Lead-Free Design Guide

Lead-free Design Guide

Section Number / Section Title / Status / Review Date
1 / Introduction / For final Review
2 / Document Scope / Finished
3 / Background / For comment
3.1 / General Solder Alloy Characteristics / For comment
3.2 / Tin Whiskers / For comment
3.3 / Silver Dendrites / For comment
3.4 / Manufacturing Processes (Background) / For comment
Soldering Processes – Through Hole / For comment
Soldering Processes SMT / For comment
Rework / For comment
3.5 / Substrate Issues (Background) / For comment
De-lamination / For comment
SIR and Dendrites / For comment
CAF / For comment
Substrate Passivation (ENIG) / For comment
Substrate Passivation (ENIPIG) / For comment
Substrate Passivation (HASL) / For comment
Substrate Passivation (OSP) / For comment
Substrate Passivation Immersion Silver / For comment
Substrate Passivation Immersion Tin / For comment
Cleaning / For comment
Conformal Coating / Needs words + expansion
3.6 / Component Selection / For comment
Material issues / For comment
Device lifetime / For comment
Obsolescence issues / For comment
3.7 / Component Storage / For comment
3.8 / The Supply Chain / For comment words
4 / The Application / For comment
4.1 / Control Level Classification / For comment but needs additions
4.2 / Classification Control Level 1 / For comment but needs additions
4.3 / Classification Control Level 2a / For comment but needs additions
4.4 / Classification Control Level 2b / For comment but needs additions
4.5 / Classification Control Level 2c / For comment but needs additions
4.6 / Classification Control Level 3 / For comment but needs additions
5 / Equipment Design
5.1 / Architecture / Needs additional words
5.2 / Technology / N/A
5.3 / Component Qualification / Needs words
6 / Manufacturing For High Reliability / N/A
6.1 / Tin Whisker Self Mitigation / For comment
6.2 / Manufacturing Processes / For comment
6.3 / Supply Chain Control / For comment
6.4 / Obsolescence Management / For comment?
7 / Design Process Flow / Needs inputs
8 / Terms and Definitions / To be updated
9 / Abbreviations / To be updated
10 / References / To be updated
11 / Appendix 1 / To be completed
SectionNumber / Section Title / Status / ReviewDate
1 / Introduction / For final Review
2 / Document Scope / Finished / 16/07/14
3 / Background / For comment
3.1 / General Solder Alloy Characteristics / Requires rework
3.2 / Tin Whiskers / For comment
3.3 / Silver Dendrites / For comment
3.4 / Manufacturing Processes (Background) / For comment
Soldering Processes – Through Hole / For comment
Soldering Processes SMT / For comment
Rework / Needsadditional words?
3.5 / Substrate Issues (Background) / Needs additional words
De-lamination / For comment
SIR and Dendrites / For comment
CAF / For comment
Substrate Passivation (ENIG) / For comment
Substrate Passivation (ENIPIG) / For comment
Substrate Passivation (HASL) / For comment
Substrate Passivation (OSP) / For comment
Substrate Passivation Immersion Silver / For comment
Substrate Passivation Immersion Tin / For comment
Cleaning / For comment
Conformal Coating / Needs words + expansion
3.6 / Component Selection / For comment
Material issues / Needs more words
Device lifetime / For comment
Obsolescence issues / For comment
3.7 / The Supply Chain / Needs additional words
4 / The Application / N/A
4.1 / Control Level Classification / For comment but needs additions
4.2 / Classification Control Level 1 / For comment but needs additions
4.3 / Classification Control Level 2a / For comment but needs additions
4.4 / Classification Control Level 2b / For comment but needs additions
4.5 / Classification Control Level 2c / For comment but needs additions
4.6 / Classification Control Level 3 / For comment but needs additions
5 / Equipment Design / N/A
5.1 / Architecture / Needs additional words
5.2 / Technology / N/A
5.3 / Component Qualification / Needs words
6 / Manufacturing For High Reliability / N/A
6.1 / Tin Whisker Self Mitigation / For comment
6.2 / Manufacturing Processes / Needs words
6.3 / Supply Chain Control / Needs words
6.4 / Obsolescence Management / Needs additional words?
7 / Design Process Flow / Needs words and chart
8 / Terms and Definitions / To be updated
9 / Abbreviations / To be updated
10 / References / To be updated
11 / Appendix 1 / To be completed

