IPC-D-279 EN.pdf - 第99页

Caution All variable resistors can suf fer 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’…

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

F-8.3 Solid Tantalum Capacitors Tantalum solid elec-

trolytics are preferred to aluminum electrolytics for stabil-

ity and reliability.

Caution

Excessive assembly soldering heat results in solder melting

and solder balls at internal connections. Some devices are

available in plastic encapsulation to limit internal heat

absorption during SM reﬂow.

Be aware of undamped reﬂections of switching transients

due to the parallel capacitance/series inductance geometry

on the power/ground distribution net of many PWAs. Visu-

alize the resemblance to the ideal conﬁguration for a delay

line with series inductors and high quality parallel capaci-

tors in the schematic and layout of SMT PWAs; the long

power and ground conductors contribute the series induc-

tance and high Q multilayer ceramic capacitors between

power and ground provide the parallel capacitance. At low

temperatures, silicon mobility increases and therefore

decreases the time rate of change of switching transients;

the edges get faster. Lossy electrolytic capacitors may have

to be applied between power and ground to dampen the

reﬂection of those switching transients.

Limitations include relatively high leakage current; voltage

range limited to 6 to 120 V. Only available polarized and

polarity must be observed; where handload or repair is

expected, mark the printed board to facilitate correct

insertion- this is also a DfM guideline.

Self healing effect of high leakage current on MnO

results

in lower incidence of shorts due to dielectric breakdown

but must use 3Ω/V current limiting resistor.

Opens due to poor solder or weld internal connections

which are damaged during vibration or shock.

F-8.4 Electrolytic Aluminum Capacitors Use where

large values are required and excess capacitance is allow-

able.

Caution

Low air pressure accelerates loss of electrolyte.

High body temperatures result in boiling of electrolyte, loss

of capacitance, increase in equivalent series resistance

(ESR), and spillage of possibly corrosive electrolyte. Some

styles are available in plastic encapsulation to prevent elec-

trolyte loss.

Storage at room or high temperature results in drying out

of electrolyte or corrosion of case. During storage, alumi-

num oxide dielectric electrochemically combines with elec-

trolyte and capacitance value decreases; connection

between electrode and aluminum dissolves in electrolyte

and opens. Operation at very severe derating conditions

also allows the aluminum oxides to ‘‘reform’’ and the

resulting capacitor is lower in voltage capability.

Low temperature can cause freezing of electrolyte.

Vacpk + DC < rated working voltage. Vacpk < DC Voltage.

No reverse voltage.

Rubber or elastomer seals cannot halt the ingress of com-

mon SMT cleaning halogenated solvents (and previously

after TH wavesoldering using the synthetic activated ﬂux/

Chloroﬂuoro-carbon cleaning combination). Rubber seal

degrades in halogenated solvent wash- the solvent is cata-

lytically degraded by the aluminum and the degradation

products include HCl. The HCl dissolves aluminium. This

halide corrosion is a major constraint on rework. Where

this kind of corrosion has occurred in OEM product, any

component cost savings were lost in the expense of the

recall, rework and repair of the product. Rubber-sealed alu-

minum electrolytics must be used in conjunction with non-

halogenated solvents, non-halogenated chemicals or water

wash to avoid the introduction of corrosives during manu-

facturing and factory rework or ﬁeld repair.

There are plastic encapsulated SM and TH versions of

rubber-sealed capacitors which seal out halides and seal in

dielectric ﬂuid.

Some aluminum electrolytics use the polar solvent

dimethyl-formamide (DMF)- a suspected carcinogen and a

solvent for and degrader of solder masks and epoxy con-

formal coatings. Better for environmental health and safety

purposes are aluminum electrolytic ﬂuids of gamma buty-

rolactone (GBL) or dimethyl acetamide which are also

polar solvents. Leaking polar electrolyte drastically reduces

the SIR of the PWA. Orient vent plugs of electrolytic

capacitors to minimize damage consequent to component

failure; face vent plugs away from substrate.

Reverse voltage results in burn out and opens.

F-8.5 Variable Capacitors Do not use variable capaci-

tors, if possible.

Caution

For greater stability, use air trimmer. Variable capacitors

require special precautions in manufacturing areas to avoid

contamination by the soldering and cleaning processes.

Sealed units require very tight process control by the sup-

plier to survive the rigors of wave soldering, 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 components which

have rubber or sliding seals around leads or shafts.

F-8.5.1 Variable Piston Capacitors Designs vary in

temperature stability.

July 1996 IPC-D-279