IPC-D-279 EN.pdf - 第70页

Appendix C Design for Reliability (DfR) of Insulation Resistance C-1.0 INSULATION RESISTANCE DAMAGE MECHA- NISMS AND FAILURE The damage mechanisms work generally in two distinct regions: at the surface and in the bulk of…

Interconnecting and Packaging Electronic Circuits,

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IPC-D-279 July 1996

Appendix C

Design for Reliability (DfR) of Insulation Resistance

C-1.0 INSULATION RESISTANCE DAMAGE MECHA-

NISMS AND FAILURE

The damage mechanisms work generally in two distinct

regions: at the surface and in the bulk of the electronic

assemblies, particularly the printed board. It has been

reported, that surface and bulk phenomena exhibit different

time constants in the response to temperature changes [Ref.

C-7: 1]. The insulation resistance for a sample circuit is the

measured integrated effect of both surface and volume

resistivity as deﬁned by ASTM [Ref. C-7: 2]. The mea-

sured bulk resistance will depend upon the nature of the

laminate, solder mask and/or conformal coating under

investigation. It will also depend upon the degree of cure

of the polymers and for printed boards on the quality of the

drilling process for the plated-through holes (PTHs) and

vias (PTVs), and will be affected by soldering ﬂux/paste

residues if they dissolve into the polymeric material during

the soldering and/or cleaning processes.

Insulation resistance measurements provide important data

in the characterization of printed board laminates, multi-

layer boards (MLBs), soldering ﬂuxes, solder masks, and

conformal coatings. Such measurements have been used to

study the effect of aging at accelerated conditions (tem-

perature, humidity and/or bias voltage) to determine any

detrimental effects on the reliability of the product.

Ohm’s law states that the magnitude of the current, I, ﬂow-

ing through a circuit with a given resistance, R, is a linear

function of the applied voltage, V, such that

V = IR (Eq. C-1)

The resistance is an extrinsic property of the material

sample dependent on the resistivity of the material and the

geometry of the sample. The resistivity, ρ, of a material is

an intrinsic material property. The resistivity is determined

from the length, l, of the sample and its cross-sectional

area, A, and is related to R by

ρ=R

(Eq. C-2)

Resistivity that measures the resistance to current ﬂow

through the bulk of a sample it termed volume resistivity,

= R

(Eq. C-3)

where t = the thickness of the bulk sample.

Surface resistivity, ρ

, measures the ability of an insulator

to resist the ﬂow of current on its surface, such that

= R

(Eq. C-4)

where A

= the surface area and l is the length of the insu-

lating strip.

C-1.1 Surface Insulation Resistance (SIR) Surface insu-

lation resistance (SIR) measurements will depend on the

nature of the surface contamination and the amount of

moisture present during the measurement. Although SIR

readings are a combination of both bulk and surface resis-

tance, 99.9 % of the current leakage for FR-4 epoxy/glass

laminate will occur on the surface of the laminate, since the

ratio of the surface resistivity to the volume resistivity is

1:1000 [Ref. C-7: 3]. Test patterns for measuring volume

resistance will either use electrodes on the top and bottom

of the substrate (for z-axis measurements) or PTVs to PTV-

patterns for measuring the x, y-resistance. SIR patterns are

typically interdigitated comb patterns such as the IPC-B-24

coupon.

Some materials, notably polyglycol, act to reduce insula-

tion resistance by absorbing water and/or forming a mono-

layer of water at lower than saturation humidity, e.g. 75%

RH [Ref. C-7:4]

C-1.2 Electrochemical Corrosion Electrochemical cor-

rosion of metallic conductors and the migration of metal

ions between anode and cathode on a printed board can

lead to circuit failure. It is important to understand the

cause of these failures in order to select materials and pro-

cesses for printed board manufacture, soldering, and clean-

ing which will minimize the occurrence of these failures.

The tendency of the metal conductor to migrate under a

bias voltage in humid conditions has been shown to

decrease across the following series of metals [Ref. C-7: 5]

Ag >Pb>Solder>Cu (Eq. C-5)

Those metals whose hydroxides are more soluble at pH 7-9

have a higher migration rate. This is related to the pH gra-

dient between the anode and cathode when a ﬁlm of mois-

ture is present.

In the case of SIR testing, the cathode is connected to the

high voltage source while the anode is connected to

ground. For electrochemical migration to occur, the path-

way must exist for ions to move from the anode to the

cathode. In the presence of moisture, the following electro-

chemical reactions can occur at the anode

O →

+2e

−

Cu → Cu

−

Cu → Cu

+2e

−

Pb → Pb

+2e

−

Sn → Sn

+2e

−

Sn → Sn

+4e

−

(Eq. C-6)

July 1996 IPC-D-279

The preferred species in the case of copper will depend on

the anion that is present. In water Cu

is the preferred

species except when C1

−

is present. In this case the forma-

tion of the CuCl

2−

will favor the formation of Cu

rather

than Cu

[Ref. C-7: 6].

