IPC-D-279 EN.pdf - 第63页

high-cycle regime as per Reference B-7:12 need to be per- formed in order to obtain an indication of the changes in the material properties from either strain-hardening or soft- ening during high-cycle fatigue. By using …

plating conditions. Under these conditions, in combination

with the non-uniform plating current densities that also get

worse with increasing aspect ratios, the copper deposit

quality rapidly deteriorates with increasing aspect ratios.

The resulting copper deposits within the PTV can have

signiﬁcantly lower ductility and strength than the copper

deposits plated at the same time external to the PTVs, e.g.

on plating mandrels. The decline in physical properties is

frequently accompanied by increased ‘dog-boning’ and

nodule formation as well [Ref. B-7: 10].

The copper deposits from standard acid copper baths in

high-aspect-ratio PTVs, even with uniform plating in the

PTV barrels and good intrinsic tensile properties, as deter-

mined by testing foil specimens from ﬂat plating mandrels,

perform only marginally in thermal cycling tests. For this

reason, special plating solutions have been developed, that

allow reduced plating current densities at the expense of

increased plating times, but producing signiﬁcantly

improved PTV copper barrel reliability.

B-1.1.2 Pyrophosphate Copper Plating Pyrophosphate

plated copper was unfortunately not part of the IPC round

robin studies [Refs. B-7: 1, 2]. It has however been shown,

that pyrophosphate copper is less susceptible to the effects

of non-uniform plating current densities and higher PTV

aspect ratios.

B-1.2 Material Properties

B-1.2.1 Tensile Properties

The tensile properties of the

PTV copper deposits are very important, both for the per-

formance of the PTVs during subsequent processing and

use, and for the DfR for the PTVs. The properties that are

needed are: (1) the tensile strength, (2) the yield strength,

(3) the modulus of elasticity, (4) the modulus of plasticity,

and (5) the fracture ductility.

It has also been found, that the electrolytically plated cop-

per deposits have a modulus of elasticity signiﬁcantly

below that of cast and rolled copper reported in material

property references. It therefore is necessary to measure the

modulus of elasticity, which can be done during the tests to

determine the yield and tensile strengths of the deposits.

The tensile strength, the yield strength, the modulus of

elasticity and the modulus of plasticity can be determined

from tensile tests. To properly determine the modulus of

elasticity, the ‘interrupted tensile test’ method should be

utilized.

It needs to be noted however, that these tensile properties

come from samples plated onto ﬂat stainless steel mandrels

and only set the upper bounds for the strength and ductil-

ity of the copper deposit inside the PTV. The evidence is

circumstantial, but very strong, that these properties are

signiﬁcantly degraded inside of high-aspect-ratio (board

thickness/PTV-diameter) PTVs [Ref. B-7: 2].

B-1.2.2 Ductility The tensile elongation is a very inaccu-

rate and subjective test for foil samples because of the

specimen geometry and the dependence on the test condi-

tions [Ref. B-7: 10, 11]. Tensile elongation for foil materi-

als is adequate for quality control and comparison pur-

poses, but it signiﬁcantly underestimates the fracture

ductility of the material by about a factor of three (3) and

gives the false indication of a ductility dependence on foil

thickness.

It is for these reasons that the test cited in References

B-7:12 and 13, was developed. This test has a high dis-

crimination power in terms of quality variations of the cop-

per deposit, thus being very valuable as a process control

tool as well as providing direct input for the all-important

ductility of the copper deposit.

In order to assess the quality of the plated copper deposits,

foil samples plated onto mandrels need to be subjected to

fatigue ductility testing [Ref. B-7: 12].

It needs to be noted however, that this ductility comes from

samples plated onto ﬂat stainless steel mandrels and only

set the upper bounds for the ductility of the copper deposit

inside the PTV. The evidence is circumstantial, but very

strong, that this property is signiﬁcantly degraded inside of

high-aspect-ratio (board thickness/PTV-diameter) PTVs

[Ref. B-7: 2].

The ductility of the copper deposit in the PTV barrel can

be determined from the performance of the PTVs in accel-

erated testing resulting in low-cycle fatigue. In Table B-1

the results from two of the tests used in the Reference

B-7:2 applied to coupons from the same sample are given

together with the stresses and strain ranges resulting from

the thermal cycling/shock excursions. Also given is the

minimum ductility resulting from the ﬁrst failures in the

tests using Eq. #1 in Section B-2.0 together with the initial

tensile strength.

The measured mandrel ductility was about 30%. Thus, the

ductility in the PTV barrels as given in Table B-1 indicates

a signiﬁcantly lower deposit ductility as compared to the

ductility of the deposit on the MLB surface.

