IPC-D-279 EN.pdf - 第85页

E-2.0 CHEMICAL E-2.1 PWA Cleanliness Where water soluble or other ﬂuxes are used, assure that the PW A is cleaned at least to the ionic contamination levels expected of a ﬁrst-pass board. Otherwise, long term failure mec…

prior to removal. Keep the heat away from neighboring

components and joints with hot air deﬂectors and IR

shielding panels.

E-1.5 Water Vapor Pressure Effects on Plastic Encapsu-

lated Components

Condensed water vapor at the inter-

faces within a plastic encapsulated surface mount compo-

nent (PSMC) rapidly turns into steam and can cause

delamination between the moulding compound and various

elements such as the leadframe, the bonding ﬁngers, or the

chip/die surface. This phenomenon normally occurs at

~220°C. In extremely susceptible components, the thresh-

old temperature is expected to be lower than 220°C. In the

worst case the delamination of the molding compound can

progress to a continuous crack between the die and the

exterior surface of the package, permitting materials such

as ﬂux and cleaning solvents to enter the package.

These materials, in the presence of water, corrode the met-

allization of the die; in the additional presence of DC bias,

dendrites can form between conductors. Even when there

are no external cracks, ionizable substances can be

extracted from the moulding compound or from the surface

of the chip (such as from phosphorous doped oxide); these

materials, together with condensed water are trapped in the

delaminated regions and, over time, result in corrosion and

dendrites (see Appendix C).

When using hot gas or IR systems to remove or replace

components, particularly if subsequent failure analysis is to

be performed on the removed component, dry the compo-

nent prior to removal. Hot air deﬂectors and IR shielding

keep the heat away from the neighboring components and

joints. Where hot gas or IR has been used to remove a

component and the body temperature has exceeded ~260°C

for 10 seconds, the re-use of that component is not advised,

particularly if the component is susceptible to plastic pack-

age cracking and has not been dried out prior to removal.

Characterize replacement parts for plastic package cracking

susceptibility by the procedures in IPC-SM-786 or IPC-

TM-650, method 2.6.20.

E-1.6 Water Vapor Pressure Effects on Printed

Boards

The glass-epoxy or glass-polymer interface can

separate or delaminate, resulting in ‘‘measling’’ when

heated above ~260°C for extended periods. This phenom-

ena occurs at the 260°C temperature for glass epoxy boards

and lower temperature in glass polyimide boards that have

been exposed to moist environments. Conductive anodic

ﬁlaments (CAF) can form low or high resistance conduc-

tive paths between the barrels of the PTVs (vias) or PTHs

(plated-through-holes) where the measling has occurred.

Most glass-polymer printed board materials should have

been evaluated at 260°C for measling during the ‘‘solder

ﬂoat test.’’ Increased susceptibility to measling or delami-

nation is noted when the polymer has a signiﬁcant moisture

content; polyimide or polyimide based substrates should be

dried prior to high temperature exposure.

When using hot gas, IR, or manual iron systems to remove

or replace components, oven dry the substrate at 125°C for

24 hours prior to high temperature exposure and minimize

printed board time at temperature. Use thermometry to

measure and control the temperature of the substrate and

the component during removal and replacement; gas stream

temperatures > 350°C have been noted in systems setup for

rapid component removal.

E-1.7 Solder Melt Temperature Effects At the tempera-

tures required to melt solder, the adhesive binding the cop-

per conductor to the printed board weakens. Aggressive use

of a soldering iron to move or remove solder or to move a

component lead by lifting, prodding or twisting before the

solder is completely melted can result in the bending, tear-

ing or complete failure of both external and internal lands

or conductors; conductor repair is then necessary. The same

land or conductor damage follows premature lifting or

twisting of a component prior to complete solder melting.

During component removal and rework/replacement, such

as on connectors and PGAs with fountain solder, minimize

the exposure of the printed board to the molten solder.

Alternatives are to employ thicker copper plating or nickel

barrier plating to minimize the effects of copper dissolu-

tion, particularly on the PTH or PTV knees.

E-1.8 Temperature Excursion (T) and Temperature

Rate of Change (T/t)

If molten solder contacts the ter-

mination of a multilayer ceramic capacitor (MLCC), mul-

tilayer ceramic inductor, or multilayer ceramic ﬁlter net-

work, during ‘‘touch up’’, rework or repair, and results in a

thermal transient exceeding 4°C/second, a crack may

result. The crack occurs in the ceramic body under the ter-

mination, hidden from visual inspection but capable of

being a site for dendriting under conditions of moisture and

bias. Most MLCC suppliers recommend a ∆T < 100°C.

