IPC-D-279 EN.pdf - 第98页

mechanical stresses under post-soldering stresses such as twisting, ﬂexing or shock and vibration during assembly operations such as TH component insertion, depaneling or testing. Identify the high stress areas of the pr…

• Design for a patch of copper foil beneath the body of

axial power > 2 Watts resistors to minimize charring of

the printed circuit board during fault conditions.

• At very high surface temperatures, radiation effects

can be signiﬁcant.

F-7.2 Trimmed Resistors The resistance value of rectan-

gular or cylindrical, thick-ﬁlm or thin-ﬁlm resistors is

adjusted by laser thermal ablation, sandblasting or abra-

sion. On cylindrical ﬁlm resistors, high speed spiral grind-

ing or abrasion is also used to adjust the resistance value.

On some rectangular thin metal-ﬁlm resistors, controlled

oxidation of the metal is used to adjust the resistance value.

The mechanical abrasion or thermal ablation resistance

value adjustment processes leave a narrow kerf. The kerf,

if bridged by conductive materials such as ﬂux, can be the

site of surface leakage. The kerf can also be the site of

voltage-induced breakdown, depending on the kerf geom-

etry. See also voltage stress and moist environment notes.

Note that the kerf on a TH Axial resistor which has been

trimmed to value is generally quite long compared to the

kerf on a rectangular SM device; hence the potential across

the kerf of the TH component is lower (less stress) than

that across the kerf of the SM component. In general, the

kerf is mechanically protected from the environment by a

plastic conformal coating. The kerf of any trimmed resistor

is best protected by a glass or inorganic coating which

covers the resistive and conductive areas; under conditions

of severe thermal or mechanical stress, even the inorganic

protective glaze can chip, crack or craze, leading to corro-

sion and failure of the resistive ﬁlm or termination.

F-7.3 Fixed Resistors Use metal oxide above 1 Watt or

wire-wound resistors above 2 Watts.

SM Metal Electrode Face bonded (MELFs) conﬁguration

components may require special ‘‘U−’’ shaped land pat-

terns to reduce rolling or may require adhesive to retain

position during reﬂow.

F-7.3.1 Metal Film Resistors Use for high stability, long

life, high frequency, reliability and accuracy.

Metal-ﬁlm resistors perform better in applications requiring

precision and stability compared to wire-wound resistors

according to warranty data.

Speciﬁc Cautions: Humidity or salt air can cause shunt

paths on surface of resistor and shorting between spirals.

Opens can be caused by mechanical damage. Higher noise

than wire-wound. Spiral cut: Opens may occasionally

occur due to too thin a resistance track due to non-uniform

resistance spiral. Operation at RF above 100 MHz may

produce inductive effects. Shorts may occasionally occur

due to protuberances on adjacent resistance spiral. Opera-

tion at > 400 MHz results in reduced effective resistance

due to dielectric effects. Critical matched resistors should

be purchased and installed as a set. Resistive ﬁlm can cor-

rode if encapsulant or conformal coating is breached; a

common point of entry for water or conductive contamina-

tion is the juncture of the terminal and overmoulding mate-

rial or conformal coating. Mechanical stresses affect this

type at temperatures < − 55°C.

Metal ﬁlms of nickel-chrome (NiCr) alloy (aluminum

doped) are susceptible to corrosion when exposed to halide

compounds such as chloride-bearing ﬂuxes if there is dam-

age to the coating during component manufacture or

printed board assembly, and subsequent exposure of the

component to corrosive materials and moisture. The alumi-

num dopant is very sensitive to basic or halide solutions

such as ﬂuxes and saponiﬁers; the presence of this dopant

should be identiﬁed for ease of failure analysis. This cau-

tion applies to TH as well as SM thin-ﬁlm resistors, both

epoxy coated and bare. Require your supplier of nickel-

chromium (nichrome) resistors to identify use of alu-

minium as an alloying agent.

Small thin-ﬁlm resistors of high value (> 100 kΩ) are the

most susceptible to moisture.

F-7.3.2 Thick-Film Resistor Networks Good tracking

between components on the same substrate.

Resistive ﬁlm can corrode if encapsulant or conformal

coating is breached. Mechanical stresses affect this type at

temperatures < −55°C.

F-7.3.3 Metal Oxide Film Resistors Resistive ﬁlm can

corrode if encapsulant or conformal coating is breached.

Mechanical stresses affect this type at temperatures less

than −55°C.

F-7.3.4 Resistor Chips

Caution

General purpose SM resistors are generally thick-ﬁlm cer-

met on ceramic substrate. Note the ESD sensitivity of bare

ﬁlms and metallic thin ﬁlms over thin oxides. See also the

comments on trimmed ﬁlm resistors.

The reduced component lead surface area of SM resistors

results in higher thermal resistance from resistive ﬁlm to

air. The smaller component and layout areas can result in

higher power densities and hence higher resistive element

temperatures. The smaller heat dissipation areas can result

in higher solder joint and substrate temperatures. Avoid

resistors that have been trimmed more than 50% of value.

