IPC-D-279 EN.pdf - 第37页

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 .…

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

respective land areas is necessary for complete solder joint

formation, and thereby contributes to solder joint reliabil-

ity. Fully automated pick-and-place equipment requires the

use of ﬁducials (which are designed into the bare printed

board) for the accurate alignment of the components rela-

tive to the substrate. Fine pitch components often require

their own set of ﬁducials near or inside the footprint to

assure accurate placement. The type of ﬁducial (circle,

cross, pound sign, etc.) that works best depends on the par-

ticular assembly equipment used. Refer to IPC-SM-782 for

ﬁducial and land pattern information.

Bent and skewed component leads may also contribute to

improper location of some leads relative to their corre-

sponding pads, leading to solder wicking and incomplete

solder joints. Checks for lead coplanarity are often done for

high reliability assemblies.

For large volume manufacturing, the placement of smaller

substrates in a panelized format with snap-out features or

scored edges provides more efficient assembly and results

in potentially less handling damage. Care must be taken to

design these features correctly to prevent damage to solder

joints and board interconnects upon separation of the indi-

vidual assemblies from the panel.

7.4 Soldering

7.4.1 Solder Paste Reflow

Solder paste reﬂow is typi-

cally performed by forced convection, infrared, vapor

phase, or laser soldering process. Important parameters that

will affect the reliability of the components and assembly

include: heating ramp rate, peak temperature in preheat,

time above the minimum reﬂow temperature (20°C above

liquidus for copper), peak reﬂow temperature and cooling

rate and duration above the glass transition temperature of

the substrate. Control of the reﬂow proﬁle is required to

minimize damage to the components and printed board and

to control the formation of intermetallic compounds at the

solder-substrate interface. The formation of appreciable

intermetallic compounds has been identiﬁed as one of the

main sources of solder joint failure. See also IPC-SM-816.

High pin count, plastic leaded chip carriers, thin quad ﬂat

packs, thin small outline packages and small outline ICs

(PLCCs, TQFP, TSOP, and SOICs) may exhibit cracking of

the plastic package upon reﬂow. See 5.3.5 and Appendix

F.6.

7.4.2 Wave Soldering The wave soldering of surface

mount components requires that the parts be correctly ori-

ented during the design phase. If this is not performed, the

terminations may be soldered using specialized wave

geometries (e.g. dual or vibrating wave), however, the trail-

ing terminations will exhibit oversized solder ﬁllets that

may stress and crack chip components.

A good solder joint can form only if the ﬂux has properly

cleaned and prepared the surfaces for the molten solder.

Therefore, activity of the ﬂux is a parameter that needs

close monitoring to prevent dewetted and nonwetted solder

joints. As with reﬂow soldering, a good soldering proﬁle is

essential to reducing solder defects and preventing thermal

shock cracking of chip capacitors.

The presence of contamination in the solder greatly

impacts the appearance and integrity of the solder joint.

Gold in sufficient quantities (> 3% by weight) can result in

seriously embrittled joints. Solder bath contaminant levels

should be regularly monitored and limited to the levels

found in IPC-S-815.

Active plastic packages such as SOICs and PLCCs may be

adversely affected by wave soldering if ﬂux seeps into the

lead frame. There is a potential for this to happen since the

lead frame and molding compound have different coeffi-

cients of expansion. Passive components may exhibit

leaching of the precious metal terminations during wave

soldering. If dwell time in the solder wave is minimized

(3-4 seconds), leaching on these parts can be prevented by

a nickel barrier underplating between the precious metal

adhesion layer and the solder coating.

7.5 Cleaning Due to the low clearance of many surface

mount components, ﬂux residues become entrapped under-

neath. Remaining ﬂux may eventually cause corrosion and

electrical malfunctioning, hence, complete removal of

active ﬂux residues are essential to long term reliability.

Following soldering, it may be necessary to initiate imme-

diate cleaning to prevent hardening of the ﬂux that makes

removal difficult. The effectiveness of the cleaning process

should be monitored by measuring ionic residues. See the

test procedures in J-STD-001.

Besides ﬂux, it may be necessary to remove a range of

other residues such as solder oils, dross particles, strippable

solder mask materials, solder balls and particulate matter.

The cleaning medium (solvent) can adversely affect the

solder mask, printed board, conformal coating, board or

component legends/marking/labels, thin or mechanically

stressed sections of plastic components. Appendix I dis-

cusses the speciﬁcs of solvent compatibility on various

plastics and metals.

7.6 Rework/Repair Repair and rework equipment are

typically one of two types: hot air devices and conductive

tips. When hot air devices are used, care should be taken

to prevent thermal damage to adjacent components and the

surrounding area of the printed board. The number of times

that a part is removed and replaced should be limited in

order to prevent internal thermal damage such as intercon-

nect separations within the substrate. Desoldering time

when using conductive tips should be limited to three sec-

onds to prevent thermal damage. See IPC-R-700 for

rework and repair methods. See 7.8.1.2 for reliability

impacts.

July 1996 IPC-D-279