Contents

1.Introduction

2.Document Scope

3.Background

3.1.General Pb-Free Solder Alloy Characteristics

Processing Temperature

Microstructure

3.2.Tin Whiskers

3.3.Silver Dendrites

3.4.Manufacturing Processes

Background

Soldering Processes – Through Hole Technology (THT)

Soldering Processes – SMT

Re-work

3.5.Substrate Issues

Background

De-lamination

Surface Insulation Resistance and dendrite growth

Conductive Anodic Filaments (CAF)

Substrate Passivation

Cleaning

Conformal Coating

3.6.Component Selection

Material Issues

Device Lifetime

Obsolescence Issues

3.7.Component Storage

3.8.The Supply Chain

4.The Application

4.1.Control Level Classification

4.2.Classification Control Level 1

4.3.Classification Control Level 2a

4.4.Classification Control Level 2b

4.5.Classification Control Level 2c

4.6.Classification Control Level 3

5.Equipment Design

5.1.Architecture

Multiple channels and voting?

5.2.Technology

Hybrid Implementation

Conventional Packaged Components

Commercial Off The Shelf (COTS) Assemblies

5.3.Component Qualification

6.Manufacturing for High Reliability

6.1.Tin Whisker Self-Mitigation:

6.2.Manufacturing Process

Storage Prior to Manufacture

Specialist Build – Hybridisation

Soldering – THT

Soldering – SMT

PCB Surface Finish

6.3.Supply Chain Control

6.4.Obsolescence Management

7.Design Process Flow

8.Terms and Definitions

9.Abbreviations

10.References

11.Appendix 1

Collation of Suggested Best-Practice

1.Introduction[AJR1]

1.1.This guide is intended to aid Electronics and SystemsallDesign design engineers (e.g. System, Hardware, Circuit, Component, and Manufacturing) in the design of high reliability electronic circuits and assemblies that make use of Lead-free materials, components and processes.

1.2.It is here assumed that the designers themselves are competent in all aspects of circuit and or system design, but that they may not be fully conversant with the detailed vagaries of Lead-free (LF) manufacturing processes, materials or components or how these impact on equipment reliability or longevity.

1.3.Whilst a great deal of historical knowledge exists relating to Tin/Lead based systems and circuits, the rapid evolution of the electronics industries towards Lead-free has, virtually at a stroke, removed changed that experience base.

1.4.The impact of Lead-free technology,when compared with Tin/Lead processes and materials, is most apparent in the following areas. Note that this is not a comprehensive list and that these areas are listed in no particular order of importance:

