IPC-D-279 EN.pdf - 第35页

interactions. Failure to understand the links between board manufacture, assembly processes and circuit design may result in a choice of solder mask which will be costly to reverse after introduction. 6.1.1 Solder Mask S…

at one temperature, there may be no stress where two mate-

rials join but at a different temperature, if there is a differ-

ence in CTEs, the same joint may well be under such con-

siderable strain that the part fractures.

Matching the CTEs at a joint is no guarantee of freedom

from the problem because electronic components are their

own heat sources and because there is a temperature differ-

ence between component and substrate. The problem is

directly related to the size of the component, the thickness

of a solder joint and the compliance of the lead. See Sec-

tion 3.4 and Appendix A. The major problem arises from

thermally induced cyclic stress in the solder joint of the

larger leadless ceramic chip carrier (LLCCC) components.

5.8 ESD Packaging Requirements All SM pick-and-

place feeder parts, sensitive or not, if they are dispensed

adjacent to ElectroStatic Discharge Susceptible (ESDS)

components, should be packaged in antistatic materials.

5.9 Specials or Custom Devices Use Precaution

Devices with unusual or exotic characteristics, extremely

tight speciﬁcations, or low volume ‘‘custom’’ processing

are not as reliable as ‘‘standards.’’ The tightened speciﬁca-

tions result in a smaller or unknown C

. The unique pro-

cess of evaluation and selection used to meet special

requirements is more vulnerable to errors. It is very diffi-

cult to demonstrate product quality and reliability (or to

improve the process) with low volume of product.

5.10 Components to Avoid or to Use with Caution

• Printed board with T

< 125°C

• Printed board with PTH and PTVs aspect ratio > 3:1

• Components not on the preferred parts list

• Components using obsolete technologies

• Components containing liquid and sealed only with

rubber

• Components with rotating seals

• Components with thick silver or gold plating or paste

as the solderable termination

• Components with corrosive or polar liquids

• Aluminum electrolytics with silver anode (obsolete

technology)

• Components with exposed moving electrical contacts

• Electro-Mechanical connections between contact ﬁn-

ishes of tin and gold (dissimilar metals)

• Film resistors trimmed more than 50%

• Multilayer ceramic components such as capacitors,

inductors, ﬁlter networks assembled into PWAs using

assembly processes with ∆T/∆t > 4°C/second.

• Solder immersion or wave soldering of surface mount

components other than simple chip resistors and chip

capacitors.

• Components with ESD susceptibility

• Variable resistors, particularly wire-wound

• Variable capacitors

• Multilayer capacitors trimmed to value

5.11 Component Selection Considerations for Military

and Space Applications

Military and aerospace applica-

tions exposed to severe environments may require pack-

ages that are more robust. Hermetic packages may be

required to be robust to a life cycle environment which

includes high relative humidity. However, under extreme

levels of shock and vibration, hermetic packages (with ﬂy-

ing internal leads) are less robust than plastic encapsulated

packages. See Appendix P.

6.0 SOLDER MASK AND CONFORMAL COATING

CONSIDERATION

Because of the ﬁne lines and close conductor spacing and

the assembly processes utilized in SMT, solder mask and

conformal coatings may be a new design requirement. Sol-

der mask performs two functions, one to limit the ﬂow of

solder paste, the other to protect adjacent traces from cor-

rosion. Conformal coating protects the solder joints and

component conductors from corrosion. Detailed descrip-

tions of solder masks and conformal coatings may be found

in Appendix N.

6.1 Solder Mask Considerations for SM The decision to

use solder mask for surface mount technology applications

is usually based upon the need to prevent migration of sol-

der away from the device pads during solder reﬂow. An

alternative to the use of solder mask is a pads-only

approach.

Typically, solder mask openings are designed 125 µm

larger than the component land or through-hole pad. This

allows for alignment tolerances during solder mask pro-

cessing. Since solder mask or its residue on surface mount

pads will reduce the size of the solder ﬁllet, it is highly

recommended that printed boards be procured which

exclude solder mask from extending onto these areas.

The ‘‘do’s’’ and ‘‘don’ts’’ of solder mask are important ele-

ments in the construction of a reliable surface mount

assembly. The type, thickness, coverage areas and proper

application are essential parameters in this complex equa-

tion. Since surface mount design begins to encroach upon

the limits of the solder mask process, it is important for the

designer to understand solder mask process capabilities.

The solder mask must be compatible with the surface

mount processes being used, especially the heat and sol-

vent resistance characteristics. The choice of a particular

solder mask should be validated with the proposed assem-

bly processes (reﬂow, wave and rework soldering; clean-

ing, conformal coating) to ensure that there are no adverse

July 1996 IPC-D-279

interactions. Failure to understand the links between board

manufacture, assembly processes and circuit design may

result in a choice of solder mask which will be costly to

reverse after introduction.

6.1.1 Solder Mask Selection Table 6-1 provides guide-

lines on solder mask selection.

6.1.2 Solder Mask Thickness Issues Regardless of the

type of solder mask used for surface mount applications,

the solder mask must be the correct thickness and consis-

tent. Solder mask plays an important role in forming a gas-

ket between the solder paste stencil and the printed board

to reduce the extrusion of paste and limiting smearing.

