IPC-7095C-2013.pdf - 第61页

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. Photoimag…

the PCB. An organic substance is deposited as part of the

process; this reduces the oxidation that would be expected

with a pure silver surface. IPC-4553 Speciﬁcation for

Immersion Silver Plating is a valuable reference document.

Immersion silver ﬁnishes are compatible with SMT, BGA

and through-hole components but it is not wire-bondable.

Silver ﬁnishes are compatible with most solder masks.

Immersion silver PCBs have a shelf life of 6 to 9 months

if they are stored properly. It provides a ﬂat surface which

reduces stencil printing and component coplanarity issues.

During soldering, the silver dissolves into the solder and

becomes part of the solder joint; it is almost undetectable

in the ﬁnal solder joint. Since the silver dissolves during

soldering, the solder joins directly to the copper surface.

Immersion silver ﬁnishes typically can withstand 4 to 5

heating cycles without affecting solderability. Immersion

silver is compatible with tin/lead and lead-free solders.

Immersion silver-coated PCBAs are prone to creep-

corrosion failure when they are exposed to poor air quality

environments; in particular, higher than normal concentra-

tions of sulfuric gases. In the presence of a more noble

metal, copper reacts with sulfur from the atmosphere to

form copper sulﬁde. Copper sulﬁde is soluble in water;

repeated exposure to sulfur and moisture results in corro-

sion that creeps out across the circuitry. Creep corrosion

has been observed on all surface ﬁnishes, but immersion

silver is more prone to reacting with sulfur and moisture

than other surface ﬁnishes. The worst creep corrosion

occurs at the copper to solder mask interface. Another con-

cern is micro voids or champagne voids that appear at the

ball to land interface (Figure 5-11); however, this problem

appears to have been mitigated with newer immersion sil-

ver chemistries.

5.3.3.6 Immersion Tin The immersion tin process uti-

lizes a displacement reaction between the copper surface

and tin ions in solution to reduce a layer of tin onto the

copper surface of the PCB. An organic substance is depos-

ited as part of the process that reduces the oxidation that

would be expected with a pure tin surface. Immersion tin

is compatible with SMT, BGA and through-hole compo-

nents but it is not wire-bondable. The chemistry and pro-

cess may not be compatible with some solder masks.

Immersion tin PCBs have a shelf life of 6 months. Previ-

ously shelf life was very limited (less than 6 months); how-

ever, in recent years it has improved considerably (see IPC-

4554). Tin dissolves into the solder and becomes part of the

solder joint; it is almost undetectable in the ﬁnal solder

joint. Since the tin dissolves during soldering, the solder

joins directly to the copper surface. Immersion tin is com-

patible with tin/lead and lead-free solders. It provides a ﬂat

surface which reduces stencil printing and component

coplanarity issues. There is some concern about tin whis-

ker formation but, with proper plating controls, it should

not occur.

Immersion tin ﬁnishes typically can only withstand 3 heat-

ing cycles without affecting solderability. Complete hole

ﬁll at wave soldering may be difficult to achieve due to

immersion tin degradation after multiple reﬂow cycles,

especially when no-clean ﬂux is used. It is a good idea to

use a nitrogen atmosphere during reﬂow soldering to limit

the amount of degradation; this will reduce the risk of hole

ﬁll problems during wave soldering. It is also a good idea

to have a nitrogen atmosphere over the solder pot during

wave soldering, if possible.

5.4 Solder Mask The solder mask is a polymer coating

which serves to mask all surfaces which are not to be sol-

dered. Unlike the laminate, which is a composite, solder

mask is commonly a homogeneous material. As the name

suggests, this solder mask is used to mask off the surface

of the board where solder is not required to prevent bridg-

ing between conductors. Because of process changes for

lead-free soldering, evaluating solder mask performance

takes on a whole new meaning.

In the past, not all boards required a solder mask because

the conductors and lands were spaced quite far apart. With

the advent of ﬁne lines and spaces, the use of a solder mask

has become almost mandatory for boards that are going to

be wave soldered. On a full SMT board where no wave

soldering is required, tenting or plugging of via holes is

done to assist drawing a vacuum on some ICT testers. Also,

the application of solder mask to block or plug a via allows

closer spacing between a via and the adjacent conductor

lines.

5.4.1 Wet and Dry Film Solder Masks Dry ﬁlm solder

mask is no longer supported by major PCB suppliers. Addi-

tionally there is an overall solder mask thickness issue that

does not allow solder mask dams smaller than 0.25 mm.

Therefore, dry ﬁlm solder mask is not suited for BGAs and

other ﬁne pitch components. Wet (screened) mask is still

available; however, it too is not suited for BGA designs

because of registration challenges.

Figure 5-11 Example of Micro Voids

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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

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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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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