Low Voiding Reliable

Solder Interconnects for

LED Packages on Metal

Core PCBs

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Outline/Agenda

• Introduction

• Assembly

• Components

• Materials

• Process

• Results of Experiments

• Conclusions

• Q & A

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Introduction – LED Packages

• Lumen maintenance requirement for LED

packages used in commercial & outdoor

residential lighting is 70% lumen maintenance for

50% of population after 35,000 hours and 3 year

warranty (Energy Star A; IESNA LM-80)

• Good thermal management is key to long term

stability of light output (brightness) and color

temperature of LED lights

– Solder joints with low voiding for low thermal

resistance

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Package Assembly

• For this study used commercially available LED

package, metal core boards and solder pastes

– Luxeon Rebel LED

– Metal Core PCB substrate w/ Aluminum core, Copper

circuitry and 2 dielectric types

– 4 solder pastes – Pb-free; no clean; zero halogen; 4

alloy types

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LED Package

• Surface mount package w/ ceramic substrate & metal interconnect

• Solderable to next level substrate - bottom side with thermal pad under LED, and (2) pads for electrical connections

• Hard silicone lens

over LED

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Metal Core PCB (MCPCB) • Aluminum core; Copper

circuitry and (2) Dielectric types

– Dielectric A: High thermal

conductivity for good conduction of heat into metal core

– Dielectric B: Low modulus for reducing solder joint strain, especially for assemblies w/ large CTE mismatch between package and board

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Solder Pastes • All with Type 3 lead-free solder powder; no-clean; zero

halogen

ID Alloy Attribute

A SAC305 Broad application range

B Maxrel Superior creep resistance,

temp cycling / vibration

performance

C SACX Plus

0807

Reduced Ag for lower cost

D SnBiAg Low melting point (<140°C)

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Assembly Process & Testing

• Assembled 36 LEDs per board; (5) boards per

solder paste and dielectric type

• Standard SMT equipment – printing, P/P, reflow

• Solder paste printing: 5 mil stencil, on-contact

printing at 2.54 cm/sec; 268 gr/cm pressure;

0.051 cm/sec stencil release

• Reflow in air with high soak profiles

• % Voids by Xray; 50% solder joints per board

measured

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Reflow Profile Test • Pre-screening test with high soak & straight ramp

profiles, SAC305 paste & Type B dielectric MCPCB

board

Str RampHigh Soak

25

20

15

10

5

0

Profile

% V

oid

s

5.875.71

Boxplot of % Voids - MCPCB with Type B Dielectric; SAC305 Paste

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Reflow Profile – SAC & Maxrel Alloy Pastes

150-200°C, 115 sec soak; 240°C peak; 67 sec TAL

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Reflow Profile – SnBiAg Alloy Paste 100-110°C, 75 sec soak; 175°C peak; 60 sec TAL

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Test Results

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Main Effects Plot for Dielectric & Paste Alloy

BA

20.0

17.5

15.0

12.5

10.0

7.5

5.0SnBiAgSACX0807SAC305Maxrel

Board Dielectric

Me

an

Paste Alloy

Main Effects Plot for % VoidsData Means

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% Voids vs Board Dielectric

BA

35

30

25

20

15

10

5

0

Board Dielectric

% V

oid

s

10.80510.465

Boxplot of % Voids

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% Voids vs Paste Alloy & Dielectric

Paste Alloy

Board Dielectric

SnBiAgSACX0807SAC305Maxrel

BABABABA

35

30

25

20

15

10

5

0

% V

oid

s 20

Boxplot of % Voids

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Void Sizes

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

% o

f J

oin

ts

ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%

Void Size as % of Joint Area

LED Assembly Voids Sizes

B Diel/ SAC305

A Diel/ SAC305

A Diel/ SnBiAg

B Diel/ SnBiAg

A Diel/ Maxrel

B Diel/ Maxrel

A Diel/ SACX0807

B Diel/ SACX0807

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Average Void Sizes

LED Assembly Average Void Size

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

SAC305 Maxrel SACX0807 SnBiAg

Paste Alloy

Averg

e V

oid

Siz

e (

% jo

int

are

a) Dielectric A

Dielectric B

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Maximum Void Sizes

LED Assembly Max. Void Size

0.0

5.0

10.0

15.0

20.0

25.0

30.0

SAC305 Maxrel SACX0807 SnBiAg

Paste Alloy

Ma

x. V

oid

Siz

e (

% jo

int

are

a) Dielectric A

Dielectric B

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Typical Xray Voids Images

Paste Alloy/

Dielectric Type

SAC305 Maxrel SACX

0807

SnBiAg

MCPCB

Dielectric A

%Voids 9.3 15.8 10.0 11.6

MCPCB

Dielectric B

% Voids 7.2 12.6 12.3 12.3

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Summary/Conclusions • (4) different metal alloy solder pastes were evaluated for % voids in

the reflowed joints formed between a LED surface mount package

and MCPCB substrates having (2) different dielectric types

• Overall, the board dielectric type had little effect on the solder joint %

voids, whereas the solder alloy had a more significant effect

• The Type A dielectric boards resulted the lowest average and

maximum void sizes, for all pastes, and the SnBiAg paste resulted in

the smallest void sizes

• Overall, >90% of the solder joints had void sizes ≤4% of the solder

joint area and less than 20% voids on average

• The SAC305 alloy paste combined with the Type B board dielectric

resulted in the lowest % voids (<8.5%)

• The boards from this study will be subjected to further tests including

electrical/optical, die shear, thermal cycling/shock, and solder joint

characterization by cross-sectioning

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For More Information Please Contact:

Ravi Bhatkal – VP, Energy Technologies

Rbhatkal@cookson.com

+1-908-791-3013

Amit Patel – Technology Analyst

Apatel@cookson.com

+1-908-791-3051