Performance of Pb-free Solder pastes at different

reflow profiles

Primary Objective

To analyze the defects of Pb-free solder pastes from two different manufacturers, subjected to three different reflow profiles, viz.,

In-Spec Profile Above-Spec Profile Below-Spec Profile

Secondary Objective

To come up with the above said reflow profiles based on the two different solder paste manufacturer’s specification sheets.

The two solder pastes selected were, Alpha OM-338-T Pb-free Indium 5.1 Pb-Free

Alpha Pb-free Specification

Ramp Rate : 0.8°C to 1.7oC/SecondSoak Temp : 130°C to LiquidusTAL : 35 - 90 sec Peak Temp : 232-250oC

Indium 5.1 Pb-free Specification

Ramp Rate : 0.5°- 2.0°C/SecondSoak Temp : 200°-210°CTAL : 30 - 90 sec Peak Temp : 229-260oC

Project Description

The above mentioned Pb-free solder pastes are subjected to three different profiles.

The performance of each paste is analyzed by examining the possible defects.

The defects are then superimposed on the reflow profile explaining the reasons for their occurrence.

Profiling Procedure

The paste and the reflow specification are collected from the paste manufacturer specification document.

The three different reflow recipes for the Pb-free solder pastes are then generated.

The zone temperature and the conveyor speed for each specification of each paste are noted down.

The thermocouple-attached copper coupons are used for profiling.

Alpha Reflow Statistics

Profile Ramp Rate(°C/Sec)

Soak Temperatur

e

TAL(Seconds)

Peak Temp (°C)

Alpha Above-Spec

2.8130°C to

217°C134.7 270.5

Alpha In-Spec

1.2130°C to

217°C64.3 241.4

Alpha Below Spec

1.1130°C to

217°C19.6 220.2

Zone Temperatures for Alpha Paste

Profile Conveyor

Speed Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone

6 Alpha

Above-Spec 38 153 187 212 249 268 281Alpha

In-Spec 36 86 116 148 193 235 256Alpha

Below Spec 35 65 94 124 164 201 240

Alpha Reflow Profiles

Indium Reflow Statistics

Profile Ramp Rate(°C/Sec)

Soak Temperatur

e

TAL(Seconds)

Peak Temp (°C)

Indium Above-Spec

1.7200°C to 210°C

124.5 268.1

IndiumIn-Spec

1.3200°C to 210°C

62.5 239.7

Indium Below-Spec

1.1200°C to 210°C

7 218.2

Zone Temperatures for Indium Paste

Profile Convey

or Speed Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

Zone 6

Indium Above-Spec 29 125 166 205 211 257 282

Indium In-Spec 31 104 139 173 217 201 254Indium

Below-Spec 35 84 110 130 161 190 240

Indium Reflow Profiles

Reflow Procedure

The Pb-free solder paste is pasted on the copper coupons using a 0.25 in diameter stencil aperture.

The first printed copper coupon was subjected to reflow with a Above-Spec profile recipe for both pastes.

The second printed copper coupon was subjected to reflow with an In-Spec profile recipe for both pastes.

The third printed copper coupon was subjected to reflow with a Below-Spec profile recipe for both pastes.

Reflow Procedure

The images of the copper coupons are taken after reflow.

The performance of the solder paste in all the three profiling conditions are analyzed by examining the possible defects using, Magnifying scope X-Ray

The images of the defects are captured and further analyzed.

Defects Analysis

Anticipated Defects

Solder Balling Colder Solder Voiding Blow Holes Non-Wetting De-Wetting Poor Wetting

Pin holes Porous Solder Disturbed Solder Solder Spatter Scratches Oxidation

Solder Balling

Small spheres of solder that remain on the board after reflow soldering adhering to the board.

Solder Balling was seen in Alpha Above-Spec Alpha Below-Spec Indium Above-Spec Indium Below-Spec

Indium Above-SpecIndium Above-Spec Alpha Above-SpecAlpha Above-Spec

Solder Balling

Solder Balling

Solder Balling

Alpha Below SpecAlpha Below Spec Indium Below-SpecIndium Below-Spec

Solder Balling

Solder Balling

Solder Balling

Solder Balling

The possible reasons for Solder Balling could be Preheat temperature too low / high Preheat ramp up rate high Excessive heating rate Solder paste viscosity too low Solder paste oxidation

Cold Solder

A solder joint with poor wetting and a grayish porous appearance is called as Cold Solder.

