IPC-D-279 EN.pdf - 第36页

the board T ype 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 typica…

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

or;

2. Apply solder paste to side 2, dispense adhesive to

side 2, place SM components, cure/bake adhesive/

paste, invert board, apply solder paste to side 1, place

SM components, reﬂow solder paste, clean, test/

repair.

With mixed technology boards containing surface mounted

and through-hole components on more than one side of the

substrate (Type 2C - Figure 7-4), there are several typical

sequence possibilities:

1. For SM components on side 2 only: Apply adhesive

to side 2, place SM components, cure adhesive,

clean, invert board, insert through-hole components

and clinch, wave solder, clean test/repair.

or;

2. For SM components on both sides: Apply solder

paste to side 1, place SM components, reﬂow solder

paste, clean, invert board, apply adhesive to side 2,

place SM components, cure adhesive, invert board,

insert through-hole components and clinch, wave sol-

der, clean, test/repair.

3. For SM components on both sides: Apply adhesive to

side 2, place SM components, cure adhesive, apply

solder paste to side 1, place SM components, reﬂow

solder paste, clean, invert board, insert through-hole

components and clinch, wave solder, clean, test/

repair.

or;

4. For SM components on both sides: Apply solder

paste to side 1, place SM components, reﬂow solder

paste, clean, insert through-hole components and

clinch, apply adhesive to side 2, place SM compo-

nents, cure adhesive, invert board, wave solder,

clean, test/repair.

7.1 Solder Paste Application Solder paste may be

applied to a substrate either by syringe application or

screen/stencil printing. The ﬁrst issue impacting reliability

at this stage is the alloy composition of the solder paste

being deposited. Solder comes in a variety of alloys includ-

ing combinations of tin, lead, silver, antimony, bismuth,

indium and cadmium. Each alloy has a distinct set of met-

allurgical properties and the selection should be based on

compatibility with the metals being joined (including

leaching and intermetallic formation), the service environ-

ment and the service/process temperatures. Reliability will

also be affected by the type of ﬂux system used in the sol-

der paste; high reliability applications should be limited to

the use of R (rosin) or halide-free RMA (rosin-mildly acti-

vated) types.

Another contributor to reliability in this assembly operation

is the volume of solder paste deposited since it will dictate

the size of the solder joint. Careful consideration should be

given to parameters that will affect solder volume such as

stencil thickness and aperture sizes, squeegee/syringe pres-

sure, metal content of the solder paste and thickness of sol-

der coating on the printed board or substrate. See Appen-

dix M for details.

7.2 Adhesive Application Adhesive used to mount com-

ponents for subsequent wave soldering is typically applied

via a syringe dispensing method (either manual or auto-

matic). The volume of deposited adhesive should be

adequate to restrain the part in place but not impinge on

land areas or affect solderability and solder joint formation.

All adhesives should be tested for their tendency to form

voids that can entrap ﬂux and affect long term reliability.

Adhesive voids may also be caused by curing the material

too rapidly. Adhesives may outgas even after complete cur-

ing.

7.3 Component Placement The placement of compo-

nents on the substrate may be accomplished either manu-

ally or by using a broad range of pick-and-place equip-

ment. Accurate positioning of components to their

IPC-279-10

Figure 7−4 Type 2C − Mixed Technology

IPC-D-279 July 1996