Process management for avionics - Aerospace and defence electronic systems containing lead-free solder - Part 22: Technical guidelines

IEC/TS 62647-22:2013(E) is intended for use as technical guidance by aerospace, defence, and high performance (ADHP) electronic applications and systems suppliers, e.g., original equipment manufacturers (OEMs) and system maintenance facilities, in developing and implementing designs and processes to ensure the continued performance, quality, reliability, safety, airworthiness, configuration control, affordability, maintainability, and supportability of high performance aerospace systems both during and after the transition to Pb-free electronics. The guidelines may be used by the OEMs and maintenance facilities to implement the methodologies they use to ensure the performance, reliability, airworthiness, safety, and certifiability of their products, in accordance with IEC/TS 62647-1:2012. This document also contains lessons learned from previous experience with Pb-free aerospace electronic systems.

General Information

Status
Published
Publication Date
24-Sep-2013
Current Stage
PPUB - Publication issued
Start Date
25-Sep-2013
Completion Date
25-Sep-2013
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IEC/TS 62647-22
Edition 1.0 2013-09
TECHNICAL
SPECIFICATION
Process management for avionics – Aerospace and defence electronic systems
containing lead-free solder –
Part 22: Technical guidelines
IEC/TS 62647-22:2013(E)
---------------------- Page: 1 ----------------------
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IEC/TS 62647-22
Edition 1.0 2013-09
TECHNICAL
SPECIFICATION
Process management for avionics – Aerospace and defence electronic systems
containing lead-free solder –
Part 22: Technical guidelines
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
ICS 03.100.50; 31.020; 49.060 ISBN 978-2-8322-1112-0

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – TS 62647-22  IEC:2013(E)
CONTENTS

FOREWORD ........................................................................................................................... 5

INTRODUCTION ..................................................................................................................... 7

1 Scope .............................................................................................................................. 9

2 Normative references ...................................................................................................... 9

3 Terms, definitions and abbreviations ............................................................................. 10

3.1 Terms and definitions ....................................................................................... 10

3.2 Abbreviations ................................................................................................... 15

4 Approach ....................................................................................................................... 16

4.1 General ............................................................................................................ 16

4.2 Assumption ...................................................................................................... 16

5 General Pb-free solder alloy behavior ............................................................................ 17

5.1 General ............................................................................................................ 17

5.2 Elevated temperature ....................................................................................... 17

5.3 Low temperatures ............................................................................................ 18

5.4 Temperature cycling ........................................................................................ 20

5.4.1 General .......................................................................................... 20

5.4.2 Solder thermal cycling failure mode ................................................ 21

5.4.3 Stress relaxation considerations ..................................................... 21

5.4.4 Ramp rate ....................................................................................... 21

5.4.5 Dwell time at elevated temperature ................................................. 21

5.4.6 Dwell time at low temperature ......................................................... 22

5.5 Rapid mechanical loading (vibration/shock) ..................................................... 22

6 System level service environment .................................................................................. 22

6.1 General ............................................................................................................ 22

6.2 Service environment ........................................................................................ 23

6.3 Electronics/electrical equipment thermal environments .................................... 23

6.3.1 General .......................................................................................... 23

6.3.2 Electronics/electrical equipment steady temperatures ..................... 23

6.3.3 Electronics/electrical equipment temperature cycling ...................... 23

6.4 Vibration and shock ......................................................................................... 23

6.5 Humidity .......................................................................................................... 24

6.6 Other environments: salt spray, fungus, cooling air quality, and fluid

compatibility ..................................................................................................... 24

6.7 Other special requirements .............................................................................. 24

7 High performance electronics testing ............................................................................. 24

8 Solder joint reliability considerations .............................................................................. 25

8.1 General ............................................................................................................ 25