Cold Solder was seen in Indium Below-Spec

Indium Below-SpecIndium Below-Spec Indium Below-SpecIndium Below-Spec

Cold Solder

Soldered Region Cold SolderCold Solder

Cold Solder

The possible reasons for Cold Solder could be Insufficient heat In adequate cleaning Excessive impurities in solder Poor fluxing action Low cooling rate

Non-Wetting

A condition whereby a surface has contacted molten solder , but the solder has not adhered to all surface leaving the base metal exposed is called as Non-Wetting

Non-Wetting was seen in Indium Above-Spec Alpha Above-Spec

Indium Above-SpecIndium Above-Spec Alpha Above-SpecAlpha Above-Spec

Non-Wetting

Non-Wetting

Non-Wetting

The possible reason for Non-Wetting is Preheat temperature high Reflow Temperature too low Presence of physical barrier between surfaces to be

joined Solder paste oxidation

Dewetting

Dewetting is a condition that results when molten solder coats a surface and then recedes to leave irregularly-shaped mounts of solder and has an appearance of water on greasy surface.

Dewetting was seen in Indium Above-Spec Alpha Above-Spec

Indium Above-SpecIndium Above-Spec Alpha Above-SpecAlpha Above-Spec

Dewetting

Dewetting

Dewetting

The possible reason for Dewetting is Excessive Reflow temperature Excessive heating rate Board contamination

Flux Splatter

Flux Splatter is a condition where we find the flux is splattered around the solder, creating aesthetic issues.

Flux Splatter was seen in Indium Above-Spec Alpha Above-Spec

The possible reason for Flux Splatter is Improper process parameter (Heat / Moisture)

Indium Above-specIndium Above-spec Indium Above-SpecIndium Above-Spec

Flux Spatter

Flux Spatter

Craters

Craters are formed due to the out gassing of various volatile substances and moisture present in the solder paste, creating aesthetic issues.

Craters were seen in Alpha Above-Spec Alpha In-Spec Alpha Below-Spec

The possible reason for Crater formation is Out Gassing of volatile substances and moisture.

Alpha Above-SpecAlpha Above-Spec Alpha In-SpecAlpha In-Spec

Craters

Crater Craters

Alpha Below-SpecAlpha Below-Spec Alpha Below-SpecAlpha Below-Spec

Craters

Crater Crater

Flux Charring

Due to excessive heat, the flux starts charring and gets deposited on to the board and also blackening of the board occurs, creating aesthetic issues.

The Flux Charring is seen in Indium Above-Spec Alpha Above-Spec

The possible reason for Flux Charring is Excessive heat

Alpha Above-SpecAlpha Above-Spec Indium Above-SpecIndium Above-Spec

Flux Charring

Flux charring

Flux charring

Scratches

Scratches are formed on the surface of the solder surface due to poor handling of components during cleaning and other processes, causing aesthetic issues.

Scratches were seen in all the specifications due to poor handling occurred during cleaning.

Uneven Surfaces

Uneven surfaces are formed due to the out gassing of various volatile substances and moisture present in the solder paste, creating aesthetic issues.

Uneven Surfaced were seen in Alpha Above-Spec Alpha In-Spec

The possible reason for Uneven surface formation is Out Gassing of volatile substances and moisture.

Alpha In-SpecAlpha In-Spec Alpha Above-SpecAlpha Above-Spec

Uneven Surfaces

Oxidation

Oxidation of solder occurs due to the excessive heat in the oven, which is seen outside as a blackened surface causing aesthetic issues.

Oxidation is seen in Indium Above-Spec Indium In-Spec

Indium Above-SpecIndium Above-Spec Indium In-specIndium In-spec

Oxidation

Por

Comparison of three specification

In-Spec Below-SpecAbove-Spec

While comparing the three profiles, The In-Spec solder is too shiny than the other two profiles.