IPC-7095C-2013.pdf - 第102页

faster to ﬂow ‘ ‘L ’ ’ or ‘ ‘U’ ’ shaped patterns (down two sides or three sides respectively). Underﬁll can be dispensed around packages on boards either with automated equipment (jet dispense or auger pumps or others) …

created more fragile package constructions, particularly in

the areas of shock and bend. Electronic devices are getting

smaller and smaller. Smaller devices tend to be carried

more and dropped more which leads to more demanding

shock and drop speciﬁcations. These factors are driving

underﬁlls and structural adhesives to be commonplace in

electronics packaging.

Polymeric reinforcement of BGA packages is rapidly mov-

ing into many electronics applications. Early adopters of

these methods include the markets such as cell phones,

MP3 players, PDAs, cameras, medical electronics, avionics

and military applications. More recent markets that are

using underﬁlls and other epoxies include laptop mother-

boards and ultra mobile PCs. Desktop motherboards and

server boards have resisted implementation of polymeric

BGA reinforcement. However, as BGA packages get more

fragile, this approach is also a possibility in these markets.

Three polymeric approaches for strengthening the BGA to

PCB interconnection are popular in the marketplace. These

include full capillary ﬂow underﬁll, partial capillary ﬂow

underﬁll, and corner applied adhesive. No ﬂow underﬁll

technologies are being developed, but High Volume Manu-

facturing (HVM) adoption of this approach has not yet

occurred. Some studies have shown that roughly a 100 to

200% improvement in package performance can be seen in

representative packages in shock and bend performance

versus packages with no polymeric reinforcement. Poly-

meric reinforcement of BGA is better than many other

approaches that have been tried. (larger land sizes, metal

deﬁned lands, alternate land shapes, etc.).

The form factor, i.e., body size of and/or ball pitch, of

BGA Packages is not the main determinant factor for their

polymeric reinforcement. The market segment in which the

BGA components are used is more critical in determining

whether corner glue or underﬁll is required or not. The

owner of the product design must determine if the product

needs additional mechanical protection for BGA packages

to meet marketplace speciﬁc reliability requirements (e.g.,

shock, bend, vibration, drop, temperature cycling, etc.)

Figure 7-11 shows a conceptual map of some typical types

of adhesive approaches overlaid into reliability require-

ments versus designed product life for a sampling of elec-

tronics devices. In this map, three distinct adhesive

approaches are conceptually grouped.

The ﬁrst grouping is assembled with the high performance

underﬁlls. These include devices that have the highest

class of expected performance in both temp cycle and

shock. Devices in this class can have expected product life

expectancies of 10-20 years or more and include avionics,

military electronics, medical devices and automotive elec-

tronics. The underﬁlls used typically have low molecular

weight resins and are highly ﬁlled with smaller particle size

ﬁllers to minimize any void formation and ﬁller segrega-

tion during ﬂow. These materials may require longer dura-

tion cure cycles and are not reworkable. In this market,

performance is the ultimate driver, not cost.

The next grouping is categorized as process oriented

underﬁlls. These products include things like cell phones

and smart phones, MP3 players, and tablet PCs. In this

market, high performance in shock is required. Tempera-

ture cycle performance is not as demanding probably

because these mobile devices run cooler with less power

consumption. In this consumer market, cost is very impor-

tant. The resulting underﬁlls used are made from resins

which ﬂow quickly and cure more quickly at lower tem-

perature. The resulting underﬁlls can sometimes even be

reworked. These attributes maintain high beat rates and

minimize scrap to keep total costs down.

The ﬁnal grouping of products uses corner glue adhesives.

(Corner glue is similar in concept to corner bond or edge

bond names used by particular manufacturers.) Devices

that use this strategy include laptop PCs, tablet PCs, net-

books, sometimes desktop PCs, and rarely servers. These

devices tend to be less portable that the prior group and, as

a result, their shock requirements are less demanding. The

corner glue strategy provides less shock performance than

the underﬁll strategies. Corner glue has the advantages

over underﬁll in that this approach is more easily

re-workable and less expensive to implement in terms of

capital, material cost and labor cost. Some corner glue

adhesives are even formulated to cure with a short expo-

sure to UV light. This can eliminate the need for more

expensive cure ovens and replace them with a bank of UV

lights.

As would be expected, high performance with these poly-

meric reinforcement strategies can only be obtained by

choosing the right material for the speciﬁc application

using experimentation. Underﬁll users should be aware that

choosing an underﬁll chemistry with cured mechanical

properties that match the use environment is critical.

Underﬁll chemistries generally increase mechanical perfor-

mance of packages (shock, bend, vibration, and drop) but

if they are not selected properly they can degrade the tem-

perature cycling performance at the same time. Hence,

margin gain in mechanical shock reliability needs to be

balanced with risk of margin loss in the temperature cycle

reliability.

7.2.2.1 Full Underfill and Partial Underfill Full underﬁll

is usually done by applying uncured liquid polymer to the

board at the edge of the BGA package and allowing the

underﬁll to ﬂow underneath the BGA package via capillary

action. Care must be taken when designing the underﬁll

dispense process to avoid the capture of a large air bubble

(void) in the interior of the BGA package. Dispense pat-

terns such as an ‘‘I’’ shaped dispense down one side of the

package are much less likely to entrap bubbles than the

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faster to ﬂow ‘‘L’’ or ‘‘U’’ shaped patterns (down two sides

or three sides respectively).

Underﬁll can be dispensed around packages on boards

either with automated equipment (jet dispense or auger

pumps or others) or with manual equipment (pneumatic

dispense through a syringe and needle). To increase the

ﬂow rate of the underﬁll and the production rate of the

manufacturing line, assembled boards are often preheated

to between 50°-110°C. Underﬁll vendors recognize that

ﬂow rate drives production rate. More recent generations of

underﬁlls are being formulated with lower viscosities and

better wetting characteristics that signiﬁcantly increase

ﬂow rates. Newer generation underﬁlls that ﬂow well with-

out preheating of PCBs are also being introduced.