8.1.1 Overview ........................................................................................ 25

8.1.2 Final solder joint composition ......................................................... 25

8.1.3 Solder wetting and final joint shape ................................................ 25

8.1.4 Strength of the PCB/PWB and component interfaces ...................... 26

8.2 Mixing of solder alloys and finishes .................................................................. 26

8.3 Pb-free terminations in tin-lead joints ............................................................... 26

8.3.1 General .......................................................................................... 26

8.3.2 Ball grid array Pb-free terminations in tin-lead joints ....................... 27

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TS 62647-22  IEC:2013(E) – 3 –
8.3.3 Flat pack and chip device Pb-free terminations in tin-lead

joints .............................................................................................. 29

8.4 Tin-lead terminations in Pb-free joints .............................................................. 30

8.4.1 General .......................................................................................... 30

8.4.2 Ball grid array tin-lead terminations in lead-free joints ..................... 30

8.4.3 Flat pack and chip device tin-lead terminations in lead-free

joints .............................................................................................. 30

8.5 Bismuth effects ................................................................................................ 30

8.6 JCAA/JG-PP testing of mixed alloy combinations ............................................. 31

8.6.1 General .......................................................................................... 31

8.6.2 Vibration ......................................................................................... 31

8.6.3 Thermal shock testing ..................................................................... 32

8.6.4 Combined environments ................................................................. 32

8.7 Pb-free solder and mixed metallurgy modeling ................................................. 32

9 Piece parts .................................................................................................................... 36

9.1 Materials .......................................................................................................... 36

9.2 Temperature rating .......................................................................................... 36

9.3 Special considerations ..................................................................................... 36

9.4 Plastic encapsulated microcircuit (PEM) moisture sensitivity level (MSL) ......... 36

9.5 Terminal finish ................................................................................................. 36

9.6 Assembly stresses ........................................................................................... 38

9.7 Hot solder dipping ............................................................................................ 38

10 Printed circuit boards ..................................................................................................... 38

10.1 General ............................................................................................................ 38

10.2 Plated through holes ........................................................................................ 38

10.3 Copper dissolution ........................................................................................... 39

10.4 PCB/PWB laminate materials ........................................................................... 40

10.4.1 General .......................................................................................... 40

10.4.2 Coefficient of thermal expansion ..................................................... 40

10.5 Surface finish ................................................................................................... 41

10.6 Pb-free PCB/PWB qualification ........................................................................ 42

10.7 PCB/PWB artwork and design considerations for Pb-free solder

applications ..................................................................................................... 42

11 Printed circuit board (PCB)/printed wiring board (PWB) assembly .................................. 43

11.1 General ............................................................................................................ 43

11.2 PCB/PWB process indicator coupons ............................................................... 43

11.3 Solder inspection criterion ................................................................................ 43

11.4 Fluxes, residues, cleaning and SIR issues ....................................................... 43

12 Module assembly considerations ................................................................................... 49

12.1 Connectors and sockets ................................................................................... 49

12.2 Heatsinks/modules ........................................................................................... 49

12.3 Conformal coating ............................................................................................ 49

13 Manufacturing resources ............................................................................................... 50

14 Aerospace wiring/cabling considerations ....................................................................... 50

14.1 Insulation temperature rating ........................................................................... 50

14.2 Cable connectors ............................................................................................. 50

14.3 Wire terminals .................................................................................................. 51

14.4 Splices ............................................................................................................. 51

14.5 Sleeving ........................................................................................................... 51

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– 4 – TS 62647-22  IEC:2013(E)

15 Rework/repair ................................................................................................................ 51

15.1 General ............................................................................................................ 51

15.2 Piece part rework ............................................................................................. 53

15.2.1 Area array rework ........................................................................... 53

15.2.2 Surface mount capacitor/resistor rework ......................................... 53

15.2.3 Through hole piece part rework ...................................................... 53

15.3 Depot level repair............................................................................................. 54

15.4 Mixed solder rework temperature profiles ......................................................... 54

15.5 Solder fluxes .................................................................................................... 54

15.6 Rework/repair cleaning process ....................................................................... 55

15.7 Inspection requirements ................................................................................... 55

16 Generic life testing ........................................................................................................ 55