At the cathode the possible reactions are

O+2e

−

→ 2OH

−

O+2e

−

→ 2OH

−

+2e

−

→ Cu

+2e

−

→ Pb

+2e

−

→ Sn (Eq. C-7)

C-1.3 Dendrite Growth In normal electrochemical den-

dritic growth, electrolytic dissolution of the metals occurs

at the anode and reduction of the metal ions by plating out

occurs at the cathode. Typical dendrites associated with a

solder-coated copper comb pattern will be lead-needles

with some tin. These appear as ‘‘tree-like’’ dendrites which

begin at the cathode. As these surface dendrites grow, their

effect on the total SIR reading is minimal until they are

very close to the anode. At the point of bridging, the den-

drite will burn out quickly due to the high current density.

For this reason, the presence of dendrites is not easily

determined by the electrical SIR readings. These readings

are not taken frequently enough to insure that a measure-

ment will be taken exactly when the dendrite bridges.

Thus, it is customary to examine SIR samples under the

microscope with back lighting after the test is terminated to

visually observe if dendrites have formed.

C-1.4 Conductive Anodic Filaments (CAF) In the late

1970 ’s a new electrochemical migration failure mechanism

was reported by Bell Laboratories [Ref. C-7: 6]. During the

study of potential failure modes associated with high volt-

age switching applications, the subsurface formation of

conductive ﬁlaments along the glass/epoxy interface was

observed at high humidity conditions under application of

a high (200-500 V) voltage. This conductive anodic ﬁla-

ment (CAF) formation involved the dissolution of copper

at the anode and the formation of a copper-containing con-

ductive ﬁlament along the glass/epoxy interface. It was

reported that these CAF contain copper associated with

chlorine or sulfur [Ref. C-7: 8]; these contaminants are

associated with the board manufacturing process. Others

have observed only chloride- or bromide-containing copper

ﬁlaments [Ref. C-7: 9]. It has been suggested that moisture

causes hydrolysis at the glass/epoxy interface [Ref. C-7: 7].

It is postulated that the absorption of moisture by the epoxy

causes swelling which can lead to a debonding between the

epoxy and the glass ﬁber [Ref. C-7: 10]. This moisture-

induced debonding can be accelerated with damage to the

bond between the glass ﬁbers and the surrounding epoxy

during the drilling process [Ref. C-7: 11]. A capillary of

moisture at these damaged interfaces is available for the

electrochemical reactions described in Eqs. C-6 and C-7

when a bias voltage is applied.

C-2.0 INSULATION RESISTANCE MODELING

In order to make predictions as to the behavior and reliabil-

ity of electronic assemblies in environments with varying

levels of temperature and humidity, data from representa-

tive accelerated high stress tests are used to extrapolate to

milder operating conditions. This extrapolation requires an

appropriate valid extrapolation model. It is not a simple

task to obtain either good accelerated test data or appropri-

ate extrapolation models. [Ref. C-7: 12].

C-2.1 Insulation Resistance Degradation SIR measure-

ments are a relatively quick way to evaluate the interaction

of processing materials with a given substrate. Reliability

assessment, however, requires signiﬁcantly more effort.

Assumptions must be made about the relationship of the

operating and use environments to the accelerating test

conditions with elevated temperatures, humidities, and bias

voltages chosen to accelerate the rate of degradation by

known mechanisms without introducing extraneous dam-

age mechanisms. If the assumptions are correct, extrapola-

tion of the results from the accelerated tests back to oper-

ating conditions will provide an estimate of the product

reliability.

To accomplish this task, a statistically signiﬁcant number

of samples must be processed and tested to failure at dif-

ferent accelerated test conditions. The effect of the tem-

perature on the rate of failure follows for these processes

an Arrhenius relationship [Ref. C-7: 13]

= k

exp

(

−E

)

(Eq. C-8)

where

= the reaction rate at a given temperature,

T = absolute (Kelvin) temperature,

= a constant,

= the activation energy of the reaction, and

k = Boltzmann’s constant.

The effect of relative humidity can be described by

expC(RH)

(Eq. C-9)

where

= the reaction rate at a given relative humidity,

= a constant,

C = described as a constant but is likely temperature

dependent (see Eq. C-10), and

b = an exponent empirically observed in the range from

1 [Refs. C-7: 1,8] to 2 [Ref. C-7: 14].

For tests with a bias voltage of 52 V, the insulation resis-

tance, IR, was found to have the following dependence on

temperature and humidity [Ref. C-7:1]

IPC-D-279 July 1996