Typical dry ﬁlm solder masks range from 75 to 100 µm

thick, leaving the mask higher than the surface mount pads

(see Figure 6-1). Thus, the stencil will rest on the solder

mask and leave a gap between the stencil and the pads

through which solder paste can ﬂow. However, typical liq-

uid photoimageable masks range from 15 to 30 µm thick.

This makes the pad surfaces the highest points on the board

so that when the stencil is lowered, it will rest on the pads.

The stencil then gaskets the opening around the pads, pre-

venting the solder paste from getting under the stencil.

Excessively thick solder mask, particularly dry ﬁlm over

traces under components with small clearance, can contrib-

ute to the formation of crevices which entrap ﬂux. In the

case where the solder mask touches the bottom of the com-

ponent, if insufficient solder paste is used, it may result in

chip component drawbridging (tombstoning), insufficient

solder ﬁllet or lack of solder joint. (See IPC-SM-782 and

IPC-D-275)

Solder joint reliability under temperature cycling or power

cycling conditions may be reduced if the solder mask

touches the bottom of the component or conformal coating

ﬁlling the printed board-component gap. (See IPC-SM-

785.)

6.2 Temporary Solder Mask and Tapes Temporary sol-

der mask and tapes are used to prevent solder and solvents

from causing problems during assembly processes. In SMT

the reﬂow techniques may expose the mask or tape to tem-

peratures which may cause thermal breakdown. This may

cause reversion of some materials into a sticky ‘‘goo’’ and

others may adhere tenaciously making them difficult to

remove. The cleaning of SM assemblies may also be more

aggressive than through-hole. It is necessary to verify that

the temporary mask selected is compatible with the assem-

bly processes.

6.3 Conformal Coatings The primary purpose of confor-

mal coatings is to provide environmental protection. Some

conformal coatings have been shown to signiﬁcantly affect

the reliability of surface mount solder joints. Parylene

(trademark of Union Carbide, chemical name Polyparaxy-

lylene) and silicone conformal coating have been shown to

improve accelerated fatigue life of SM solder joints by

approximately a factor of two or three. However in thermal

shock, some silicone coatings have been reported to

decrease life in thermal shock.

7.0 ASSEMBLY PROCESSES AND DESIGN FOR MANU-

FACTURABILITY

While there are only a few processes used in fabricating an

assembly with SMT components, these processes directly

impact the formation of the solder joint that provides the

reliable attachment of the part to the substrate. It is essen-

tial that the designer of surface mount assemblies under-

stand the manufacturing processes involved, including their

impact on the reliability of the completed assembly. Design

for manufacturability requires that the designer have a clear

view of the impact and limitations of the fabrication steps.

IPC-CM-770, Printed Board Component Mounting,

includes the following classiﬁcation scheme for surface

mount assemblies:

Type 1 —Components (mounted) on only one side of

Table 6−1 Solder Mask Guidelines

RECOMMENDED FOR THESE SM SOLDERING

PROCESSES

SOLDER MASK TYPE

APPLICATION

METHOD THICKNESS

TENTED

VIAS

STANDARD

PITCH

REFLOW

FINE

PITCH

REFLOW

WAVE

SOLDER

Liquid Screenprint 8-50 µm N* N N N

Dry Film Hot roll Lamination 60-100 µm Y Y N Y

Liquid Photoimageable Open Screen Coat

Curtain Coat

Roller Coat

Electrostatic Spray

15-30 µm N Y Y Y

Combination (liquid

photoimageable* dry

ﬁlm cap layer)

Lamination 50-75 µm Y N Y

*The majority of via holes can be ﬁlled by this process but not tented.

IPC-D-279 July 1996

the board

Type 2 —Components (mounted) on both sides of the

board

Class A —Through-hole component mounting only

Class B —Surface mounted components only

Class C —A mixture of through-hole and surface

mounting

The typical assembly with surface mount components on

only one side of the substrate (Type 1B - Figure 7-1) is

assembled by one of the following sequences:

1. Apply adhesive, place SM components, cure adhe-

sive, wave solder, clean, test/repair.

or;

2. Apply solder paste, place SM components, reﬂow

solder paste, clean, test/repair.

If the assembly contains both surface mount and through-

hole components of a single-sided assembly (Type 1C Fig-

ure 7-2), it may be assembled using one of the following

sequences:

1. Apply adhesive, place SM components, cure adhe-

sive, wave solder, clean, insert through-hole compo-

nents and clinch, wave solder, clean, test/repair.

or;

2. Apply solder paste, place SM components, reﬂow

solder paste, clean, insert through-hole components

and clinch, wave solder, clean, test/repair.

When only surface mount components are mounted on

both sides of an assembly (Type 2B - Figure 7-3), there are

typically two assembly options:

1. Apply solder paste to side 2 (bottom), place SM

components, reﬂow solder paste, clean, invert board,

apply solder paste to side 1 (top), reﬂow solder paste,

clean, test/repair.

IPC-279-06

Figure 6−1 Impact of Solder Mask Thickness on Stenciling

IPC-279-07

Figure 7−1 Type 1B Assembly − All SMT Components

IPC-279-08

Figure 7−2 Type 1C Assembly − Mixed Technology Assembly

IPC-279-09

Figure 7−3 Type 2B − All SMT Components

July 1996 IPC-D-279