B-1.2.3 Fatigue Behavior It has been found that

annealed electrolytic acid copper strain-hardens upon the

application of cyclic strain loads during fatigue tests [Ref.

B-7: 11]. This results in high-cycle fatigue lives that are

longer than expected based on the initial material proper-

ties.

Conversely, annealed pyro-phosphate-plated copper strain-

softens as the result of the application of the cyclic strains

during the fatigue testing. Therefore, fatigue life tests in the

July 1996 IPC-D-279

high-cycle regime as per Reference B-7:12 need to be per-

formed in order to obtain an indication of the changes in

the material properties from either strain-hardening or soft-

ening during high-cycle fatigue. By using larger bend man-

drel diameters than are used for the fatigue ductility test,

the samples are subjected to high-cycle fatigue to assess

the material behavior of the copper plating under extended

fatigue loading.

B-1.3 Damage Mechanisms and Failure

B-1.3.1 PTV Quality

The quality of the PTVs, together

with the severity of the thermal expansion loading, is the

critical aspect in the reliability of PTVs.

Even good quality PTVs will eventually fail and will typi-

cally do so as the result of a PTV barrel fracture near the

center of the barrel as is illustrated in Fig. B-1.

PTV failures can also occur as the result of PTV shoulder

fractures (see Fig. B-2) and internal land fractures. Failures

of these types typically are the consequence of an inad-

equate material choice for the vendor copper foils [Ref.

B-7: 14] used for the laminates or some processing error.

Of the ∆T≈180°C thermal excursion during the soldering

process, ∆T≈100°C is below T

at a mismatch in the

Table B-1 Estimates of Tensile Properties of Copper Deposit Inside the PTVs [Reference B-7:10]

Test

∆T

[°C]

Fatigue Life

[cycles]

Barrel Stress

[MPa/ksi]

Strain Range

∆ε

max

(eff)

[%]

Copper

Strength

[MPa/ksi]

Minimum

Ductility

[%]

IEC Hot Oil

Thermal Shock

235 32 219/31.8 4.5 281/40.7 20.6

Temperature

Cycle/Shock

190 150 177/25.7 2.2 281/40.7 23.3

Figure B−1 Cross-Section Schematic of a PTV With a Barrel Fracture Near the Center of the MLB

IPC-D-279 July 1996

CTEs of ∆α≈20-69 ppm/°C, whereas ∆T≈80°C is above T

at a mismatch in the CTEs of ∆α≈168-331 ppm/°C. This

results, for a 1.60 mm thick MLB, in an expansion mis-

match between ~25 and 50 µm, providing the severe load-

ing conditions that can lead to fractures in the PTV barrels

or shoulders as well as the cracking of inner copper layers

near the MLB surface.

The most typical failure mode, however, is the fracture of

the PTV copper barrel, which is the primary subject here.

In Section B-2.0 on reliability modeling, an attempt is

made to distinguish between three different quality aspects

of PTVs. One of these is related to the quality of the PTV

walls as the result of the drilling and desmear processes,

and two to the plating quality in terms of the plating uni-

formity ‘dog-boning’ and the reduced material properties in

the center of the PTV.

B-1.3.2 Impact of Assembly Processes and ESS Proce-

dures

The most severe stress condition and threat to reli-

ability for PTVs takes place during the solder reﬂow pro-

cesses necessary to make circuit board assemblies. Because

of the large differences in the CTEs for the copper and the

MLB resin, the larger the temperature excursions the larger

is the resulting thermal expansion mismatch causing

stresses in the PTV copper barrel and shoulders.

The large temperature excursions during the soldering and

solder reﬂow processes combine with the difference in the

CTE between the glass-reinforced epoxy layers surround-

ing the PTV and the plated copper of the PTV barrel to

create tensile stresses in the copper barrel and bending

stresses at the PTV shoulder due to the PTV land rotating

as a result of the large z-direction (the direction perpen-

dicular to the plane of the MLB) expansion of the epoxy.

The CTE of FR-4 in the z-direction is typically 38-97

ppm/°C below the glass transition temperature, Tg, and

186-349 ppm/°C above T

[Refs. B-7: 15-20], whereas

electrodeposited copper foils have a CTE of about 17±2

ppm/°C.

Environmental Stress Screening (ESS) procedures, in order

to be effective, need to resemble solder reﬂow excursions

in their severity. Therefore, ESS cycles have a similar

impact as do solder reﬂow excursions.

It is during these excursions to solder reﬂow temperatures

during solder reﬂow operations or ESS procedures that

PTV barrel cracks can initiate due to overstressing and

Figure B−2 Schematic Cross-Sectional View of a PTV with a Shoulder Fracture in a printed board

July 1996 IPC-D-279