16.1 Thermal cycling, vibration, and shock testing ................................................... 55

16.2 Other environments ......................................................................................... 56

16.2.1 Salt fog ........................................................................................... 56

16.2.2 Cooling air quality ........................................................................... 56

16.2.3 Fluid compatibility ........................................................................... 56

16.2.4 Generic humidity ............................................................................. 56

17 Similarity analysis .......................................................................................................... 56

Annex A (informative) Equipment service environmental definition ....................................... 58

A.1 Steady temperature service environments ........................................................ 58

A.2 Service cyclic temperature environments ......................................................... 58

Bibliography .......................................................................................................................... 59

Table 1 – Review of piece part surface finish and potential concerns .................................... 33

Table 2 – Elements promoting and supressing tin whiskers ................................................... 34

Table 3 – Elements promoting and supressing tin pest.......................................................... 35

Table 4 – Piece part lead/terminal and BGA ball metallization tin whisker and tin pest

propensity ............................................................................................................................. 37

Table 5 – PCB/PWB metallization tin whisker and tin pest propensity ................................... 42

Table 6 – Piece part terminal and BGA ball metallization solder process compatibility

risk ...................................................................................................................................... 44

Table 7 – PCB/PWB finish solder process compatibility risk .................................................. 46

Table 8 – Piece part terminal and BGA ball metallization reliability risk ................................. 47

Table 9 – PCB/PWB metallization reliability risk .................................................................... 48

Table 10 – Relative rigidity of IPC-CC-830 conformal coating categories .............................. 50

Table 11 – Process temperatures of mixed alloys ................................................................. 55

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TS 62647-22  IEC:2013(E) – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PROCESS MANAGEMENT FOR AVIONICS –
AEROSPACE AND DEFENCE ELECTRONIC
SYSTEMS CONTAINING LEAD-FREE SOLDER –
Part 22: Technical guidelines
FOREWORD

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The main task of IEC technical committees is to prepare International Standards. In

exceptional circumstances, a technical committee may propose the publication of a technical

specification when

• the required support cannot be obtained for the publication of an International Standard,

despite repeated efforts, or

• the subject is still under technical development or where, for any other reason, there is the

future but no immediate possibility of an agreement on an International Standard.

Technical specifications are subject to review within three years of publication to decide

whether they can be transformed into International Standards.

IEC/TS 62647-22, which is a technical specification, has been prepared by IEC technical

committee 107: Process management for avionics.
---------------------- Page: 7 ----------------------
– 6 – TS 62647-22  IEC:2013(E)
The text of this technical specification is based on the following document:
IEC/PAS 62647-22 .

This technical specification cancels and replaces IEC/PAS 62647-22, published in 2011. This

edition constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous

edition:
a) Coherence with IEC/TS 62647-1 and IEC/TS 62647-2 definitions.
b) Reference to IEC 62647 documents when already published.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
107/205/DTS 107/218/RVC

Full information on the voting for the approval of this technical specification can be found in

the report on voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 62647 series, published under the general title Process

management for avionics – Aerospace and defence electronic systems containing lead-free

solder, can be found on the IEC website.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
___________

IEC/PAS 62647-22, which served as a basis for the present document, has been derived from GEIA-HB-0005-2.

---------------------- Page: 8 ----------------------
TS 62647-22  IEC:2013(E) – 7 –
INTRODUCTION
0.1 General

The global transition to lead-free (Pb-free) electronics impacts the aerospace, defence, and

high performance (ADHP) industry and other industries having high reliability applications in

various ways.
0.2 Transition to Pb-free

In addition to the perceived need to replace the tin-lead solders used as an interconnect

medium in electronic and electrical systems, the following variations to established practice

will need to be considered:

• components and printed circuit boards (PCBs)/printed wiring boards (PWBs) will need to

be able to withstand higher manufacturing process temperatures;

• printed circuit boards (PCBs)/printed wiring boards (PWBs) will need to have robust

solderable lead-free (Pb-free) surface finishes;

• manufacturing and inspection techniques are needed that yield repeatable reliability

characteristics;

• at least initially, Pb-free alloys used within the equipment should be restricted to those that

are compatible with tin-lead soldering systems;

• a maintenance strategy should be developed that will facilitate the support repair of new

and existing equipment throughout a long life time which can be higher than 20 years.