Opens occur in the SM ceramic component body due to

damage from mechanical stresses such as overload from

vibration, shock or ﬂexure of the PWA during service life.

Opens occur in the SM ceramic component body and in the

body-to-termination interface due to damage from

IPC-D-279 July 1996

mechanical stresses under post-soldering stresses such as

twisting, ﬂexing or shock and vibration during assembly

operations such as TH component insertion, depaneling or

testing. Identify the high stress areas of the printed board

and avoid placing large, susceptible components in these

areas. Minimize PWA ﬂexure by providing adequate sup-

port and vibration damping in the ﬁxturing.

Hard pick and place (P/P) tooling can cause mechanical

shock during SM pick and place; test probes landing on

SM component body or component terminations may also

result in mechanical shock damage during testing. These

tooling shock cracks generally occur in the middle of the

component, although test probes can cause cracking and

spalling at the impact site.

The EIA 1210 size should be an upper bound for reﬂow

attachment of leadless ceramic and ferrite components to

polymer-glass substrates; these components include lead-

less chip carriers (LCCs), multilayer ceramic capacitors

(MLCCs), chip resistors, inductors and networks. At or

above this size, compute a solder joint ‘‘Figure of Merit’’

to evaluate the impact of package size and lack of lead

compliancy on solder joint reliability on glass-epoxy sub-

strates. For speciﬁc guidelines, see Appendix A.

F-7.4 Variable Resistors Variable resistors (pots) have

electrical and mechanical failure modes associated with the

wiper in addition to all of the frailties ascribed to ﬁxed

resistors. The wiper contact resistance can vary due to

wear, contamination or corrosion, wiper current, or vibra-

tion. The wiper position can shift due to shock or vibration,

so that the wiper setting is affected by shipping or portable

use. The current ‘‘rating’’ of the wiper is often an absolute

maximum number which is guaranteed only to the extent

that the resistor will not self destruct; the wiper current

contributes noise voltage. The maximum power rating is

based upon heat dissipation over the entire resistive ele-

ment and is speciﬁed with ‘‘maximum resistance

engaged.’’

Variable resistors require special precautions in manufac-

turing areas to avoid contamination by the soldering and

cleaning processes. Sealed units require very tight process

control by the supplier to survive the rigors of wave solder-

ing, SM solder reﬂow processes and high pressure cleaning

processes. If you must use variable components, hand-

install (backload) the parts or process them sealed prior to

and during soldering/cleaning. Avoid variable SMT compo-

nents which have rubber or sliding seals around leads or

shafts.

TH variable resistors generally experience soldering tem-

peratures only on the terminations during wave solder. SM

variable resistors experience high surface and internal com-

ponent temperatures during solder reﬂow. Elastomers used

in the seal area can be degraded by this heat exposure. The

thermoplastics used in the housing and rotor can see very

high surface temperatures during solder reﬂow processing.

A small change in the nominal dimensions of an ‘‘O’’-ring

was found to lead to the ingress of ﬂux into a sealed pot.

The supplier’s ‘‘standard’’ bubble test was found to be

inadequate to detect the problem before the problem was

found after assembly.

If halogenated solvents, activated ﬂuxes or saponiﬁers leak

past the seal, enter the component cavity, dissociate and are

then exposed to moisture, one result is lowered insulation

resistance. If there is electrical bias present, we have seen

migration of the thick ﬁlm electrodes (generally of silver)

and shorting. Metal migration tracks (dendrites) are very

fragile; the ‘‘short’’ can disappear with minimum mechani-

cal movements and may be noted as an ‘‘NTF.’’ A change

in ﬂux application method on a wave soldering machine

was found to be responsible for the ingress of ﬂux into a

sealed pot.

Silver dendrite growth has been observed inside potentiom-

eters due to intrusion of high pressure wash water past

improper, cracked or heat-distorted seals.

F-7.4.1 Enhancing Variable Resistor Reliability

1. Do not use a variable resistor, if possible; replace

rotating or sliding variable resistors with solid state

switches and programmable resistors. Elimination of

a component which requires adjustment on test is a

DfM guideline.

IF YOU MUST USE A VARIABLE RESISTOR,

2. Minimize the effects of contact resistance variations

on performance; use the wiper in a potentiometer

mode; draw low current through the wiper- but too

low wiper current can also cause problems.

3. Minimize the effects of contact resistance variations

on performance; incorporate a low pass ﬁlter capaci-

tor after the wiper to minimize noise voltages.

4. Count on no more than 1 degree of arc (0.3%) reso-

lution with a 1-turn pot. Don’t expect inﬁnite wiper

position resolution or perfect stability.

5. Consider the maximum current speciﬁcation as well

as the maximum power speciﬁcation.

6. Use sealed variable resistor components where the

value must be varied by hardware means and where

the component must survive an inline cleaning pro-

cess.

7. Pay close attention to the control exercised by the

supplier on raw materials and process, particularly on

items such as rubber seals and their associated

dimensions.

