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Solder Pastes

Technical Needs, Gaps and Solutions

The technology issues surrounding Solder Paste, the associated needs, technology status of those needs, as well as gaps and challenges to overcome, are summarized below in Table 1. The time period considered is from 2024 to 2034. Assessments of to address these gaps and challenges is presented in Table 2 as a set of potential solutions for each set of issues.

Technology Status Legend

For each need, the status of today’s technology is indicated by label and color as follows:

In-table color + label key

Description of Technology Status

Solutions not known

Solutions not known at this time

Solutions need optimization

Current solutions need optimization

Solutions deployed or known

Solutions deployed or known today

Not determined

TBD

Table 1. Solder Paste Gaps, and Today’s Technology Status with Respect to Current and Future Needs

TECHNOLOGY ISSUE

ROADMAP TIMEFRAME

TODAY (2024)

3 YEARS (2027)

5 YEARS (2029)

10 YEARS (2034)

Solder Paste

SOLDER PASTE ISSUE #1: Need high surface insulation resistance(SIR)/electronic control module (ECM)/ flux reliability for no-clean. 50 um board pad lines and spacings used today. 20 um lines and spacing are in the 3 to 5 year horizon. SIR test coupons are looking down to 200 um spacing. Current IPC SIR test methods have spacings of 300 um to 400 um

NEED

  • Maintain High SIR flux materials for smaller spacings compared with current materials (Compare IPC SIR spacing with other standard spacings). Ensure the standard addresses situations where the material is encapsulated.

  • Compatible with conformal coatings.

CURRENT TECHNOLOGY STATUS

Solutions need optimization

Some high SIR flux materials are available but limited.

GAP

Lack of suitable alternatives or development. Flux activators may not be evaporated during reflow (e.g., thermal quad-flat no-leads (QFN) pad). At what point will the board spacing be so small that no clean materials cannot be used and we will just use water soluble pastes?

CHALLENGE

Compatibility with conformal coating/underfills/solder mask, etc. Obtaining high SIR flux materials.

SOLDER PASTE ISSUE #2: Need for ultra-low residue paste to be compatible (SIR/ECM adhesion) with encapsulation materials (e.g., conformal coating, underfill, etc) which does not need to be cleaned. (similar to Issue #1)

NEED

Low amount of flux residue (how much is low amount: % reduction in residue reduction) while still maintaining good wetting and other properties in the paste?) 

CURRENT TECHNOLOGY STATUS

Solutions need optimization

Some low residue materials available. Need to reflow in higher purity Nitrogen (<100 ppm O2) (not mainstream). Typical residue amount today for regular paste is 6%. Low residue pastes the value is 2 to 3%. The higher the flux residue amount, the more sensitivity to the property of the underfill material.

GAP

Ultra-low residue paste with good wetting and reflowability in air especially with finer powder sizes (Type 5 and smaller)

CHALLENGE

Balance between low residue and good wetting for this type of paste

SOLDER PASTE ISSUE #3: Need excellent printing paste for low area ratios/small apertures for Type 4 solder paste

NEED

Excellent printing with low aspect ratio (AR) (<0.55-0.6) with Type 4 paste

CURRENT TECHNOLOGY STATUS

Solutions need optimization

Use Type 5 paste with Nitrogen reflow, step down stencil, nano-coatings, printing machine update

GAP

Lack of Type 4 paste available with good printing at low AR.

CHALLENGE

Better printability with Type 4 pastes.

SOLDER PASTE ISSUE #4: Need development of low temperature alloys used in the solder paste

NEED

Reduced warpage, lower energy costs/usage

CURRENT TECHNOLOGY STATUS

Solutions need optimization

  • Lower temperature alloys available but generally brittle- adhesives/underfills used to improve mechanical drop shock performance especially for high input/output (I/O)/ large BGA components. Soldering coil onto board with low temperature solder (e.g., vibration concerns requiring underfill).

  • Some epoxy flux paste used but some printability challenges with fine pitch components.

GAP

  • Improve ductility of low-temperature SnBi-based lead-free alloys.

  • Combination of low temperature solders with underfill/adhesive materials.

  • Development of composite/nano-materials.

CHALLENGE

Low temperature alloys such as SnBi are generally brittle. Electromigration of Bi in SnBi alloys.

