IPC-D-279 EN.pdf - 第112页

Silicon Controlled Rectiﬁer (SCR) with I O < 0.175 A at 100°C ambient temperature Precision V oltage Regulator Diodes: Line or Load V oltage Regulation < 0.5% Microwave Devices (Schottky barrier diodes, point conta…

Graceful recovery from any ESD induced glitch

Disable interrupts during critical periods such as backup

Token establishment entering subroutine; check token on

leaving SR

Memory/Registers

No-Op codes in unused memory locations

Error Traps

Unused interrupt locations trapped to error routine

Characterize CMOS for ‘‘latchup’’ susceptibility

H-2.2.2 Printed Board Design Guidelines

Reduce electromagnetic (EM) coupling of ESD

ﬁelds with

minimum loop area (Items with EMI are also used for EMI

purposes)

EMI As much power and ground plane as possible, rather

than conductors

EMI Power and ground grids, tied together with vias at

intervals < 60 mm, rather than conductors

EMI Power and ground conductors close together, rather

than spread apart

EMI No large hole (slot antenna) in power or ground

plane with dimension > 50 mm, particularly near

ESD ground point

EMI Vulnerable lines very close to ground or power con-

ductor; or above ground or power grid; or above

ground or power plane

EMI Unavoidable long conductors/loops treated by trans-

position

EMI Power and ground decoupled frequently with Multi-

layer Ceramic Capacitors (MLCC) [RF shunt or

bypass] ~60 mm apart

EMI ESD/chassis ground plane may be simple stamped

aluminum foil, 0.025 mm thick, or a laminated heat

sink or very large capacitor to inﬁnity

ESD charge injection

Keep ESD/chassis ground conductors separate from

circuit ground on printed board; join ESD/chassis

grounds beyond card-edge

Uninsulated portion of ESD/chassis ground on

printed board with length/width aspect ratio < 5:1

and separated from circuit conductors by more than

2mm

Uninsulated conductors/components, particularly

those with sharp edges, > 20 mm from user access

H-2.2.3 Components

Reduce EM coupling with short conductors

EMI Components close together

EMI Components with densest interconnections closest

together

EMI Common buss for power, ground, and signal fed

from center of printed board rather than edge of

printed board

I/O components as close as possible to the related I/O

connector

Keep susceptible components and their conductors in

those printed board areas with infrequent access;

avoid printed board edges

Keep components and conductors away from ‘‘ﬂoat-

ing’’ metal parts, particularly those with sharp edges/

burrs, e.g. screws, stampings

Tie structural metal parts to ESD/chassis ground; RFI

fences and boxes may be tied to analog ground; digi-

tal bypassed busses may be tied to logic ground

Panel components, such as LEDs, switches, latches

and keyboard separated from user by ESD/chassis

ground or guard ring

Use differential input/output transmitter-receivers for

rejection of ESD induced common mode noise

System RESET line NOT connected to long input

lines

Floating inputs tied with resistor either high or low

(do not use same resistor for all- See DfTestability)

Sensitive inputs ﬁltered/protected as close to IC as

possible

EMI Too much capacitance in one package adds series

inductance

EMI Ferrite beads not allowed to touch each other,

ground, power or signal lines.

EMI Lowest speed/ frequency/ rate of rise/fall components

practical

EMI Avoid edge triggered logic

EMI ASICs with output stages tailored for rate of rise/fall

EMI Connector separated from input circuits by ESD pro-

tection networks and EMI ﬁlter(s)

H-2.2.4 Cable

EMI Use shield 0.025 mm thick and preferably of 100%

coverage type and connect HF chassis ground to the

shield at both ends of the cable.

If necessary, use an MLCC (1- 10 nF) as a logic

ground

If necessary, add ferrite bead to signal line(s) at

receiver end

EMI If possible, do not add ferrite bead to shield ground

EMI As necessary, treat extra lines in cable: clip off or

remove or connect electrically in parallel.

