IPC-7095C-2013.pdf - 第62页

damage the via wall resulting in open vias. However , applying the via caps after the surface ﬁnish has been applied can degrade some surface ﬁnishes (e.g., OSP , ImAg, ImSn) due to the thermal exposure that is necessary…

5.4.2 Photoimageable Solder Masks Photoimageable

solder masks provide accurate registration, are easy to

apply, encapsulate the circuit lines totally, have excellent

durability, and are cheaper than dry ﬁlm.

Photoimageable masks can be either screened on or applied

by a process called curtain-coating in which the board is

passed at high speed through a curtain or waterfall of sol-

der mask.

The photoimageable mask may contain solvent along with

photopolymer liquid. If the solvent is added in the mask,

the liquid mask is screened on, solvent is dried off in an

oven, and then the mask is exposed to UV light by off-

contact or on-contact methods. (If no solvents are used, the

liquid is 100% reactive to UV light.) The off-contact

method requires a collimated light system to minimize dif-

fraction and scatter in liquid. This makes the system very

expensive. The on-contact approach needs no collimated

UV light source, and the system is relatively cheaper.

5.4.3 Jettable Solder Mask A new solder mask applica-

tion technology is becoming available to allow application

of solder mask in very ﬁne pitch components. This technol-

ogy uses digital inkjet printer to print solder mask layer on

PCB or other substrates. The solder mask material is

applied directly on the substrate using digital data and set

of inkjet print heads with a resolution of 750 DPI.

The solder mask layer is implemented using one step as

opposed to using a four step process with the traditional

way of applying the solder mask. The solder mask material

is a specially-developed jettable ink. The ink is a solvent-

less hybrid system which is both UV and thermal curable.

Some of the advantages of the jetting system is the ability

to print narrow surface mount dams between ﬁne-pitch

lands and tight control of the solder mask thickness. A few

disadvantages are that an end user must qualify the ink and

the material at the moment is only available in green color.

5.4.4 Registration of Board to Panel Image for Solder

Mask

Registration between individual boards within a

multi board panel becomes critical for any surface mount

application. This is especially true when the board is made

in a pallet format to assist the assembly process and

through-put characteristics. Board manufacturers inherently

build printed boards in a manufacturing panel format;

assemblers also want to take advantage of the multiple

board array format when they complete their assembly.

The positioning and orientation of individual boards on any

panel is usually at the discretion of the board manufacturer.

The manufacturer optimizes the use of the material in the

panel and the tolerance conditions that can be achieved

with the material used to build a particular board. It is a

well known fact that organic materials are prone to move-

ment (i.e. growth and/or shrinkage); thus, the board manu-

facturer, based on their knowledge of materials and their

predicted dimensional change movement, will commonly

adjust the photo tool of the circuit deﬁnition and the solder

mask deﬁnition to compensate for material stretch or

shrinkage depending on the circuit, the board size and the

particular properties of the selected solder mask material.

It is important to understand that assembly companies fre-

quently build their stencils based on a process of ‘‘step-

and-repeat’’ where elements of an individual board are

repeated to match the arrangement of the boards on the

subpanel. It is vital that the exact layout used by the board

manufacturer is understood to accurately provide the rela-

tionships of a land pattern for a BGA on one board with all

other land patterns of sister boards in the same panel.

Inconsistent arrangement of the assembly array can result

in misprints when stenciling solder paste onto the panel for

surface mount assembly.

5.4.5 Via Protection

5.4.5.1 Encroached vias

The encroached via concept is

one that permits solder mask being on the land without

ﬁlling the via plated-through hole. Encroachment vias take

the primary solder mask opening and adjust it so that it is

slightly larger than the via hole size.

This concept will permit any outgassing or cleaning of the

via plated-through hole, provide more surface coverage,

and increase adhesion between the solder mask and copper

of the annular ring. It will also provide a larger web

between the land and the via, and thus, should minimize

solder mask removal during BGA removal for rework.

5.4.5.2 Via Tenting, Plugging and Filling Via tenting,

plugging and ﬁlling (conductive or nonconductive) are

techniques used to cover or ﬁll via holes with solder mask.

These processes serve different purposes. Via tenting, plug-

ging or ﬁlling is normally performed on boards that use

both reﬂow soldering and wave soldering. Via tenting,

plugging or ﬁlling is also recommended under certain

speciﬁed conditions, such as for boards where exposed vias

under BGAs are exposed to a wave solder. The concern is

based on the fact that, when a board with BGAs on the ﬁrst

side is processed through wave soldering, a large amount

of heat can transfer from the vias. This can be very signiﬁ-

cant because BGAs can have very high via densities

beneath them. The BGA’s joints can potentially reﬂow

again in the wave and a second reﬂow without ﬂux could

result in cold solder joints or open joint conditions.

Via ﬂooding and capping processing may be performed

after a surface ﬁnish has been applied. For OSP and ImAg

(immersion silver) ﬁnishes, via capping may be done after

the surface ﬁnish is applied because the harsh chemicals

that are used to clean the copper surface can become

trapped around the via cap. These trapped chemicals can

January 2013 IPC-7095C

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damage the via wall resulting in open vias. However,

applying the via caps after the surface ﬁnish has been

applied can degrade some surface ﬁnishes (e.g., OSP,

ImAg, ImSn) due to the thermal exposure that is necessary

to cure the via cap material. Both of these issues must be

taken into account when making a decision to ﬂood or cap

vias. Most PCB suppliers ﬂood or cap vias before the sur-

face ﬁnish is applied.

