Printed circuit boards for lead-free soldering: Materials and failure mechanisms

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IVF Industrial Research and Development Corporationwww.ivf.se

Temadag Miljö och Elektronik12 April 2007 Oslo

Per Johander, Per-Erik Tegehall, Abelrahim Ahmed Osman, Göran Wetter and Dag Andersson

IVF Industrial Research and Development Corporation Argongatan 30, SE 431 53 Mölndal Sweden Tel +46 31 706 6142, Fax +4631-276130

Printed Circuit Boards for Lead-Free Soldering: Materials and Failure Mechanisms

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CCH3

CH3

OHHO CH2 ClCHO

CH2+

Bisphenol-A Epichlorhydrin

Epoxy

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EpoxyThe properties are determined by the:• Resin type• Crosslinking Agent• Degree of crosslinking Tg• Thermal decomposistion temperature of the resin Td• Adhesion between resin and reinforcement• Adhesion of resin to metal foil• Additive fillers CTEThe resin types used for laminates today are mostly based on di-,

tetra- or multifunctional epoxy resins. The curing system is dicyandiamide, novolac or hydroxyl.

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Wei

ght L

oss

Decomposition Temeperature

Temp

Td

CTE

-Z30-70 ppm

150-300 ppm

Tg

Glass TrasitionTemperature

Temp

Two critical temperatures Tg and Td

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CH2 CH2

CH2O CHO

CH2 CH2O CHO

CH2 CH2O CHO

CH2

Epoxy Phenolic Novolac Resinn

CH2O CHO

CH2

CH2O CHO

CH2

CHHC

CH OCHO

CH22

CH OCHO

CH22

Tetra functional Epoxy Resin

HC

CH OCHO

CH22

Tris functional Epoxy Resin

CH2O CHO

CH2

CH2O CHO

CH2

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New Halogen free Reactive flame retardants

OO PH

DOPO

OOPO

NN

NNH

NH 2

Diphenyl phosphato benzo guanamin

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Polymerisation reaction Epoxy - DOPD

CCH3

CH3

CH2O CHO

CH2CH OCHO

CH22

OO PO

CCH3

CH3

CH2O CHO

CH2CH OCHO

CH22

HCH2CH

OCH2

HO

CCH3

CH3

CH OCHO

CH22

OO PO

HH

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Failure DescriptionConductive Anodic Filament (CAF) growth is a conductive copper-containing salt created electrochemically that grows from the anode toward the cathode subsurface along the epoxy/glass interface. CAF is associated primarily with mechanically drilled holes where the mechanical drilling disrupts the glass reinforcement fibers in glass bundles permitting the absorption of subsequent processing chemistry between fibers and epoxy.

From IPC-9691, User Guide for the IPC-TM-650, Method 2.6.25, Conductive Anodic Filament (CAF) Resistance Test (Electrochemical Migration Testing).

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Electrochemical ReactionsReactions at the anode:H20 → ½O2 + 2H+ + 2e-

Cu → Cuz+ + ze-

Reactions at the cathode:2H20 + 2e- → 2OH- + H2

½O2 + H20 + 2e- → 2OH-

(Cuz+ + ze- → Cu)The reactions are basically the same that occur during dendrite formation except that reduction of copper ions may not occur.

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CAF formed between Via Holes

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CAF formed between Conductors

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Standard for CAF Testing

IPC-TM-650, Method 2.6.25, Conductive Anodic Filament (CAF) Resistance Test: X-Y Axis.The test board shall have alternating rows of connected vias. Section A has in-line hole to hole and Section B has staggered hole to hole.

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Test Patterns on Section A

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Pattern 1via to via270 μmVertical

Pattern 2via to via510 μm Vertical

Pattern 3via to via270 μm

Horizontal

Pattern 4via to via510 μm

Horizontal

Pattern 5BGA

34x34Full array

1,0 mm pitch

Pattern 6BGA

34x34Full array

1,0 mm pitch

A B C D E F G H J K L

Layout component side Vertically viasPatterns 1 and 2

Horizontal viasPatterns 3 and 4

Layers 2 and 4 BGA components.

Layers 3 and 5, BGA components

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Acceptance Criterion for Method 2.6.25

• A more than a decade drop in the insulation resistance after the stabilization time.