Some soldering irons, set at 425°C, with sufficient thermal

mass in the tip, and with a tip wet with solder will transfer

heat so rapidly that the ceramic under the termination will

crack. Preheating of the component above 150°C may be

required to avoid this kind of cracking but may result in

damage to other components or the printed board. The

issue of thermal shock cracking for MLCCs is worse for

capacitors with high values and high thickness; other

parameters which correspond to thermal shock susceptibil-

ity are high layer count, high dielectric constant or low

working voltage rating. Require your component supplier

to provide data regarding the ∆T and ∆T/∆t to which the

particular capacitor (dielectric type, value, working voltage

case style, temperature coefficient of capacitance) is robust;

this data may restrict your choice of component values

(parameters) or suppliers.

July 1996 IPC-D-279

E-2.0 CHEMICAL

E-2.1 PWA Cleanliness

Where water soluble or other

ﬂuxes are used, assure that the PWA is cleaned at least to

the ionic contamination levels expected of a ﬁrst-pass

board. Otherwise, long term failure mechanisms may

result. Corrosion stress cracking of the solder joints may

occur, together with dendrites on the surface of the printed

board and loss of Surface Insulation Resistance perfor-

mance (particularly important in high impedance linear

applications). See also the comments on cleaning under

‘‘Conformal Coating’’.

E-3.0 MECHANICAL

E-3.1 PWA Flexure

Excessive ﬂexure of the PWA due to

shock, vibration or handling during rework or repair can

result in a cracked component body, a detached termina-

tion, or an overloaded and failed solder joint.

E-3.2 Tooling Impact Impact of a hard tool on a ceramic

component body or termination can result in a cracked or

fractured component body.

During component removal, there should be no mechanical

stress on neighboring components. Some removal tech-

niques and tools have been observed to use a neighboring

component as a pivot or fulcrum.

E-4.0 ELECTRICAL

E-4.1 Electrostatic Discharge (ESD)

Observe ESD pre-

cautions while handling, testing, and transporting the PWA

to avoid the introduction of infant and latent defects.

E-5.0 SMT FAILURES/STRESS CONDITIONS

E-5.1 Component Derating Reference Conditions

Component derating is commonly referenced to the abso-

lute maximum ratings deﬁned by the manufacturer’s speci-

ﬁcation or data sheet. Each of the several maximum ratings

(e.g. power, voltage, current, temperature) must be applied

individually and not in combination with any other abso-

lute maximum rating. Where experience dictates, more

conservative references may be used.

E-5.2 The Most Important Stress and Some Precau-

tions

The most universal stress is temperature. The criti-

cal point at which to evaluate temperature is the active site

or junction or ﬁlm, not the ambient temperature; this deﬁ-

nition of the critical temperature takes into account the

temperature rise in the component due to heat generated

within the component. Many of the common failure

mechanisms in electronic components double their failure

rate contribution with an increase of only 10°C.

Caution The absolute maximum ratings or the

temperature/thermal derating of components usually state

or imply a maximum operating and/or storage temperature

(whether junction or hot-spot) and various electrical values

based upon DC power conditions measured in a still ambi-

ent at 25°C. However, to determine whether this assump-

tion is true for speciﬁc components, you may have to ques-

tion the manufacturers’ engineering staffs.

The physical conﬁguration of the test environment is often

not speciﬁed; this failing most affects low, short-leaded

components such as those in SMT because of unstated sig-

niﬁcant factors such as the orientation of the component,

the printed board conductor conﬁguration, the use (or non-

use) of sockets and the velocity of the airstream immedi-

ately adjacent to the component under test.

E-5.3 Failure Modes/Failure Mechanisms A failure

mode is the failure of a component to perform its electronic

or mechanical function. An electronic component fails due

to the underlying chemical, physical or mechanical mecha-

nisms.

For instance, an integrated circuit (IC) can fail due to an

open failure mode on a given pin or lead.

The responsible failure mechanisms can include: external

lead contamination leading to an open solder joint; lifted

bond wire at the internal package lead; corroded wire in the

package; resistive and brittle intermetallic compound

(IMC) growth at the wire/package interface; broken bond

wire (due to cyclic fatigue from external temperature

cycling, cyclic fatigue from internal power cycling, tensile

overload, material coefficient of thermal expansion incom-

patibles), melted bond wire (due to high currents); lifted

bond wire at the integrated circuit die (due to a poorly

executed bond, corroded bond pat metallization, contami-

nation on bond pad metallization, excessive IMC develop-

ment); open IC die metallization (due to poor oxide step

coverage, electrical overstress, electromigration, corrosion,

stress from dielectric layers, or stress from molding com-

pound movement). Note that these mechanisms can be the

result of simultaneous or sequential exposure of the com-

ponent to the physical/mechanical, chemical or electrical

stresses.

