IPC-D-279 EN.pdf - 第128页

of the solder . The conditions require that the termina- tion ﬁnish and the lands be solderable, the solder paste/ﬂux is suf f iciently active with no moisture in the paste during reﬂow , and the reﬂow conditions be well…

Appendix M

Solder Joint Variability

M-1.0 SOLDER JOINT VARIABILITY

Appendix A describes the design parameters which deter-

mine the mean cycles to failure. The Weibull parameter

describes the variability in the response of the solder

attachment and the individual solder joints due to manufac-

turing or processing conditions as well as the inherent vari-

ability in wear-out processes.

• Open surface mount solder joints have been found

after a period in service. These ‘‘cold’’ or ‘‘dry’’ solder

joints were characterized by a component termination

and substrate land which were mechanically touching

but not completed with a permanent solder connection

with intermetallic compound (IMC) formation can

open during service. Tin-lead to tin-lead mechanical

contacts (under light mechanical loading and not sol-

dered) oxidize and open in service under temperature

cycling or vibration/shock conditions. This failure

mode is similar to the ‘‘fretting corrosion’’ described

in these appendices.

Failed joints have been traced to

• planarity (or coplanarity) problems on the component

termination

• planarity (or coplanarity) problems on the substrate

land

• solderability problems on the component termination

and on the substrate land

• improper selection of solder paste/ﬂux.

• improper solder reﬂow processing.

Difficulty in inspection has resulted in ‘‘dry’’ solder joints

that are seen on J-lead solder joints. These ‘‘dry’’ joints are

insidious because they can be intermittent and be reported

as ‘‘NTF;’’ however, gull-wing terminations are not

exempt. Gull-wing leads are preferable to J-leads only

because they are not hidden under the component and sol-

der joints can be inspected and reworked more easily.

Some solder joints to gull-wing terminations have been

found to be open due to excessive IMC formation and

mechanical disturbance during solder solidiﬁcation. Some

companies use an electric or air driven plunger to ‘‘tap’’

PWAs during functional test to detect ‘‘cold’’ or ‘‘dry’’ sol-

der joints. Some repair technicians use a pencil ‘‘tapper’’ to

detect these solder joints. ‘‘3D’’ optical or X-Ray systems

may be fast enough and deﬁnitive in their detection of

defective joints.

• for the ideal SM solder joint of uniform thickness (for

minimum dispersion in the Nf or cycles to failure), the

surface of the solder paste on the lands of the printed

board must be coplanar (in the same plane), all the

leads of the SMT component must also be coplanar

and contact the solder paste simultaneously. If there is

a gap between any lead and the solder paste, a defec-

tive joint is likely.

Coplanarity and solder volume issues include:

• Maintenance of component lead coplanarity requires

appropriate shipping and storage containers, such as

trays rather than tubes for Ceramic Leaded Chip Car-

riers to prevent lead interlocking and damage and stor-

age of parts to be used for repair/rework or ‘‘kitting’’

in the original container rather than loosely ‘‘binning’’

them,

• Some of the requirement for perfect coplanarity is alle-

viated because the component leads sink into the sol-

der paste and into the molten solder; this requires that

cumulative coplanarity (between the substrate and the

component leads) apply over the dimension of the

component to ~100 µm. Solder paste thickness for

Fine Pitch and extra Fine Pitch (FP, XFP) is less than

that for the coarser pitch of 0.5 - 1.2 mm components

and therefore the cumulative coplanarity requirement

is tightened to ~75 µm, again over the dimension of

the component.

Large, uninterrupted stencil openings result in thin

paste in the middle of the opening; paste uniformity is

aided by assembling the large opening from a multi-

plicity of narrower openings with the long axis paral-

lel to squeegee blade travel.

• Planarity of the substrate lands is lost, particularly in

the case of Fine Pitch and extra Fine Pitch (FP, XFP),

when the substrate is processed through hot air solder

leveling or hot air leveling (HASL or HAL). These

processes leave uneven deposits of solder on the pads

with the distribution often a function of the location of

the pad on the panel.

• The volume of solder in the joints can be reduced by

via and feedthrough holes/pads, as well as large area

conductors or other large lands very close to compo-

nents lead pads; these features tend to draw solder

away from (steal or thieve) the intended solder joint

and should be covered with solder mask or separated

with narrow conductors, covered or not with solder

mask. The presence of a large number of solder balls

or a number of large solder balls also indicates a

reduction in the solder available for joints.

• The ideal SM solder joint is metallurgically joined to

the component lead and to the substrate land with no

voids in the joint nor sign of non-wetting or dewetting

IPC-D-279 July 1996

116

of the solder. The conditions require that the termina-

tion ﬁnish and the lands be solderable, the solder

paste/ﬂux is sufficiently active with no moisture in the

paste during reﬂow, and the reﬂow conditions be well

speciﬁed and executed.

• The large number of small, narrow, closely spaced

leads on a given component does not allow the evalu-

ation or measurement of the component lead solder-

ability; often, the production reﬂow operation demon-

strates the marginal solderability of the termination

ﬁnish on one (1) lead of one component with 68 leads.

Similarly, the large number of very small pads on a

given printed board does not allow the evaluation of

the solderability of the printed board. Some PWAs fail

because of one (1) failed joint out of several thousand.

Confounded with component termination solderability

is the preferential wetting of the termination by the

ﬂux and molten solder when soldering temperature is

achieved by the termination before the land.

• Repeated HAL or HASL (hot air solder leveling) pro-

cessing of the printed board can result in exposed

copper-tin IMC which rapidly oxidizes and has

extremely poor solderability.

