IPC-7095C-2013.pdf - 第100页

7.2 Post-SMT Processes 7.2.1 Conformal Coatings Conformal coatings are used to protect the parts from surface moisture and thus corro- sion. Conformal coatings should be speciﬁed to meet the requirements of IPC-CC-830 an…

7.1.7.1 Flux Residues that Require Cleaning (Clean)

Those residues that require cleaning can be separated by

the chemicals necessary to remove the residue, typically

either solvent/surfactant, basic water, or deionized water

cleanable. Selection of a cleaning solvent should follow a

careful evaluation of technical, economic, and environmen-

tal considerations.

In general, cleaning under BGAs is difficult because the

tight gap between the board and components may entrap

ﬂux which may be difficult to remove during cleaning.

Entrapped ﬂux residues that require cleaning are typically

highly corrosive so this situation can lead to serious reli-

ability concerns. However, if proper care is taken in select-

ing the cleaning processes and equipment, and if the sol-

dering and cleaning processes are properly controlled,

cleaning under BGAs can be successful. In addition, if

no-clean pastes are used, the stencils are required to be

cleaned to ensure good printing. It does need to be empha-

sized, however, that good washer process control is essen-

tial when using aggressive water-soluble ﬂuxes.

When establishing a cleaning process for water-cleanable

solder pastes, it is necessary to verify cleanliness speciﬁ-

cally with low proﬁle BGAs or CSPs, and BGA connec-

tors. It is common to utilize a method such as surface insu-

lation resistance (SIR) to verify that ﬂux residues are

completely removed between the BGA balls and to estab-

lish your cleanliness process settings. Other methods such

as ion chromatography or ionography are recommended to

be used to monitor the process after qualiﬁcation.

7.1.7.2 Flux Residues that Do Not Require Cleaning (No-

Clean)

The use of no-clean ﬂuxes has increased due to

the environmental concerns of using ﬂuxes that require

cleaning, the disposal of used solvents and the cost of the

cleaning equipment. However, no-clean is not a drop-in

process. No-clean ﬂuxes are generally not as active as other

types of ﬂux and hence the soldering results may be less

than desired unless adequate steps are taken. Some

no-clean ﬂuxes require nitrogen reﬂow environments to

produce acceptable results; however, most currently avail-

able no-clean ﬂuxes can be reﬂowed in air.

No-clean ﬂux residues must be sufficiently inert so that

they will not damage the PCB or components. These resi-

dues are often mildly ionic and/or acidic and can poten-

tially cause product failure. The possible failure modes

caused by those residues include:

1. Corrosion of the PC board.

2. Shorting between metal traces due to dendritic growth.

3. Functional degradation of the ICs due to ionic contami-

nation diffusing into the active junctions of the die.

The ﬁrst two failure modes are surface phenomena. The

effects can be detected on the board surface, and the incu-

bation time is relatively short. These failure mechanisms

are detected with the surface insulation resistance (SIR)

test or the electrochemical migration test (ECM). The third

failure mode depends on how long it takes for the external

ions, if any, to diffuse through the package encapsulation

into the die area. Once these foreign ions reach the silicon

die, they may impact its function. The diffusion time is

usually on the order of years in a normal operating envi-

ronment. Typically, some form of acceleration testing, such

as SIR or ECM, is required in order to measure this effect

within a reasonable test period. In addition, all no-clean

ﬂuxes used on a PCA such as solder paste, wave solder ﬂux

and even rework ﬂuxes can all remain on the PCA and can

become mixed. It is recommended that these ﬂuxes be

tested in combination, as well as individually, to under-

stand the complete cleanliness picture.

7.1.8 Package Standoff The package standoff is one of

the prime parameters determining the reliability of the

BGA solder joints. Package standoff for a BGA is deﬁned

as the distance between the land on the bottom of the pack-

age substrate and the land on the top of the board surface.

This distance varies depending on the type of solder ball:

the high lead ball type stays a fairly consistent size; the

eutectic solder ball reduces the package standoff height.