This document will establish guidelines for the use of Pb-free solder and mixed tin-lead/lead-

free alloy systems while maintaining the high reliability standards required for aerospace

electronic and electrical systems. Currently the largest volume of lead (Pb) in many of these

electronic systems is in the tin-lead eutectic (Sn-37Pb) and near eutectic alloys

(Sn-36Pb-2Ag, Sn-40Pb) used in printed circuit board/printed wiring board assemblies, wiring

harnesses and electrical systems. High-lead solder alloys are not specifically addressed in

this document; however, many of the methodologies outlined herein are applicable for their

evaluation.

A good deal of the information desired for inclusion in this technical guidelines document does

not exist. A large number of lead-free (Pb-free) investigative studies for aerospace and high

reliability electronic and electrical systems are either in progress or in the initiation stage. The

long durations associated with reliability testing necessitates a phased release of information.

The information contained herein reflects the best information available at the time of

document issuance. It is not the goal of this document to provide technical guidance without

an understanding of why that guidance has technical validity or without concurrence of the

technical community in cases where sufficient data is lacking or conflicting. The document will

be updated as new data becomes available.

Further complicating matters is the fact that no single alloy across the supply base will be

replacing the heritage tin-lead eutectic alloy and that it is not likely that qualification of one

alloy covers qualification for all other alloys. Given the usual requirement for long, high

performance electronic service lives, any lead-free (Pb-free) alloy will need to have

predictable performance when mixed with heritage tin-lead alloys. Lead-free (Pb-free) alloys

containing elements such as bismuth (Bi) or indium (In) that can form alloys having melting

points within the equipment’s operating temperature range will need to be considered very

carefully before use. Although lead-free (Pb-free) solder alloys are still undergoing some

adjustments, it appears that the Sn-Ag-Cu family of alloys will be used for surface mount

assembly and either Sn-Ag-Cu, Sn-Cu or Sn-Cu-Ni (Sn-Cu stabilized with nickel) alloys will be

---------------------- Page: 9 ----------------------
– 8 – TS 62647-22  IEC:2013(E)

dominant in wave solder applications. In addition, some applications are using the Sn-Ag alloy

family [1] [2] [3].

The majority of the lead-free (Pb-free) solder alloys being considered have higher melting

temperatures than tin-lead eutectic solder. In order to make use of the lead-free (Pb-free)

solders, changes to the molding compound, die attach and printed circuit board (PCB)/printed

wiring board (PWB) insulation systems are being introduced to accommodate the 30 °C to

40 °C higher (54 °F to 72 °F higher) processing temperature. Thus, not only is the lead-free

(Pb-free) transition changing the solder alloy, but a significant portion of the electronic

packaging materials are changing as well. The higher melting point, greater creep resistance

and higher strength of the lead-free (Pb-free) alloys have driven a significant amount of study

into the thermal cycling and mechanical vibration/shock assessments of these new alloys.

The consumer electronics industry has invested considerable resources to ensure that lead-

free (Pb-free) solder will perform adequately for their products. Creep resistance of lead-free

(Pb-free) alloys can vary considerably from heritage tin-lead solders. The creep/stress

relaxation performance of the solder depends on the stress level, temperature and time for a

specific solder material and joint composition. Therefore, one needs to establish what the

acceleration factor is between a particular test condition and application. The interpretation of

the results of a head-to-head testing needs to be assessed in terms of the anticipated service

conditions with respect to these acceleration factors. Thermal preconditioning prior to thermal

cycling should be considered in the lead-free (Pb-free) solder assessment plan particularly as

it relates to changes in solder microstructure. Modeling/analysis is needed to properly

compare the tin-lead and lead-free (Pb-free) alloy performance and correct for the stress

relaxation differences obtained for the various piece parts and thermal cycling condition

...

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