• High manufacturing process temperatureare usually required for LF manufacturing processes and this increase in thermal stresscan introduce significant component degradation and use-life issues.
• Generally the poor wetting characteristic of Lead-free solders hasresulted in the need to use more active (acidic) fluxes. select flux compatible with the Pb-free solder and the cleaning process if needed
• In order to properly activate the appropriate fluxes amore stringent control of the processing temperatures is required. OR Lead-free solders have higher melting temperatures requiring increase solder process temperatures with more stringent process temperature controls being required.
• More active flux residues that are more difficult to clean off. OR Fluxes and flux residues for Lead-free solder processes require improved cleaning chemistries/procedures/processes. Subsequently, the compatibility of flux and cleaner needs to be checked as well.
• The effect of the cooling cycle on the joint crystallography is enhanced[MC2].[u3]
• More brittle solder joints which have a much shorter use-life than that expected from the use Tin/Lead solder[MC4][AJR5]. [u6]This[AJR7] is particularly the case for circuits used in the more demanding mixed thermal and mechanical environments.
• Joint embrittlement [u8]tends to increase with time ultimately resulting inan open circuit joint failure due to brittle fracture[MC9]. Note that this is an additional dominant failure mode over the creep mechanism that occurs with Tin/Lead solders.
• The solder joints grow (Tin) whiskers[u10][AJR11][AJR12][MC13].
• A significant number of LF components have Tin passivated leads which over a long product lifetime will grow (Tin) whiskers[MC14] if subjected to certain conditions (e.g. high heat, humidity, localized static stress, etc.).
• The incompatibility[MC15] material and strength propertiesof[AJR16] some LF materials can result in lower lifetime joints, poor thermal / mechanical performance and / or the earlier onset of whisker growth.
• The printed circuit board (PCB) substrate thermal characteristics create greater joint stress at the LF process temperatures. OR Greater stress due to higher lead-free soldering process temperature results in the need to select appropriate PCB (e.g. materials) to withstand the temperature gap (vs. Sn-Pb process). Do we add something on considering PTH reliability and failure modes change from SnPb to LF. The second choice is provided based on Catherine’s dis-agreement with the original statement.
• The quality of the substrates has degraded and in a large minority of cases failure due to Conductive Anodic Filaments (CAF) is now apparent. OR The increased process temperatures of Lead-free solder processing and the current state of laminate material characteristics increase the potential of having Conductive Anodic Filament (CAF) failures on printed circuit assemblies. AJR follow-up: Soften it up….refer to CAF section, discuss approaches
• Lack: an increase of the tombstoning phenomena has been observed with lead-free solder. Appropriate pad design and improvement of assembly process can decrease it. (Dale Lee comment 16 June 2015: The reverse of this is true. We less tombstone issues with lead free solder due to paste range during reflow process with all other thing being equal).
• Lack: Lead-free assembly create more voids than leaded solder. These voids can lead to the solder joint fissure and/or decrease the thermal dissipation. (Dale Lee comment 16 June 2015: With proper soldering process profile development, voiding is similar to tin/lead.)

1.5.The normal evolutional drivers of the electronics industries, commercial pressure and political expediency, has also contributed in making the design of high reliability equipment much more demanding. Whilst this trend continues, the interactions between components and materials need more than ever to be considered holistically. OR [“Genoa” proposed change] Above all the transition to Lead free technology, mainly achieved via Tin based alloys which have been known for decades, is not a total revolution, but therefore we have to face with both:

• The re-apparition of old failure mechanisms (which have historically disappeared with improvement of processes and materials)
• The apparition of new failure mechanisms linked to the new Lead free technology limits

As today there is no real Lead free field experience return for harsh environment, it is important to remind the main potential failure mechanisms that can occur and to introduce or propose adequate mitigation solutions, guidance or best practices that can be used during:

• design phases
• selection of components, material, PCB
• implementations of the assembly process

1.5.1.6.to reduce the risks of Lead free electronics used in ADHP products (Aerospace,Defense and High Performance

1.6.1.7.This document is intended to facilitate the implementation of a justifiable design process.

2.Document Scope

2.1.This document identifies those processes that may be used for designing electronic equipment andin mitigating the potential deleterious effects of Lead-free materials, processes etc. within electronic systems

2.2.This document is applicable to Aerospace, Defence and High Performance (ADHP) electronic applications and which includes equipment that may have historically contained Tin/Lead materials and made use of the associated processes but now have, or are about to be, migrated toa Pb-free status.

2.3.In addition to design guidance, Tthe guidelines contained herein may be used by ADHP manufacturers (at all levels)Original Equipment Manufacturers (OEMs) and / or maintenance facilities to develop and implement the methodologies they have chosen to useneeded to assure the performance, reliability, airworthiness, safety, and / or certifiability of their products, in accordance with associated performance specifications/standards. Document GEIA-STD-0005-1, reference 1. [Mods per Jeff Rowe recommendation.]