• The move to ‘‘low residue,’’ ‘‘leave on’’ or ‘‘no-clean’’

ﬂux technologies results in ﬂuxes which are milder

and less able to penetrate oxidized termination and

land ﬁnishes.

• The effectiveness of the solder paste is sensitive to

many factors such as mean solder particle size, particle

size distribution (particularly the ﬁnes content), oxide

content of the solder particles, moisture content of the

ﬂux vehicle (as manufactured or after any subsequent

exposure to moist air). The SM process, if not properly

controlled, can exacerbate the degradation of the paste

through such process deviations as excessive drying

(loss of tackiness) prior to placement of SM compo-

nents, re-use of paste from day to day, or opening of

the paste jar without proper warming of the contents to

room temperature. Insufficient preheating prior to

reﬂow can result in inadequate activation of the ﬂux

while excessive or prolonged preheating prior to

reﬂow can result in oxidation of the solder metal par-

ticles and exhaustion of the ﬂux. Many of these factors

contribute to solder balling.

Solder balls are reported to result in short circuits by

bridging component pads of an MLCC connected to

Vcc and Gnd, resulting in a charred printed board.

Solder balls can be the result of improper handling and

processing of the solder paste or the injection of sol-

der through a via or PTH. The one or more solder

joints on the PWA may lack the solder in the balls and

solder joint reliability may be compromised.

• Some joints may not achieve the required temperature

for correct solder reﬂow under infrared (IR) conditions

because of the differences in thermal mass of compo-

nents (such as high pin count connectors or sockets

and those PGAs with affixed heatsinks), thermal shad-

owing effects from overhanging portions of taller com-

ponents and the component leads termination ﬁnish;

similarly, to avoid the heat sinking effects of ground

and power planes, component lead connections to

power and ground planes should be made using ther-

mal relief (isolated) pads connected to the larger

planes by thin conductors. Conductors to ﬁne pitch

land patterns also have solder thieving and heat sink-

ing effects during the reﬂow process.

• Formation of excessive brittle copper-tin IMC at tem-

peratures encountered in SM processes, including

rework/repair degrades the ability of the solder joint to

withstand temperature cycles. Photographs of these

IMCs are in Solder Joint Reliability and in Metallurgy

of Solder Joints in Electronics; Scanning Electron

Microscope (SEM) photographs of solder joints are in

Soldering in SMT Technology.

• Solder joints are subjected to thermo-mechanical stress

during cool down after mass reﬂow soldering where

the entire printed board is heated; this effect is exacer-

bated where the design includes components which are

long, stiff, or possess a low coefficient of thermal

expansion (CTE). Examples of these components

include RFI fences, ground bars, power/ground/ﬁlter

distribution laminates, and board stiffeners. Variation

of the time at temperature during reﬂow will be

reﬂected in the incidence rate of failed joints due to the

stress arising from cool-down. The cool-down phase of

the process can cause warping of printed boards or can

ﬂex warped boards such that solder joints and compo-

nents are stressed and broken; stresses may be reduced

if stress relief bends and curves are designed into the

part. A long, heavily ﬁlled SM connector which is not

mechanically restrained and in which the contact

inserts are captured may fall into this category. Hot bar

reﬂow at AT&T of 600 pin connectors, laser soldering,

single point TAB bonding and scanned light energy

sources have been used to address the issue of heating

the entire printed board.

Note that a 4-sided ‘‘box’’ of rigid components such as

a complete RFI fence will be robust with respect to

mechanical ﬂexure but will result in a ‘‘z’’ axis flexure

in the center of the ‘‘box’’ during SM reﬂow and sub-

sequent thermo-mechanical solder joint stress upon

cool down.

The hard, brittle IMCs embedded in the much softer solder

matrix require changes to the metallurgical sawing/

polishing/etching techniques normally used for homoge-

neous materials.

July 1996 IPC-D-279

117

The solder joint quality is dependent on the volume of sol-

der present. Typically, a stencil or screen will be used to

deposit the proper volume of solder paste. While the per-

formance range of the various SMT processes vary, studies

have shown that over 63% of defects identiﬁed after reflow

originate during the solder paste screening (or stenciling)

step. Correct solder paste volume is controlled by the sten-

cil or screen emulsion) thickness, aperture opening size,

solder mask height and stencil process parameters (such as

squeegee hardness and pressure).

While either stencils or screens may be used for surface

mount leaded parts of 1.2 mm pitch and greater, stencils

are preferable for ﬁne-pitch work. Although stencils cost

more and require longer lead times for construction, they

clog less frequently, and provide longer wear life and a

greater degree of control.

The challenge for the screen or stencil designer is to pro-

vide for the correct volume of solder paste to the corre-

sponding land area while preventing bridges caused by

excessively wide apertures. Actual aperture sizes and sten-

cil thicknesses must be determined from solder volume

calculations or experimentally to suit the paste type, board,

tinning thickness and inspection expectations.

If the surface mount assembly contains a mix of various

pitch packages on the same board, it will be necessary to

engineer a stencil which can deposit various amounts of

solder without compromising the integrity of any compo-

nent’s attachments. There are at least four alternatives in

this situation:

(1) step-down stencils which have a thinner foil thick-

ness in the ﬁne-pitch land areas

(2) stencils which have the land apertures reduced in

only the ﬁne-pitch areas

(3) modiﬁed land stencils which have aperture openings

on alternating ends of the ﬁne-pitch lands and

(4) stencils which use fancy shaped apertures (such as

tear-drops, triangles, etc.).

The option chosen must be based upon design, as well as

manufacturing considerations.

IPC-D-279 July 1996

118