H-2.3 CLASS 1: Sensitivity Range 0 to 1,999 Volts

Metal Oxide Semiconductor (MOS) devices, discrete,

including capacitors

Integrated Circuits (IC)

Very High Speed Integrated Circuits (VHSIC)

Charge Coupled Devices (CCD)

Surface Acoustic Wave (SAW) devices

Operational Ampliﬁers (OP AMP)

Junction Field Effect Transistor (JFET)

IPC-D-279 July 1996

100

Silicon Controlled Rectiﬁer (SCR) with I

< 0.175 A at

100°C ambient temperature

Precision Voltage Regulator Diodes: Line or Load Voltage

Regulation < 0.5%

Microwave Devices (Schottky barrier diodes, point contact

diodes, and other detector diodes), Frequency > 1 giga-

Hertz

Thin-Film Resistors

Thick-Film Resistors where the ESD ﬁeld across the ﬁlm >

2 kV/mm

Hybrids utilizing Class 1 parts

H-2.4 CLASS 2: Sensitivity Range 2,000 to 3,999 Volts

Devices or Microcircuits when identiﬁed by Appendix A

Test Data as Class 2

Metal Oxide Semiconductor (MOS) devices, discrete

Integrated Circuits (IC)

Very High Speed Integrated Circuits (VHSIC)

Operational Ampliﬁers (OP AMP)

Junction Field Effect Transistor (JFET)

Precision Resistor Networks (Type RZ)

Hybrids utilizing Class 2 parts

Low Power Bipolar Transistors, P

< 100 mW with IC <

100 mA

H-2.5 CLASS 3: Sensitivity Range 4,000 to 15,999 Volts

Devices or Microcircuits when identiﬁed by Appendix A

Test Data as Class 3

Metal Oxide Semiconductor (MOS) devices, discrete

Integrated Circuits (IC)

Very High Speed Integrated Circuits (VHSIC)

Operational Ampliﬁers (OP AMP)

Junction Field Effect Transistors (JFET)

Small Signal Diodes with power < 1 watt or IO < 1

Ampere

General Purpose Silicon Rectiﬁer Diodes

Silicon Controlled Rectiﬁer (SCR) with IO > 0.175 A at

100°C ambient temperature.

Low Power Bipolar Transistors with 350 mW<P/T/<100

mW and 400 mA>I/C/ >100 mA

Optoelectronic Devices (LEDs, Phototransistors, optocou-

plers)

Resistor Chips

Piezoelectric Crystals

Hybrids utilizing Class 3 parts

H-2.6 CLASS ‘‘4’’: Sensitivity Range 16,000 Volts

CONSIDERED NON-ESD SENSITIVE.

The above values are considered ‘‘default’’ values for the

part type; where vendor and part speciﬁc data exists and

where the database structure permits vendor speciﬁc data,

that data is to override the default value.

July 1996 IPC-D-279

101

APPENDIX I

Solvents

I-1.0 INTRODUCTION

Surface Mount Printed Wiring Assemblies (PWA) are sub-

jected to solvents (including water) and chemicals during

manufacture, rework, repair and service. These agents

include those used in soldering (alcohols, glycols and other

solvents in ﬂux vehicles at temperatures approaching

150°C), in cleaning the assembly after solder (saponiﬁers,

neutralizers, hot water, terpene, chloroﬂuorocarbon (CFC)

mixtures, hydrochloroﬂuorocarbon (HCFC) mixtures and

other halogenated solvents and blends at moderate process

temperatures, during removal of conformal coatings with

various chemicals, and during service (hydraulic and cool-

ing ﬂuids and fuels in military applications; alcohols and

halogenated hydrocarbons during cleanup). These solvents

and chemicals can adversely affect the solder mask (SM),

printed wiring board (printed board), conformal coating

(CC), printed board or component legends and markings as

well as degrade thin or mechanically stressed sections of

plastic components. Section 7 of Electronic Materials

Handbook, Volume 1, Packaging, 1989, discusses the vari-

ous conformal coating (CC) chemistries. Where the adhe-

sion of the CC, solder mask (SM) or marking to the under-

lying layer has been weakened (as evidenced by swelling

or wrinkling), abrasion or high velocity water or high

velocity solvent may lift the overlying material. Where the

intimate adhesion of the CC to the SM or printed board

laminate is disturbed between traces which are DC biased,

electrochemical corrosion and dendriting may occur under

moist environment conditions. See IPC-TR-476, IPC-SM-

840, and IPC-CC-830 for discussion on dendrites, solder

masks and conformal coating compatibilities. See also the

section in these guidelines on various other solder mask

and conformal coating issues.