2.4.This document, in part, is based oncontains lessons learned from previous experience with Pb-Free systems in a variety of applications. Thisexperience gives specific references to solder alloys and other materials, and their expected applicability to various operating environmental conditions. They are intended for guidance onlyandcannot be considered guarantees of success in any given application.

3.Background[AJR17]Add some introductory remarks here about the intent of this section, e.g. may include some manufacturing experiences (but not all inclusive) and how they may be impacted by design. Provided solely as information to the designer. ALSO: ED. NOTE: Consider consolidating these next sub-sections into short paragraphs that summarize the “delta” of CAF between SnPb and pb-free.

3.1.General Pb-Free Solder Alloy Characteristics

3.1.1.The Tin-Silver-Copper (SAC) and Tin-Copper(Sn-Cu)Pb-free alloys have generally been found to be stronger and more creep resistant than Tin-Lead (Sn-Pb) alloys [3] but conversely exhibit an additional failure (wear-out) mechanism, that of brittle fracture. OR Tin-Lead alloys differ greatly from Pb-free materials with respect to material and physical properties. These changes manifest themselves in behavior under such service/environmental conditions such as thermal cycling, mechanical shock, vibration, tensile strength and shear strength.

3.1.2.Lead-free solders tend not to ‘wet’ as well as the traditional Sn-Pb alloys requiring a more active (acidic) flux to[AJR18] be used in the soldering process[MC19]. (Dale Lee comment 16 June 2015: “I don’t agree with this statement, lead free still wets well, depending on attachment finish, it does not spread like Sn-Pb.”) OR Likewise, Tin-Lead and Pb-free materials will differ in manufacturing/production process behaviors, e.g. wetting, reflow, etc.

3.1.3.Whilst in-service Lead-free solders, in assembly, have the propensity to grow Tin whiskers[u20] in certain use environments[AJR21][AJR22][AJR23]. (M. Miller/Polina input 8-20-2015 WE NEED DAVE HILLMAN TO COMMENT ON THISWhilst in-service Lead-free )solders have the propensity to grow Tin whiskers[u24][AJR25][AJR26][AJR27].

3.1.4.The melting point of the heritage Sn-Pb eutectic alloy solder is 183 °C (361 °F) whilst that of some Pb-free solders (SAC family of solders for example)iscan be typically 30+°Chigher (221°C to 227°C). Other families of Pb-free solders can have different melting point ranges as well.

Processing Temperature[AJR28] 9/2/2015 LEFT OFF HERE

3.1.5.This The higher melting temperature of Pb-free alloys results in a typical 30 to 40 ºC (54 to 72 °F) increase in the required processing temperature *(or higher temperatures depending on type of soldering process or lead free alloy ) as compared to that used with Sn-Pb alloys. [*Parenthetical addition recommended by Dale Lee on 16 June 2015.]

3.1.6.The melting points of the common eutectic solder alloys are collated in the table (1) below[AJR29].

[AJR30]

Table 1. Melting Points of Sn-Pb and some Pb-free Alloys
Solder Alloy / Proportions / Melting point temperature / Reference
Sn-Pb / 63 - 37 / 183 °C (361 °F) / 10
SAC / Sn - 3.5Ag - 0.9Cu[MC31][AJR32] / 217.2 ± 0.2 °C
(423 ± 0.36 °F) / 8
Sn-Cu / Sn-0.7Cu / 227 °C (441 °F) , / 7
Sn-Ag / Sn-3.5Ag / 221 °C / 7
Sn / 100% / 231.9 °C (449.4 °F)

Table 1: Melting points[u33].

Microstructure

3.1.7.The Pb-free alloy microstructure differs substantially from the lamellar/colony structure of eutectic Sn-Pb[AJR34].

