IPC-D-279 EN.pdf - 第84页

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…

Appendix E

Environmental Stresses

The printed board assembly starts its exposure to environ-

mental stresses during bare board fabrication, assembly and

repair/ rework before it even enters the service environ-

ment. These stresses can be thermal, chemical, mechanical

or electrical in nature. It is important to understand the

impact of environmental stresses to assure the reliability of

the assembly during its service life.

E-1.0 THERMAL

E-1.1 Effects of Rework and Repair

‘‘Touch up’’ is the

application of heat and solder to a solder joint which is

deemed cosmetically imperfect. Rework is the correction

of a defect before the SM PWA leaves the plant. Repair is

the correction of a defect found in the ﬁeld. Information on

rework and repair may be found in IPC-R-700. Each cor-

rection requires the heating of one or more solder joints

above the liquidus temperature of lead-tin eutectic solder

(183°C), typically above 210°C, and may involve the

removal and replacement of a component. Note that this

temperature of 183°C is well above several critical tem-

peratures in a SM PWA.

E-1.2 Glass Transition Temperature for Printed Boards

The glass transition temperature (T

) of most epoxy-glass

boards is ~125°C. For multi-functional epoxy glass boards,

is ~160°C. T

is greater for polyimide glass printed

boards which have a T

of 250°C. There are many other

printed board materials that fall between these two

extremes. Exceeding the T

of the printed board results in

signiﬁcant expansion of the printed board in the z axis,

hence in stresses in the barrels of plated through holes and

vias; the result can be intermittent or permanent opens. The

intermittent opens can generally be detected during tem-

perature changes of the PWA (see Appendix B).

Printed boards with a lower T

such as epoxy-glass boards

when heated locally, during component removal or compo-

nent replacement, to temperatures > T

, will tend to bulge

in the heated area. The solder joints of the replacement

component will be under tension and may fail when the

assembly cools down.

When using hot gas or IR systems to remove or replace

components, minimize printed board time at temperature.

During component removal, there should be no component

lifting or twisting during removal until solder is liquid on

all lands. This minimizes stress on the leads where they

enter the component package (with the result of cracked

plastic package body, damaged glass to metal seals), as

well as stress on the lands and their adhesive bond to the

printed board (with the result of lifted lands).

E-1.3 Intermetallic Compound Growth The copper in

the printed board conductors and component leads reacts

more rapidly with the tin in the solder to form a brittle

intermetallic compound (IMC) such as Cu

Sn and Cu

at higher temperatures. The original eutectic solder will be

transformed into a lead-rich layer. In the worst case, the

surface is exposed and oxidized; this lead oxide rendering

that surface unsolderable.

A solder joint kept at temperatures exceeding 150°C will

result in a lead-rich joint which is less ductile than one with

eutectic solder and the fatigue life of the joint will be

degraded. The IMC layer will tend to act as a shear plane

under overload conditions. These comments apply to

neighboring joints during component removal or replace-

ment and to ‘‘touch up’’ of a joint; some data indicate that

solder joints which undergo a ‘‘touch up’’ are less reliable.

If the molten lead is removed from the land by wiping, the

exposed IMC layers can rapidly oxidize and become unsol-

derable.

If the heat source is solder (wave or fountain), the interme-

tallics will be carried away and in the extreme, the copper

land will be dissolved.

When using hot gas or IR systems to remove or replace

components, keep the heat away from neighboring compo-

nents and joints. Hot air deﬂectors and IR shielding panels

may be required for component protection.

E-1.4 Glass Transition Temperature for Plastic Encap-

sulating

The T

range of most molding compounds used

in the plastic encapsulation of semiconductors and passive

networks is in the range of ~150°C to 180°C.

Exceeding the T

of the moulding compound of the com-

ponents during component removal results in internal

stresses to the metallization and passivation of the silicon

chip. This damage can hinder failure analysis of the

removed component and can also be suffered by a neigh-

boring component. Disruption of the metallization can

result in opens and dysfunction of the chip. Damage to the

passivation can result in long term corrosion of and opens

in the metal. Exceeding the T

can also cause delamination

of the molding compound from the surface of the silicon

chip or the surfaces of the leadframe; condensed moisture

can pool in the delaminated sites and lead to dendrites and

corrosion. Where the delamination coincides with bonding

pads and wire bonds, bond shear can occur.

When using hot gas or IR systems to remove or replace

components, if subsequent failure analysis is to be per-

formed on the removed component, dry the component

IPC-D-279 July 1996

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