This is also known as ball collapse height. For example,

when 1.27 mm tin/lead BGA is soldered on the package,

it collapses 0.1 mm and when it is soldered on the board,

the balls ‘‘collapse’’ additional 0.2 mm. So overall, for

1.27 mm ball you can expect 0.25 mm to 0.30 mm reduc-

tion from the pre-reﬂowed value. The high lead and copper

balls do not collapse because they do not melt. The stand-

off height on lead-free BGAs should be veriﬁed during the

set up of your process. It is recommended to establish the

process to reproduce the standoff height for speciﬁc com-

ponents.

Factors that determine the post-reﬂow BGA package stand-

off from the board include the BGA package weight, the

ball size, the ball material, the land size and land conﬁgu-

ration (solder mask deﬁned or nonsolder mask deﬁned).

Standoff height decreases with increased package weight.

However, for packages with a large ball count, the package

weight may have less effect on the standoff height. One

study on the relationship between these two parameters

discovered that increasing the package weight by 5X

decreased the standoff height by only about 0.05 mm for a

615 ball, 1.27 mm ball pitch BGA package.

Larger ball sizes will lead to larger package standoff

heights due to the larger volume of solder in each ball.

Standoff heights are inversely proportional to the land

diameters. For nonsoldermask deﬁned (NSMD) lands, a

solder mask relief around the land may reduce the standoff

height, because the solder will wet out along the conduc-

tors as well as along the edges of the land. This is shown

in Figure 7-10.

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7.2 Post-SMT Processes

7.2.1 Conformal Coatings

Conformal coatings are used

to protect the parts from surface moisture and thus corro-

sion. Conformal coatings should be speciﬁed to meet the

requirements of IPC-CC-830 and should be speciﬁed on

the master assembly drawing. When UL requirements are

imposed, the coatings shall be approved by UL for use by

the printed board manufacturer.

The designer should be cognizant of compatibility issues.

Conformal coating is an electrical insulation material

which conforms to the shape of the circuit board and its

components. It is applied for the purpose of improving sur-

face dielectric properties and protecting them from the

effects of a severe environment. Conformal coatings are

not required on surfaces or in areas that have no electrical

conductors (see IPC-2221, Section 4.5.2).

Conformal coatings may be any of ﬁve types. The thick-

ness of the conformal coatings shall be as follows for the

type speciﬁed:

• AR - acrylic resin, 0.03 to 0.13 mm

• ER - epoxy resin, 0.03 to 0.13 mm

• UR - urethane resin, 0.03 to 0.13 mm

• SR - silicone resin, 0.05 to 0.21 mm

• XY - paraxylene resin, 0.01 to 0.05 mm

There are three primary chemical categories in use for con-

formal coatings. These are: silicone elastometers, parylene,

and other organics. All conformal coating types provide

various levels of protection from solvents, moisture, corro-

sion, arcing and other environmental factors that can jeop-

ardize the circuit operation.

Conformal coatings may also be used in greater thicknesses

as shock and vibration dampening agents. This type of

application brings with it the risk of mechanical stress to

glass and ceramic-sealed parts during cold temperature

excursions. Using this material may require the use of buf-

fer materials.

Caution should be taken to prevent underﬁlling BGAs with

conformal coating materials. Testing has shown that when

completely underﬁlling BGAs with conformal coating

materials (other than paraxylene) solder joint fatigue fail-

ures resulted during thermal cycle testing due to ‘‘Z’’ axis

expansion. Thus, the use of conformal coating as an under-

ﬁll is not recommended.

Conformal coating should not be confused with encapsu-

lants. Encapsulants are used primarily to protect the bare

die as a part of the chip component package. Plastic encap-

sulants provide the protection of the plastic BGA from

external sources. The compatibility issues of encapsulants

and thermal coatings are very similar.

7.2.2 Use of Underfills and Adhesives BGAs may

require the use of adhesives to further strengthen the pack-

age to PCB interconnection. In recent years the implemen-

tation of lead-free solders and the reduction in pitches has

IPC-7095c-7-10a,b,c,d

Figure 7-10 Effect of Having Solder Mask Relief Around the BGA Lands of the Board

Solder Mask Relief Around Land ~0 mm 0.75 mm

Top view of land illustrating

increase of effective land diameter

due to trace connections

Cross-sectional view of land with

solder ball joint illustrating the sol-

der wetting down the edge of the

land when there is solder mask

relief away from the land edge

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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

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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