SOLDER PASTE ISSUE #5: Need low voiding pastes during reflow

NEED

Lower voiding on BTC/QFN type components (< 20% in air convection reflow oven)

CURRENT TECHNOLOGY STATUS

Solutions need optimization

Typically, 30-50% in air convection oven, expected to gradually reduce to 20-30% over a ten-year period. Use of vapor phase/vacuum reflow ovens to reduce voiding to less than 5%. Use higher standoff height to help to have voids to escape. Use Nitrogen (<1000 ppm 02). Profile development. Challenges with voiding around thermal vias. REACH chemical restrictions may reduce the amount of flux materials which can be used.

GAP

Need more development of low voiding pastes.

CHALLENGE

Development of low voiding paste: what to change in the flu without losing other properties of the flux (e.g., wetting, pin testability, etc).

SOLDER PASTE ISSUE #6: Need large process window for paste for reflow process

NEED

Survive high temperature and time with long preheat soak times. (230°/235°C to 260°C temperatures for SnAgCu) (Soak time 45 sec to 120 sec)

CURRENT TECHNOLOGY STATUS

Solutions need optimization

Some paste materials can survive high temperatures and long soak times. Protect smaller components from excessive heat.

GAP

Development of a paste to address this wide temperature variation.

CHALLENGE

Large BGAs and small components on a large thermal mass board. Cold joints on large components and flux exhaustion/vaporization on small components. Development of a paste material which is tolerant to high temperatures and long soak times which have good printing and other properties.

SOLDER PASTE ISSUE #7: Need a no-refrigeration paste

NEED

Move development of no-refrigeration paste with a larger process window. Reduce logistics issue during transport and lower transport costs. Ease of storage in production/warehouse.

CURRENT TECHNOLOGY STATUS

Solutions need optimization

There are a limited number of solder paste materials available that have a shelf life up to 12 months at room temperature. Customers ask for long shelf life but it is not a major priority compared to other properties such as printing and reflow.

GAP

Not a wide range of no refrigeration paste for all applications/alloys. Trade off with their use in terms of voiding/slump and other paste performance.

CHALLENGE

Some of the materials available tend to slump. Matching performance requirements of refrigerated pastes.

SOLDER PASTE ISSUE #8: Need high reliability alloy pastes  (need better definition of high reliability: shock, vibration, available transfer capability (ATC), etc.)

NEED

One high reliability alloy for all reliability requirements

CURRENT TECHNOLOGY STATUS

Solutions need optimization

High reliability alloy pastes are available but different alloys give different performance.

GAP

Challenge that one alloy does not fit all needs. Product-dependent requirements. (TC vs vibration vs mechanical shock)

CHALLENGE

-

-

-

-

SOLDER PASTE ISSUE #9: Need no-clean paste which can be cleaned more effectively and use of more benign cleaning materials

NEED

No-clean flux residues that are more effectively cleaned with more benign materials.

CURRENT TECHNOLOGY STATUS

 Solutions need optimization

Cleaning of no-clean is with solvents. Some pastes can be cleaned with more benign materials. High reliability and harsh environment segments will push for this.

GAP

Layer for no-clean residue to be removed with more benign materials.

CHALLENGE

If a no-clean is susceptible to being cleaned with DI water only, then it will be a reliability issue. Development of no-clean paste which still has good overall performance and can be cleaned with more benign cleaning materials.

SOLDER PASTE ISSUE #10: Need low-temperature solder paste with high SIR/ECM reliability for bottom terminated component (BTC)-type components

NEED

Higher amount of activators remaining with low temperature solder reflow profiles. Need to ensure activators are consumed and encapsulated so it does not cause a reliability issue. Review and update IPC standards to replicate BTC conditions in the field.

CURRENT TECHNOLOGY STATUS

Solutions need optimization

Low-temperature solders paste meet current SIR requirements, but SIR requirements do not account for low standoff/covered components (50 um standoff height for BTC components versus 200-300 um for BGA components)1

GAP

Little work with SnBi paste reliability with BTC type components. Outgassing issue under part.

CHALLENGE

Lack of progress in this area. Investigate different amounts and types of activators due to increased oxidation of Bi compared with Sn.

Newer BTC components designs may affect SIR results.

SOLDER PASTE ISSUE #11: Need test vehicle/test method development and modelling for SIR testing of no-clean solder pastes

NEED

Smaller spacings, high and very low voltages. Low stand-off BTC components. Need modelling for SIR testing and reliability. Standardized reflow profile and atmosphere

CURRENT TECHNOLOGY STATUS

Solutions need optimization

  • IPC B-52 test boards availability for SIR compatibility tests for pastes and other materials with BGA and other components.

