip chip and thin organic

substrates have significantly different

CTEs. CTE mismatch for flip chip on

thin substrates has been a challenge for

more than 10 years. Controlling defects

in mass reflow caused by warpage- and

CTE stress-related cracks has long been

the focus of reflow process engineers.

Other reflow techniques, such as ther-

mal compression bonding, are available.

However, these processes are expensive

and have low throughput, which makes

mass reflow a preferred process.

Existing mass reflow methods to

alleviate the effects of CTE mismatch

consist of a solid carrier acting as a heat

spreader, a cover plate to attempt to

hold down the substrate during reflow

and the use of a slow cooling process to

control the rate of contraction. This

method is being challenged as miniatur-

ization continues.

The use of a reduced or negative

pressure (pressure below atmosphere)

below the carrier throughout the

entire reflow process enhances the

contact of the substrate on the carrier

and helps to keep the substrate flat.

This enhances localized contact

below the chip, where a cover plate is

not effective. More effective heat

spreading produces greater tempera-

ture homogeneity and reduces the

impact of abrupt expansion and con-

traction due to CTE mismatch.

Ensuring contact with the carrier

helps to keep the substrate flat and

reduces deformation.

Negative Pressure Reflow

To demonstrate experimentally,

this requires two stages. The first

stage is to ensure effective contact of

the substrate to carrier. The second is

to control the heating rates for the

Reducing CTE Mismatch Defects in

Flip Chip Reflow

By Patrick Gao, Shoubing Ni, Thomas Tong, and Joe Yang, BTU International, Inc.

Reprinted with permission from the May, 2019 issue of ...

Figure 1: Negative pressure reflow design.

reflow process. The objective of negative pressure reflow is to

generate a continuous suction from the start of heating to the

end of the cooling process within the heated chamber. A slow

cooling rate profile is adopted to minimize the impact of heat

spikes.

Figure 1 demonstrates the gas flow to generate effective

suction with edge rail conveyors. Redirection of gas flow on

the bottom of the process produces a suction box effect, and a

pressure below atmosphere, directly below the conveyor and

product

— hence the name “negative pressure reflow.”

The carrier design is important for uniform suction and

heating. A large suction hole size creates localized heat spikes.

The recommended layout is 0.04 in. (1 mm) diameter hole grid

below the substrate. There are window and mesh top covers

used in standard reflow ovens without suction. Mesh has the

advantage of holding down the edges of each chip but has less

flexibility and costs much more to produce.

Window covers hold down the edges of the sub-

strate and allow different layouts of chips on the

substrate. The center of the substrate is subject-

ed to deformation, as there is nothing ensuring

proper contact of the substrate to the carrier.

Pressure should be measured below the

carrier with a room temperature fixture over-

laid onto a ramp to spike (RTS) slow-cool tem-

perature profile

— measured in the same oven.

If the suction remains effective throughout the

process, it ensures intimate contact of the sub-

strate with the carrier above the Tg of the sub-

strate where the deformation is at its worst.

As temperature increases, suction force will

be reduced. This is a normal phenomenon as air

density increases with higher temperature,

reducing the efficiency of suction. With a normal

tin-silver-copper (SAC) reflow profile, the suction

reduction will be about 30 percent. However, this

is still sufficient to keep the hotter substrate held

down on the carrier. This can be demonstrated

with a thermocouple (TC) mounted on the carrier,

just below the TC on the top side of the substrate.

The top of Figure 2 shows the heating of the substrate as

humps, due to the absence of the heat spreading effect as the

substrate lifts off from the carrier as it deforms. With suction,

effective heat spreading occurs, which evens

out the temperature.

Advanced Thermal Control (ATC)

Below are the heat transfer formulas related to mass

reflow ovens.

Convection heat delivered:

Target heat absorbed:

This produces S-curve heat transfer characteristics and

the heating rate will reduce when the target temperature

approaches the convection temperature. Common parameters

for mass reflow ovens based on the parameters for the above

transfer are: M = mass of the product and car-

rier; C

p = specific heat capacity of carrier; A =

exposure area; t = exposure time; H = coeffi-

cient of convection; and T = temperature.

Control of the heating rate can be

achieved by adjusting the reflow parameters.

The mass (M) of the product is an important

factor. It is necessary to reduce excess area

and weight of the carrier to minimize effects of

forward heat sinking and backward conduc-

tion, which impact the thermal uniformity

along the direction of flow.

It is typical to see the rear of the product

with a higher temperature than the leading

edge. The rear will act as a heat sink for the

leading edge, while the leading edge will con-

duct heat to the rear as it gets hotter. This

effect can be reduced with smaller tempera-

ture setting differences between the zones and

avoiding high ramp rates, but may require

longer zones or a longer oven.

With the oven size, conveyor speed and the product and

carrier mass held as constant, the next parameter to investi-

gate is the coefficient of convection (H). The variables for con-

vection heat transfer will be the area of coverage and impinge-

ment pressure. Plenum orifice size and distance from target

will determine the impingement force on the product. By opti-

mizing the orifice size, distribution and distance from product,

the thermal transfer rate can be optimized

to a preferred condition. This creates a con-

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Figure 2: Window cover without suction (top)

and with suction (bottom).

Figure 3: Profile comparison — standard vs. ATC.

trolled heating and cooling rate for the reflow process.

Figure 3 (page 67) displays the profiles with similar set-

tings and some fine-tuning to accommodate the critical areas

to control reflow and CTE differences. Unformed joints are

not sensitive to CTE mismatch during heating. The area of

critical control starts from the reflow ramp at zone 5 to the

end of the oven cooling zones. The calculations for heating

and cooling rates are made over five second intervals.

The control of the time above liquidus (TAL) is believed

to influence the spread size of the solder joint. This deter-

mines the area of interface between the substrate and bump

and will be more prone to cracks if the timing is too long.

Typical TAL values are between 50 and 60 seconds.

It can be seen in Figure 3 that the cooling rates achieved

with the controlled cooling from ATC are more consistent,

even as those compared to the standard PYRAMAX TrueFlat

oven. These spikes over short durations of five seconds are

called micro spikes. How critical are these micro spikes to the

reflow process?

If the pitch is wide, such as

≥

120 micron copper pillars

or simply by using soft solder bumps, the effect of these

spikes may be less detrimental. With further miniaturiza-

tion, brittle ELK dielectric with copper pillars and much

finer pitch means less flexibility and more sensitivity to

CTE-related cracks. Such products will require tighter con-

trol of the cooling rate.

TrueFlat negative pressure reflow technology ensures

that thin substrates on carriers are kept flat to reduce die tilt

and in direct contact for effective heat spreading during the

reflow process. ATC complements TrueFlat technology in

reducing micro spikes for optimal control of CTE mismatch-

related defects.

Contact: BTU International, Inc., 23 Esquire Road,

North Billerica, MA 01862  978-667-4111

Web: www.btu.com



See at NEPCON China,

Booth 1E50, and at SMTconnect,

Hall 4 Booth 551

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