E-6.0 OVERVIEW OF STRESSES

E-6.1 Common Stresses and Component Response to

Stress

Thermal stresses include static high and low ambi-

ent temperature and cyclic ambient temperature; power

cycling results in cyclic internal temperatures. Cyclic tem-

peratures result in thermo-mechanical stresses. Electrical

stresses include static and cyclic power; static, cyclic and

transient voltage; current and current density; electrostatic

discharge (ESD) and electro-magnetic interference (EMI).

Chemical stresses include moisture or humidity, corrosive

IPC-D-279 July 1996

gas ambient, hydrolyzable dust and sand, solvents and

residual hydrolyzable contaminants from the fabrication

and assembly processes and handling. Mechanical stresses

include sand and dust, mechanical shock, vibration,

connection/removal of connectors, and unrelieved strain

due to warped printed circuit boards. Other stresses include

low atmospheric pressure/high altitude/vacuum, electro-

magnetic radiation, ionizing radiation such as X-rays and

alpha particles. A special but not uncommon case of a com-

bined stress is that of temperature- humidity- bias.

E-7.0 IMPORTANCE OF TEMPERATURE AS A COMPO-

NENT STRESS FACTOR

Temperature is the greatest single contributing stress to

component failure.

E-7.1 Temperature-Related Reversible/Temporary

Changes in Component Parameters

Examples of revers-

ible changes include:

• increased resistance in metallic resistors

• changes in capacitance, dielectric constant, power fac-

tor, and equivalent series resistance (ESR) of capaci-

tors

• decreased inductance of inductors and ﬁlters, particu-

larly near Tc (Curie temperature)

• increased gain of bipolar transistors

• decreased breakdown voltage and increased saturation

(leakage) current of P-N junctions

• increased output resistance of bipolar and ﬁeld effect

transistors

• decreased viscosity and load carrying ability of lubri-

cants.

High temperature exposure during SM assembly process-

ing (particularly solder reﬂow) or service also results in

reversible

• expansion of materials (particularly of polymers above

) with the possibility of the jamming of moving

parts and the ‘‘bi-metallic’’ bowing of materials

• loss of control and a long recovery period in tempera-

ture stabilized components such as crystal oscillators.

E-7.2 Temperature-Related Irreversible/Permanent

Changes in Component Parameters

Examples of irre-

versible effects of exposure to high temperatures include

the following:

• oxidation (particularly of lubricants and contacts)

• corrosion (particularly in the presence of moisture and

hydrolyzable contaminants)

• Intermetallic compound (IMC) formation particularly

in the case of SMT and Tape Automated Bonded sol-

der joints and other bi-metallic joints.

• grain growth in multiphase alloys such as eutectic

lead-tin solder

• diffusion of alkali metals in semiconductor devices

resulting in device instability

• diffusion of halogenated solvents through rubber seals

of non-solid electrolytic capacitors resulting in internal

corrosion and subsequent component failure

• evaporation and subsequent loss of high vapor pres-

sure fractions of silicone compounds, greases and ﬂu-

ids

• evaporation and subsequent loss of plasticizers in plas-

tics

• evaporation and transportation of silicone and plasti-

cizer compounds to the mating surfaces of separable

contacts such as those in relays and connectors and

cold ﬂow of materials such as polytetraﬂuoroethylene

(PTFE).

• evaporation and subsequent loss of ﬂuid in non-solid

electrolytic capacitors has caused losses on compo-

nents and instruments which have been in storage at

room temperatures. Evaporation is accelerated at

higher temperatures and higher altitudes. Some alumi-

num electrolytic capacitors rated for operation from

−55°C to +85°C or +105°C contain ﬂuids such as dim-

ethyl formamide (DMF) which has a boiling point of

67°C. Replacement of the DMF with dimethyl aceta-

mide (BP = 74°C) or gamma butyrolactone (GBL) (BP

= 97°C) reduces the susceptibility of the component to

high temperatures. Because these solvents have ﬂash-

points very close to their boiling point, the solvent

change also reduces the ﬁre hazard. Changing from

DMF to dimethyl acetamide or GBL also reduces the

toxicity of any leaking electrolyte.

High temperature exposure during SM assembly process-

ing (particularly solder reﬂow) also results in irreversible

• internal and external SM integrated circuit package

delamination and cracking, mechanical damage to the

surface of the die and its microinterconnections as well

as and potential failure due to corrosion of the die

metallization. Passive component networks are also

susceptible to this delamination, cracking, and corro-

sion. See appendix D for a summary and IPC-SM-786

for details.

• temporary softening and a degradation in resistance to

cut-through of insulating polymers (such as those in

capacitors) with a permanent loss of dielectric with-

stand strength and mechanical strength and a long

period to recover some of the lost properties.

• stress cracking of susceptible polymers, such as trans-

parent nylon optical components under internal

July 1996 IPC-D-279