IPC-D-279 EN.pdf - 第97页

• Design for a patch of copper foil beneath the body of axial power > 2 W atts resistors to minimize charring of the printed circuit board during fault conditions. • At very high surface temperatures, radiation ef fec…

boron trioxide, B

into the silicone dioxide layer, form-

ing a boro-phospho-silicate glass (BPSG).

Corrosion and opens occur on IC bond pads where the MC

is delaminated, particularly in the corners of the chips in

PSMC. Corrosion-related opens occur on any bondpads in

TH packages where delamination, cracking or MC voiding

has occurred, particularly on bonding pads associated with

centrally located lead ﬁngers.

Intermittent opens have been observed in PSM, as well as

in PTH packages where the delamination or cracking

occurs at the tip of the lead ﬁnger, causing wedge bond lift

or wire break. Delamination can also occur at the ball

bond/pad site. The separation of the wedge bond from the

lead ﬁnger, the ball bond from the bond pad, or the broken

wire ends from each other are microscopic. The separation

may be very sensitive to changing temperature and is often

intermittent. The product may fail during temperature

ramps, but may be functional at the endpoint temperatures.

F-6.3 PSMC Delamination and Thermal Resistance

(

Degradation)

Delamination and debonding of the die attach paddle/heat

spreader from the MC results in heat transfer by conduc-

tion through the gases in the voided region with an increase

in the θ

. There is no data on the effect of various degrees

of delamination/ debonding on the magnitude of θ

. Fur-

ther the increase in θ

appears to be: greater for thinner

packages; greater for packages of thermally conductive

MC; expected to ﬁrst appear in the corners of the die attach

paddle; more signiﬁcant for the package surface where heat

is conducted to the printed board or to a heatsink by such

means as thermally conductive adhesive; more signiﬁcant

for those components where heat dissipation or chip tem-

perature uniformity is critical and where variations in these

parameters affect performance or reliability.

F-7.0 RESISTORS

When resistors overheat or fail, the printed board under-

neath is often thermally degraded (with color changes in

the laminate or coatings as well as decreases in resistivity

of the laminate) or even charred, leading to catastrophic

failure and smoke. The pyrolytically generated gases may

or may not ignite. Printed board damage may be minimized

by placing a copper pad under the resistor; the additional

copper reduces the peak temperature experienced by the

printed board during overload of the resistor.

Above the critical value of resistance (where maximum

rated power is dissipated at maximum rated voltage), use

voltage derating rather than power derating.

Through-hole (TH) resistors are generally spaced apart

both because of their bulk and their need for PTH lands on

the substrate. Surface mount (SM) resistors can be

squeezed together because of their small size and because

of the small surface area required for lands on the sub-

strate. This opportunity for higher spatial density is rarely

refused by the designer; the result is higher power density

on the substrate.

TH and SM resistors dissipate their heat primarily through

their leads, which are both electrically and thermally con-

ductive, to the substrate conductors and thence to the air.

Because of the reduced solder land area on the SMT sub-

strate, the joint temperature will probably be higher in the

SMT version of a TH product.

TH resistors have a relatively large surface area available

to dissipate the heat to the air; in contrast, the active area

of the SM device is relatively small. Inexpensive TH resis-

tors are generally metal or carbon ﬁlm. A relatively large

volume or mass of material is available to dissipate both

steady state and pulsating peak heat. Inexpensive SM resis-

tors are generally thick ﬁlm in nature and printed on one

surface of a rectangular ceramic (generally alumina) body

of low thermal conductivity.

The SM active element has a very small thermal mass; the

active area of the SMT component can rapidly respond to

increased power input with increased active element tem-

peratures. Some resistor suppliers provide ratings on their

product performance as a function of duration of stress at

given temperatures and during assembly processing. SM

resistor power dissipation ratings should be approached

with caution; the data is generally obtained by the manu-

facturer in an undeﬁned environment possibly described by

a single resistor and an environmental chamber with mov-

ing air of unspeciﬁed velocity. Thermal ﬁnite element

analysis (FEA) and IR scans are recommended for veriﬁ-

cation of hot spot location and magnitude. The presence of

multiple heat dissipating elements in close proximity

reduces the power dissipation allowable for any given ele-

ment. No SM resistor manufacturer has provided allowable

active ﬁlm or component surface temperature data.

F-7.1 A Checklist for Power Resistors

• Locate resistors for favorable convection cooling.

• Provide mechanical clamping or thermoset heat trans-

fer material to improve conductive heat transfer from

power resistors to heat sink or chassis.

• Use short leads whenever possible so that traces and

leads provide sufficient ‘‘heat dissipation’’ capability-

unless physical distance from the printed board is

required to minimize heat rise in the printed board.

• Individual power resistors over 10 cm long mounted

with axis horizontal to minimize hotspots along length;

average temperature of vertical and horizontal

mounted resistors is about the same.

• Groups of power resistors mounted with axes vertical.

Stagger resistors horizontally so that they don’t direct

hot airﬂows to their vertical neighbor.

July 1996 IPC-D-279

• 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