  • No standard IPC test method for low standoff BTC components.

  • Some BTC components have heights as low as 50 um. Current IPC B-25 spacings is 319 um. IEC test coupon with comb spacing of 200 um.

  • Design options to improve outgassing are limited in high density products. (Include high and maybe low voltages—high voltages from 300 V to 3000 V) Current testing uses voltages in the 5 V range. 

  • No models developed for SIR reliability.

GAP

Outgassing issue under parts affecting reliability. Standards do not account for low and high voltages.

CHALLENGE

Some BTC components have heights as low as 50 um. Have solder paste with limited outgassing process.

SOLDER PASTE ISSUE #12: Need improved flux residue reliability after partial cleaning/spot cleaning for reflowed soldered pastes

NEED

Cleaning method for localized/spot cleaning at room temperature.

CURRENT TECHNOLOGY STATUS

 Solutions need optimization

Chemicals are available for localized/spot cleaning which are expensive. Low-cost alternatives such as isopropyl alcohol (IPA) are used which may not be effective.

GAP

  • Development of cleaning solutions for localized/spot cleaning which are lower cost (comparable to IPA).

  • Meeting flux reliability requirements.

  • Development of fluxes to meet reliability requirements without heating-

  • Need to define pre-condition (e.g., 24 hours at room temperature). Benign flux residue even after partial cleaning (flux chemistry, volume, process cleaning condition dependent).

CHALLENGE

How do we correlate partial cleaning with SIR reliability?

SOLDER PASTE ISSUE #13: Regulatory impacts on all materials development

NEED

More transparency in understanding regulations impact.

CURRENT TECHNOLOGY STATUS

 Solutions need optimization

Some companies are asking for full disclosure (IPC 1752). EU Reach legislation which is updated every 6 months.

GAP

Need to have more impact into regulations to streamline process

CHALLENGE

Many regulations that are being developed are changing. This information is not readily available from and to flux developers/formulators. Multiple questionnaires from customers require more staff at suppliers, Increased time and cost. Safety data sheet  (SDS) variations in different regions. Increased requirements.

SOLDER PASTE ISSUE #14: Reduction in powder sizes affecting processability

NEED

Reduced powder sides more oxidation (Type 5 and higher types). Flux needs to remove more oxidation. Improved slumping performance, cleanability, reduced solder balling, bridging. Improved shelf life.

CURRENT TECHNOLOGY STATUS

  Solutions need optimization

For Printing: Type 3 and 4 and down to Type 5 and 6. For Jetting: Type 6 and 7 are available and even down to Type 8. Use of Nitrogen increasing. Increased cost with smaller powder sizes. AR needs.

GAP

Development of smaller powder sizes in paste with good wetting/ solderability. Improved hot slump performance.

CHALLENGE

Increase solder wetting with smaller powder sizes without solder paste negatively impacting reliability.

SOLDER PASTE ISSUE #15: Removal of solder paste flux build up in reflow equipment- flux management (This content is covered in the INEMI Surface Mount Technology Printing and Reflow group roadmap. Refer to this roadmap section for further details.)

SOLDER PASTE ISSUE #16: Soldering to hard-to-solder materials (aluminum heat sinks/plates) and copper plates (high-heat sink material) and RF Shields (Ni, Ni/Al)

NEED

Increased solderability of difficult to materials with current low activity no-clean solder pastes.

CURRENT TECHNOLOGY STATUS

Solutions need optimization

  • Challenge to remove oxide and removal at lower temperatures due to heat sinks.

  • Thermal issues.

  • Development of alloys to improve wetting.

GAP

Lack of solder pastes developed which are low activity which are capable of removing oxides (e.g., solder alloy, Al, Cu, Ni, etc.) at normal soldering temperatures.

CHALLENGE

Difficult to solder to. Have increased flux activity which could be a reliability issue if no-clean.

SOLDER PASTE: ISSUE #17:  High current carrying capacity (HCCC) (6 to 8 amps, 3KA/cm2 to 4KA/cm2) solders, which exhibit minimal electromigration and have higher conductivity than current SnAgCu, SnPb and SnBi solders.

NEED

(1) HCCC solder which exhibits minimal resistance and microstructural changes with high constant or pulsed (current on and off) current.