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Factors Affecting CAF Formation• Humidity

• Spacing

• Voltage

• Ionic contamination

• Mechanical stress, for example during drilling and soldering

• Glass to resin bond strength

• Via to glass fibre alignment, warp or fill direction

• PCB properties

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Test Conditions in Method 2.6.25• 65 ± 2°C or 65 ± 2°C

• 87 +3/-2% RH

• One Mohm current limiting resistors in series for each current path

• 96 h stabilization at test conditions prior to applying bias

• Test duration minimum 500 h

• Bias two times the maximum operating voltage

• 100 VDC test voltage with same polarity as bias

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Board designation

No. of boards

Conditions

Boards A 11 MSL 3, 30°C and 60% RH for 192 h

Boards B 5 85°C and 85% RH for 1068 h + bake at 125 °C for 12 h

Boards E 6 85°C and 85% RH for 984 h + drying in a cabinet at 46°C and 4% RH for 1752 h

Boards F 6 MSL 1, 85°C and 85% RH for 168 h

Experimental matrix for absorption and desorption tests.

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Board designation

No. of boards

Preconditioning Soldering

Boards A 11 MSL 3, 30°C and 60% RH for 192 h

3 lead-free soldering processes

Boards B 5 85°C and 85% RH for 1068 h + bake at 125 °C for 12 h


Boards C 4 None 3 lead-free soldering processes

Boards D 4 None None

Experimental matrix for the CAF test

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0

0,1

0,2

0,3

0,4

0,5

0,6

0 50 100 150 200 250Time (h)

Wei

ght %

JEDEC level 1

JEDEC level 3

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 500 1000 1500 2000 2500 3000Time (h)

Wei

ght %

Dry Cabinet 46°C 4% Rh

Bake 125 °C

85 °C 85% Rh

Weight increase/decraese due to humidity absorption/desorption in the printed circuit boards at various conditions

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a)

b)

(a) SAM and (b) X-ray images of a test board after exposure to JEDEC level 1 followed by 3 reflow soldering cycles.

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Via hole resistance - Boards A

0

1

2

3

4

5

6

7

8

9

0,00 100,00 200,00 300,00 400,00Time (h)

log

R

Pattern 1 Vertical 270 µm Pattern 2Vertical 510 µmPattern 3Horizontal270 µm Pattern 4Horizontal510 µm

MSL 3, 30°C and 60% RH for 192 h3 lead-free soldering processes

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Via hole resistance - Boards B

0

1

2

3

4

5

6

7

8

9

0,00 100,00 200,00 300,00 400,00Time (h)

log

R

Pattern 1 Vertical 270 µm

Pattern 2Vertical 510 µm

Pattern 3Horizontal270 µm


85°C and 85% RH for 1068 h + bake at 125 °C for 12 h3 lead-free soldering processes

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Via hole resistance - Boards C

0

1

2

3

4

5

6

7

8

9

10

0,00 100,00 200,00 300,00 400,00Time (h)

log

R





No humid pre conditioning


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Via hole resistance - Boards D

0

1

2

3

4

5

6

7

8

9

10

0,00 100,00 200,00 300,00 400,00Time (h)

log

R





No humid pre conditioning

No lead free soldering

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Resistance between Layer 2 & 4 and via holes

0

2

4

6

8

10

12

0,00 50,00 100,00 150,00 200,00 250,00 300,00 350,00 400,00Time (h)

Log

Res

ista

nce

Boards D

Boards C

Boards A

Boards B

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Resistance between Layer 3 & 5 and via holes

0

2

4

6

8

10

12

0,00 50,00 100,00 150,00 200,00 250,00 300,00 350,00 400,00Time (h)

Log

Res

ista

nce

Boards D

Boards C

Boards B

Boards A

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Photographs of CAF (from IPC 9691)

Source: IPC-9691

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O

Epoxy Glass Fiber

Si

C

H2O

H2OO

C

O

Si

C

O

C

H HO

Epoxy Glass Fiber

Si

C

O

Si

C

O

Si

C

O

Si

C

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Printed circuit boards for lead-free soldering: Materials and failure mechanisms