Solvents under pressure, including water, can mechanically

remove or chemically dissolve lubricants needed for the

proper operation of switches, potentiometers and other

moving components.

During servicing, alcohols and halogenated hydrocarbons

may be applied to the PWA during cleanup. The PWA may

be exposed to hydraulic ﬂuids and fuels in military appli-

cations.

The D-limonene (terpene) based solvents have question-

able compatibility with, or are not recommended for, short

term contact at room temperatures with rubbers of nitrile,

ethylene-propylene, butyl, natural, neoprene or silicone

(which swells); plastics (and their alloys and blends) such

as polystyrenes (PS), polycarbonates (PC), polysulfones,

polyvinyl chloride (PVC), polyallomer, polyurethane

(PUR), ABS, low density polyethylene (LDPE), and poly-

vinylidene ﬂuoride (PVDF); nor with metals such as cop-

per and brass. D-limonene solvent can also weaken the

adhesion of printed board marking; this effect may be of

signiﬁcance when high velocity water and solvent washes

and rinses are employed. If D-limonene is trapped under

low clearance components, resulting in prolonged expo-

sure, softening of the conformal coating or solder mask and

degradation of the component plastic may occur. A general

rule of thumb is that a material which survives CFC and

chlorinated solvent cleaning and is water resistant should

be compatible with D-limonene.

The common halogenated cleaning solvents decompose at

high temperatures or in the presence of catalytic metal sur-

faces. Extremely high halide levels have been found in

droplets of water ﬂoating on the solvent surface in the cold

sump of vapor degreasing systems; in these cases, the

water absorbing cartridge had failed or the solvent stabili-

zation additives were exhausted. The halides deposit onto

assemblies which are ‘‘cleaned’’ in the cold sump. These

halogenated solvents can diffuse through the rubber seal of

aluminum electrolytic capacitors; the result is the dissocia-

tion of the solvent inside the component, the release of

HCl, the corrosion of the aluminum foil, and failure of the

capacitor. A solution is the use of capacitors where the

elastomeric seal is augmented by a hermetic, epoxy or

other polymer seal effective in greatly reducing the diffu-

sion rate of the solvent.

I-2.0 MATERIALS AFFECTED

Acrylate and epoxy solder mask materials should not be

exposed for long durations to solvents and solvent systems

containing methylene chloride, tetrahydrofuran, butyrolac-

tone, N-methyl 2 pyrrolidinone, d-limonene, ethylene gly-

col ether, propylene oxide glycol ether, methyl alcohol,

dimethylsulfoxide (DMSO) or dimethylformamide (DMF);

these are constituents of systems designed to remove con-

formal coatings based upon acrylate and epoxy chemistries.

DMF has been used as an electrolyte in aluminum electro-

lytic capacitors; its use has diminished because of its car-

cinogenic properties and its low ﬂash point. Butyrolactone

is used as an electrolyte in aluminum electrolytic capaci-

tors, replacing DMF. The susceptibility to solvents differs

between dry ﬁlm SM and liquid photoimageable SM mate-

rials and between epoxy and acrylate SM materials.

Conformal coating (CC) materials differ in their response

to solvents, depending upon their chemistry and curing

mechanisms. Some systems are cured by heat or drying

such as epoxy, acrylate, polyurethane, silicone, and ﬂuo-

ropolymer. Other CC materials are cured by ultraviolet

(UV) light and are based upon resin systems such as epoxy,

IPC-D-279 July 1996

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