(2) Consensus on Electromigration Test Methodology applicable to second level interconnects (SLIs)

CURRENT TECHNOLOGY STATUS

Solutions not known

Sn-Ag-Cu (SAC) solder has moderate current carrying capacity with microstructural changes resulting in opens after a moderate length of time under 4 to 6 A/cm2 current densities. BiSn solder has low current carrying capacity (<4 A/cm2) due to bismuth segregation at one end of the solder joint under high current densities.

GAP

Solder with conductivity in the 25-20 % Industrial Automation and Control System (IACS) range with minimal microstructural changes and resistance increase under high (constant and pulsed) current exposure, (Copper- 100% IACS, Solder- 15, Bi metal-3).

CHALLENGE

  • SAC solder has conductivity in the 15-16% IACS range and BiSn solder in the 4-5%IACS range. To get to 25-30% IACS goal addition of copper (100% IACS) is necessary. Hence, solders with significant amount of copper particles within them need to be developed. 

  • New HCCC solders have to be compatible with solder paste manufacturing process (dual or multiple metallurgy powder solder paste) as well as surface mount technology (SMT) solder paste printing and reflow process. (This can be synergized with warm assembly (WA) solder need, < 140°C peak reflow temperature assembly.)

SOLDER PASTE: ISSUE #18: Solder paste for Warm Assembly (<140°C peak reflow temperature and >100°C peak) with re-melt temperature above temperatures used in the field.

NEED

Driven by increased warpage of flip chip BGAs (FCBGAs) and thinner PCBs, heat sensitive components (e.g., co-packaged optics (CPOs) and sensors also a factor after assembly

  • Driven by increased warpage of FCBGAs and thinner PCBs, Heat sensitive components (e.g., CPOs) and sensors also a factor.

  • Advent of very large (>100 mm square) Area array components exacerbated warpage and thermal issue during reflow soldering.

CURRENT TECHNOLOGY STATUS

Solutions not known

Ultra Low Temperature solders exist such as those based on Sn-In (Mp  118°C) or Sn-Ga, but these solders have melting points very close to use temperatures of the electronic products  in the field (Thomologous effect).

GAP

Need for a range of WA temperature solder materials that can be used for reflow soldering assembly of components below 140°C temperature, but which re-melt at temperatures above ~170°C, for example.

CHALLENGE

Identification of of transient liquid-phase sintering (TLPS)-like and supercooled metallic powder particulate technology materials for preliminary assessment in board assembly.

Development and testing of TLPS-like and supercooled metallic powder particulate technologies to meet assembly yield requirements for large, high-dynamic warpage packages and assembly temperature below optical and photonic components temperature exposure limit.

Approaches to address Needs, Gaps and Challenges

Table 2 considers approaches to address Solder Paste needs and challenges. The evolution of these is projected out over a 10-year timeframe using technology readiness levels (TRLs).

In-table color key

Range of Technology Readiness Levels

Description

2

TRL: 1 to 4

Levels involving research

6

TRL: 5 to 7

Levels involving development

9

TRL: 8 to 9

Levels involving deployment

Table 2. Solder Paste Potential Solutions

TECHNOLOGY ISSUE

EXPECTED TRL LEVEL*

Solder Paste Materials

POTENTIAL SOLUTIONS

 TODAY

(2024)

3
YEARS

(2027)

5
YEARS

(2029)

10
YEARS (2034)

SOLDER PASTE MATERIALS ISSUE #1: 
Need High SIR/ECM/ flux reliability for no-clean for smaller spacings (< or = 200 um).

Flux development

4

5

7

8

SOLDER PASTE MATERIALS ISSUE #2: 
Need for ultra-low residue paste to be compatible (SIR/ECM/adhesion) with encapsulation materials (conformal coating, underfill, etc.) which does not need to be cleaned. (Similar to Issue #1)

Flux development

4

5

7

8

SOLDER PASTE MATERIALS ISSUE #3:
Need printing paste for low area ratios/ small apertures (AR: <0.55-0.6) for Type 4

Type 4 paste printing development

7

7

7

9

SOLDER PASTE MATERIALS ISSUE #4: 
Need development of Low temperature alloys (MP: 138°C temperature range) as alternatives for SnBi alloys used in the solder paste

Addition of dopant elements (optimizing combination and percentage by weight (wt%))

5

6

7

8

SOLDER PASTE MATERIALS ISSUE #5: 
Need low voiding pastes during reflow for BTC/QFN components

Lead-free SnAgCu paste development for < 20% voiding

3

4

5

6

SOLDER PASTE MATERIALS ISSUE #6:
Need large process window for paste for reflow process for extra-large thicker boards (with fixtures).