8. Hand load variable components as necessary to

enhance the reliability of your PWA.

9. Be sensitive to ‘‘NTF’’ in returns.

July 1996 IPC-D-279

Caution

All variable resistors can suffer movement of the wiper on

the resistance element as the result of shock or vibration. In

critical applications the resultant change in output voltage

can constitute a ‘‘failure’’ of the resistor. Non WW units

become noisier with wear life and will suffer resistance

change due to humidity. Power ratings for all variable

resistors are based upon the engagement of the maximum

resistance by the wiper. Excessive currents can be drawn

when less than maximum resistance is engaged, resulting

in a burn out of the resistance element. Some pots use sil-

ver thick-ﬁlm conductors; dendrites and shorts can form if

water leaks past the ‘‘O’’-ring seal during cleaning.

F-8.0 CAPACITORS

Capacitors suffer from high frequency and high voltage

effects of sinusoids; performance characteristics derived at

low frequency may not extrapolate to higher frequency; the

same is true for characteristics obtained at low voltage.

F-8.1 Multilayer Ceramic Chip Capacitors

Caution

Keep the MLCC termination electrodes ∆T/∆t < 4°C/

second and ∆T of < 100°C from preheat to peak body tem-

perature under SM reﬂow, wave solder and hand solder

process conditions to avoid cracking of the internal dielec-

tric layers. Components with the highest values of dielec-

tric constant and largest numbers of dielectric layers are

most susceptible to thermal shock and impact stresses.

Shorts occur due to silver electrode metal and end termina-

tion metallization migration under temperature/ humidity/

bias stress. Reducing the volume of the solder ﬁllets or

narrowing the pad sizes to less than the component lead

width.

Opens occur in the component body due to damage from

mechanical stresses such as overload due to vibration,

shock or ﬂexure of the PWA during service life.

Opens occur in the component body and in the body-to-

termination interface due to damage from mechanical

stresses under post-soldering stresses such as twisting, ﬂex-

ing or shock and vibration during assembly operations such

as TH component insertion, depaneling or testing. Identify

the high stress areas of the printed board and avoid placing

large susceptible components in these areas. Minimize

PWA ﬂexure by providing adequate support and vibration

damping in the ﬁxturing.

Hard pick and place tooling can cause mechanical shock

during SM pick and place; test probes landing on compo-

nent body or component terminations may also result in

mechanical shock damage during testing. These tooling

shock cracks generally occur in the middle of the compo-

nent, although test probes can cause cracking and spalling

at the impact site.

The EIA 1210 size should be an upper bound for reﬂow

attachment of leadless ceramic and ferrite components to

polymer-glass substrates; these components include lead-

less chip carriers (LCCs), multilayer ceramic capacitors

(MLCCs), chip resistors, inductors and networks. At or

above this size, compute a solder joint ‘‘Figure of Merit’’

to evaluate the impact of package size and lack of lead

compliancy on solder joint reliability on glass-epoxy sub-

strates. For speciﬁc guidelines, see Appendix A.

TH versions of MLCC (disc caps; dipped rectangular; axial

leaded ceramic) may demonstrate weakening of internal

low temperature solder connections due to assembly sol-

dering heat.

Be wary of suppliers of ‘‘precision’’ MLCC based compo-

nents who adjust the value of their part by abrading or oth-

erwise removing dielectric and conductor material; the

commonly used plastic coating is not a reliable replace-

ment for the original ceramic layers and silver migration is

likely. In ‘‘hi-k’’ capacitors, dissipation factor and dielectric

constant are affected by DC voltage, AC voltage, and tem-

perature.

For conductive adhesive attachments to SM components,

do not use nickel, tin and lead for ﬁnal component termi-

nation ﬁnishes; the oxides of these metals are not conduc-

tive. Oxygen and water can diffuse through epoxy and oxi-

dize the underlying component termination ﬁnish. The

initial ESR of nickel terminated MLCC capacitors is higher

when epoxied than when soldered; the Q factor is lower for

epoxied devices. The increase in ESR of these MLCC

capacitors was also signiﬁcantly higher after aging at

100°C/100% RH for 4 hours or at 155°C for 16 hours.

Epoxy cure temperatures higher than the melting point of

tin-lead solder will cause the solder to ﬂow away from the

cured epoxy interface. Epoxies are expected to be more

elastic compared to solder, but −55/+125°C temperature

cycles with 10 minute dwells showed similar fatigue curves

out to 100 and 1000 cycles for both epoxied and soldered

capacitors. Silver oxide is conductive and gold and palla-

dium do not tarnish or oxidize; use silver and these noble

metals, historically used for hybrid component termination

ﬁnishes.

F-8.2 Plastic Film Capacitors Excess body temperatures

during SM reﬂow will result in dielectric ﬁlm softening

and dielectric ﬁlm thinning with:

• loss in voltage capability

• increase in capacitance and leakage current

• increase in intermittent or permanent shorts

• increase in ESR may increase

In the extreme case, the result may be intermittent internal

solder joints.

IPC-D-279 July 1996