There are presently eight different methods of via plugging/

capping identiﬁed in IPC standardization. These are shown

in Figure 5-12. It is important to realize that the choice of

tented, plugged and ﬁlled vias for via protection can have

direct impacts on the subsequent assembly processes.

Table 5-4 provides the pros and cons of tenting, plugging

and ﬁlling vias. The choice among the options presented

will depend on the capabilities of both the fabricators and

the assemblers. To avoid complication during assembly, it

is imperative that all involved in the manufacturing process

understand the trade-offs among the options.

5.5 Thermal Spreader Structure Incorporation (e.g.,

Metal Core Boards)

When structural, thermal, or electri-

cal requirements dictate, a conductive constraining core or

metal core can be inserted into the layer structure. It is rec-

ommended that the board circuitry layer conﬁguration is

made symmetrical about the the core as the center. It is

possible to create structures that are asymmetrical (i.e.,

having a different number of layers to either side of the

core); however, plated-through holes going through the

entire stack may be less reliable due to the differences in

expansion on either side of the metal or constraining core

(see Figure 5-13).

One reason for asymmetrical design is the separation of

electrical functions from mechanical or thermal functions,

but this advantage might be offset by board distortion or

warping during soldering operations (or during thermal

cycling in the intended operating environment) due to dif-

ferences in the thermal expansion of materials.

Some compensation can be achieved by having additional

copper planes added to the back of the interconnection

product. The extra copper plane may increase the expan-

sion coefficient slightly and make soldering more difficult

due to the need to put more energy into the board to assure

proper solder joint formation; however, a positive effect is

that it enhances thermal conductivity.

5.5.1 Lamination Sequences As previously noted, more

desirable constructions are those where circuit layers are

symmetrical about the core chosen to serve at the center of

the board. By so doing, individual multilayer circuits can

be produced separately, each with their own laminating

sequence. For example, a four layer board might be manu-

factured having vias through the entire four layers and this

can be duplicated for use on either side of the core.

To achieve mechanical constraint within a chosen and use-

ful range, the total thickness of the core in the multilayer

should be approximately 25% of the board’s total thick-

ness. Constraining core board is more often used because

the core layers may be imaged, etched, and connected to

the plated-through hole. The thicker center core must be

machined. Better thermal cycle survival has been shown in

some studies for structures having two constraining cores

in the board rather than one.

Another conﬁguration is to have a special constraining core

board made by bonding a multilayer board to each side of

a thick metal core after each of the boards has been com-

pleted. The composite board is then sequentially drilled,

plated, and etched to form plated-through hole connections

between the two boards. Coupons should be provided to

test the integrity of the composite structure.

5.5.2 Heat Transfer Pathway Metal core boards add sig-

niﬁcantly to the thermal mass of the assembly. This may

force the preheating soldering process to be operated at

abnormally high limits. These designs should be thor-

oughly evaluated under production conditions prior to

release. Laminate ruptures and discoloration and grainier

textured solder are typical effects that have been observed.

The heat transfer path between components and the planes

are usually accomplished through either direct contact with

the plane or through thermal vias positioned under the

component and connected to the thermal core or plane in

its position.

Table 5-3 Via Filling/Encroachment to Surface Finish Process Evaluation

Surface Finish Tenting Flooding Capping Plugging Encroaching

HASL Okay Okay Okay Okay Okay

OSP Okay Not Recommended Okay Okay Okay

ENIG Okay Okay Okay Okay Okay

ImAg Okay Not Recommended Okay Okay Okay

ImSn Okay Not Recommended Okay Okay Okay

IPC-7095C January 2013

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One side Type a Two sides Type b

Tented Via (Type I Via)

A via with a dry ﬁlm mask material applied

bridging over the via wherein no additional

materials are in the hole. It may be applied

to one side.

(Type I-a) or both sides (Type I-b) of the via

structure (IPC-4761, Figure 5-1).

Tented and Covered Via (Type II Via)

A Type I via with a secondary covering of mask

material applied over the tented via. The material

may be applied to one side (Type II-a) or both

sides (Type II-b) of the via structure (IPC-4761,

Figure 5-2).

Plugged Via (Type III Via)

A via with material applied allowing partial

penetration into the via. The plug material

may be applied from either one side (Type III-a)

or both sides (Type III-b) of the via structure

(IPC-4761, Figure 5-3).

Plugged and Covered Via (Type IV Via)

A Type III via with a secondary covering of

material applied over the via. The plug and

secondary covering material may be applied

from either one side (Type IV-a) or both sides

(Type IV-b) of the via structure (IPC-4761,

Figure 5-4).

Filled Via (Type V Via)

A via with material applied into the via targeting

a full penetration and encapsulation of the hole

(IPC-4761, Figure 5-5).

Filled and Covered Via (Type VI Via)

A Type V via with a secondary covering of

material (liquid or dry ﬁlm soldermask) applied

over the via. The covering material may be

applied from either one side (Type VI-a) or

both sides (Type VI-b) of the via structure

(IPC-4761, Figure 5-6).

Filled and Capped Via (Type VII Via)

A Type V via with a secondary metallized

coating covering the via. The metallization

is on both sides (IPC-4761, Figure 5-7).

Partially Filled Via

The illustrations show a via ﬁlled with

nonconductive epoxy and then plated

over with copper. This is commonly

used for via in pad applications

(IPC-4761, Figure 5-8).

Figure 5-12 Via Plug Methods

January 2013 IPC-7095C

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