Capillary underﬁll ﬂow time when dispensed using an ‘‘I’’

pattern can be approximated with the following equa-

tion:(1) (see Figure 7-12).

T = (3µL

)/(hγ cos Θ) where,

T = for underﬁll to ﬂow across the package in seconds

µ = underﬁll viscosity

L = distance for underﬁll to ﬂow

h = gap between parallel surfaces

Θ = wetting angle of ﬂuid to surfaces

γ = surface tension of underﬁll

Voids in underﬁll are common, especially at the intersec-

tion of the solder ball and the PCB and the solder ball and

the package substrate. General consensus is that small

voids in the bulk underﬁll have no signiﬁcant effect on

performance in shock, bend, or temperature cycling. There

is no industry standard for allowable voids in underﬁll.

However, most underﬁll users recognize that any voids in

the underﬁll that connect adjacent solder are risky. (Solder

has been shown to creep through voids and short adjacent

solder balls in temperature cycling.) Moderate sized voids

(i.e., getting larger than half of the diameter of a solder

ball) are a grey area in terms of industry acceptance. Anec-

dotal evidence says that these moderate sized voids do not

have signiﬁcant adverse effects but some underﬁll users

would like to eliminate them from occurring in their pro-

cess. Figure 7-13 shows examples of small halo voids,

medium sized voids and large voids, respectively.

IPC-7095c-7-11

Figure 7-11 Map of Underﬁll Adhesive Usage for BGA and Other Packages

Increasing Reliability Requirements

Designed Product Life in years

low end phones

smart phones

3 7 10 12 205

Solid state drives

MP3 players

tablet PCs

net book PCs

laptop PCs

desktop PCs

servers

automotive

computing

avionics

military–

missile

guidance

other high end

electronics

medical

electronics

HIGH

PERFORMANCE

UNDERFILLS

PROCESS

ORIENTED

UNDERFILLS

CORNER

GLUES

NO ADHESIVES

IPC-7095c-7-12

Figure 7-12 Flow of Underﬁll Between Two Parallel Sur-

faces

µ,

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Proper ﬁllet height is desirable for maximum underﬁll per-

formance. The presence of a ﬁllet that extends between

25% and 100% of the way up the side of the package near

its midpoint is considered acceptable in most applications.

Keepout zones for other devices and open vias are required

around the periphery of the BGA being underﬁlled. A con-

servative rule for keep outs is 1.5X the height of the top of

the substrate of the BGA package from the PCB surface on

the nondispense sides of the package and 6.0 mm on the

dispense side of the BGA package.

Underﬁlls packages are cured in ovens. One desirable

method for curing these boards is using a single pass

through a standard SMT oven run at a lower than reﬂow

temperature. Many underﬁll chemistries can be cured in

5-20 minutes at 120-165°C which may be conducive to this

approach. Batch ovens may also be used. Underﬁll vendors

have been introducing new formulations that cure at lower

temperatures over less time.

Historically, underﬁll epoxies have not been practically

reworkable in an HVM environment. This was more

acceptable initially in devices such as early generation cell

phones where the cost of each board was relatively low and

scrapping a few boards was not a signiﬁcant cost penalty.

However, underﬁll is penetrating more and more high

value markets. As a result, underﬁll vendors are developing

chemistries that are more reworkable in HVM.

Partial or corner only underﬁll is done by dispensing

underﬁll in dot or ‘‘L’’ shaped patterns near the corner of

BGA packages. The underﬁll ﬂows into a roughly arc

shaped pattern and envelops several solder balls deep at

each corner (see Figure 7-14).

This method has the advantage over full underﬁll in that

much less underﬁll material can be used, and also that

underﬁll ﬂow time can be greatly reduced which can help

increase the production rate associated with the dispense

step. As expected, partial or corner only underﬁll does not

have quite the strength improvement of fully underﬁlled

packages; however, in many situations the performance

improvement gained with partial underﬁll is more than

enough to meet the market requirements for the package/

board being protected. (An experimental case showed a

1.5X increase in shock level where the onset of mechani-

cal damage was sustained for a partial corner underﬁll

BGA versus the same nonﬁlled BGA. This is very signiﬁ-

cant.)

Some manufacturers of mobile PC motherboards have used

partial underﬁll to increase the strength of the BGAs on

their boards.

7.2.2.2 Corner Applied Adhesive Corner applied adhe-

sive (sometimes called corner glue, corner bond or corner

tack) is a method for applying glue only to the corner

and/or outer edges of a BGA package. The theory is that by

re-enforcing the area of the package that is under the most

stress, the solder balls farthest from the center of the pack-

age, package performance can be improved. Corner applied

adhesives do not improve package performance as much as

conventional full capillary ﬂow underﬁll, but often the

advantage gained is signiﬁcant and enough to meet market

requirements. The corner glue approach has been widely

adopted by markets that require high mechanical require-

ments (shock, vibration, and bend) on large BGA packages

(20 x 20 mm to 45 x 45 mm). The mobile PC motherboard

market matches this description.

A photograph of a BGA package mounted with corner glue

is shown in Figure 7-15.

Corner glue can be applied either directly to the PCB prior

to BGA package placement and reﬂow or to the assembled

Figure 7-13 Examples of Underﬁll Voids - small, medium and large; upper left, lower left and left of solder balls, respectively

Figure 7-14 Example of Partial Underﬁll - package was

pulled from the PCB and dark underﬁll can be seen in the

corners

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