3.1.8.The[AJR35] microstructure of Sn-Ag and Sn-Ag-Cu is comprised of relatively large β phase Sn dendrites,in-betweenwhich there are lamellar arrays of β-Sn, Ag3Sn and Cu6Sn5 Error! Reference source not found.phases.

In[u36][u37]some[MC38] studies large[AJR39] Ag3Sn platelets have been observed. The solidification behavior strongly influences the solid microstructure. The Sn-Pb eutectic solder joint requires only 2ºC of undercooling to begin the solidification of the Pb onto the Cu or Ni substrate. In contrast, the eutectic Sn-Ag-Cu system begins solidification with the formation of the Ag3Sn. Unfortunately, the presence of Ag3Sn does not facilitate the nucleation of the β-Sn and significant under-cooling can occur. An undercooling of 18 °C (32 °F) was reported for β-Sn. The formation of large Ag3Sn intermetallic plates in liquid Sn was observed during slower solidification rates for SAC alloys having 3.5 and 3.8 wt% Ag and not with the 3.0 wt% Ag alloy. These plates are expected to change the mechanical response of the system. The Ag3Sn intermetallic is not brittle, and the plate may stop or re-direct the crack. If the plate is in the same direction of the shear load, life can be reduced, but it is not common to see plates oriented parallel to the PCB or piece-part pads. The presence of Ag3Sn plates is of greater concern for flip chip and wafer scale chip pack solder joints. The volume fraction of β-Sn dendrites in the solidified solder is dependent upon cooling rate and alloy composition. The grain size of the β-Sn is relatively large with respect to the solder joint size. A BGA solder joint can be comprised of as few as 10 to 30 β-Sn grains and even fewer for wafer level chip scale packageand flip chip joints. Since dispesedintermetallics in a SAC alloy tend to increase the hardness and stiffness of the solder, a greater volume fraction of Sn dendrites generally results in a solder joint with decreased stiffness. Reduced solder stiffness can be beneficial in some high stress shock applications because the solder does not impart as much stress on the pad intermetallic or the pad laminate interfaces. Presently, some investigators are evaluating SAC alloys with reduced Ag and Cu content (SAC-L) in an effort to obtain improved drop shock performance of BGA assemblies. Unfortunately, the melting temperatures of SAC-L alloys are greater than the traditional SAC alloys and their thermal cycling characteristics require evaluation.

The three main Pb-free solders are based on the Tin rich Sn-Cu, Sn-Ag or the Sn-Ag-Cu (SAC)families of alloys. Sometimes small alloy additions of Ni, Ge, In, and Sb[AJR40], are made to these basicalloys in an effort to alter dissolution, solidification, mechanical properties or wettingcharacteristics. The melting point of pure Sn is 231.9 °C (449.4 °F) and the addition of 37%Pbto the Sn reduces the melting temperature to the eutectic point of 183 °C (361 °F). Similarly, theaddition of Ag and Cu to Sn reduces the melting temperature but not to the same extent as Pb.The Sn - 3.5Ag - 0.9Cu ternary eutectic SAC alloy melting temperature is 217.2 ± 0.2 °C (423 ±0.36 °F) [4], the Sn-0.7Cu eutectic alloy melts at a temperature of 227 °C (441 °F) [5], and theSn-3.5Ag eutectic melts at 221 °C [5]. These Pb-free solder melting temperatures areconsiderably higher than Sn-Pb eutectic. The higher melting temperature of Pb-free alloys,results in a 30 to 40 ºC increase (54 to 72 °F increase) in processing temperature as compared tothe temperatures used to process heritage Sn-Pb alloys. Higher melting temperatures result inincreased amounts of base metal dissolution (see section 10.2 Copper Dissolution) and increasedshrinkage stresses on components during cooling. An additional consideration is that the SACalloys have generally been found to be stronger and more creep resistant than the heritage Sn-Pbsolders at typical electronic use temperatures [2] [3].