Paste development

4

5

6

7

SOLDER PASTE MATERIALS ISSUE #7:
Need a no refrigeration paste

Paste development

5

6

7

8

SOLDER PASTE MATERIALS ISSUE #8:
Need high reliability alloy pastes (need better definition of high reliability: shock, vibration, ATC, etc.

Industry definition of high reliability by product type (company versus industry standard differences- shock, vibration, ATC)

5

6

7

7

SOLDER PASTE MATERIALS ISSUE #9:
Need no-clean paste which can be cleaned more effectively and use of more benign cleaning materials

Paste development

5

6

7

7

SOLDER PASTE MATERIALS ISSUE #10:
Need low temperature solder paste with high SIR/ ECM reliability for BTC type components with activators consumed and encapsulated.

Paste development

5

6

7

7

SOLDER PASTE MATERIALS ISSUE #11:
Need test vehicle/ test method development and modelling for SIR testing of no-clean solder pastes

Test vehicle development (Industry standardization)

5

5

7

8

Test method development

5

5

7

8

Modelling for SIR testing and reliability

2

4

6

8

SOLDER PASTE MATERIALS ISSUE #12:
Need improved flux residue reliability after partial cleaning/ spot cleaning for reflowed soldered pastes

Development of cleaning method for localized /spot cleaning at room temperature to remove flux residue.

4

5

6

7

Development of low-cost chemicals for localized spot cleaning.

4

5

6

7

Development of fluxes to meet reliability requirements without sufficient heating—Define pre-condition (e.g., 24 hours at room temperature) and define component spacings/ voltage differential

5

6

6

7

Development of benign flux residue even after partial cleaning

5

6

6

7

Understanding the correlation of partial cleaning of flux residue with SIR reliability (Modelling)

2

4

6

8

SOLDER PASTE MATERIALS ISSUE #13:
Regulatory impacts on all materials development

Transparency in understanding regulations impact and timelines

4

5

6

7

Streamline regulation process

2

3

4

5

Feedback from industry into regulation process

2

3

4

5

SOLDER PASTE MATERIALS ISSUE #14:
Reduction in powder sizes (Type 5 and higher) affecting processability

Flux development

6

7

7

7

SOLDER PASTE MATERIALS ISSUE #15: 
Covered in the INEMI Printing and Reflow group roadmap: Refer to this roadmap section for further details)

(This content is covered in the INEMI Surface Mount Technology Printing and Reflow group roadmap. Refer to this roadmap section for further details.)

SOLDER PASTE MATERIALS ISSUE #16:
Soldering to hard-to-solder materials (aluminum heat sinks/ plates) and copper plates (high heat sink material) and RF Shields (Ni, Ni/Al)

Flux development ((Cu)

6

7

7

7

Flux development (Al)

5

6

6

7

Flux development (Ni)

6

6

7

7

SOLDER PASTE MATERIALS ISSUE #17:
High current carrying capacity (6 to 8 amps, 3KA/cm2 to 4KA/cm2) solders, which exhibit minimal electromigration and have higher conductivity than current solders

Development of novel metallurgies for solder paste particles particularly containing highly conductive elements such as copper

2

5

7

9

SOLDER PASTE MATERIALS ISSUE #18:
 Solder Paste for Warm Assembly (< 140°C Peak reflow temperature), i.e., even lower than LTS , but re-melt above 170°C

Development of novel solder metallurgies and reflow / rework processes

2

5

7

9

Conclusions

  1. Need for high SIR/ECM/ flux reliability for no-clean for smaller spacings (< or = 200 um)

  2. Need for excellent printing paste for low area ratios/small apertures for Type 4 solder paste

  3. Development of low-temperature alloys used in the solder paste

  4. Low voiding pastes during reflow

  5. High reliability alloy pastes (with a better definition of high reliability: shock, vibration, available transfer capability (ATC)

  6. Regulatory impacts on all materials development with more transparency in understanding regulation impact and timelines

  7. Reduction in powder sizes (Type 5 and higher) in solder paste and its effect on processability

References

  1. M Bixenman, M. McMeen and L. Diamond, Requirements for Soldering Fluxes Research using the B-53 Test Board, IPC APEX EXPO 2023.

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