Cleaning Printed Circuit Assemblies Design & Process Control

Transcription

1 Cleaning Printed Circuit Assemblies Design & Process Control Bob Willis bobwillisonline.com Your Delegate Webinar Control Panel Open and close your panel Full screen view Raise hand for Q&A at the end Submit text questions during or at the end

2 Bob Willis Involvement in Lead-Free Process Development Bob Willis has been involved with the introduction and implementation of lead-free process technology for the last seven years. He received A SOLDERTEC/Tin Technology Global Lead-Free Award for his contribution to the industry, helping implementation of the technology. Bob has been a monthly contributor to Global SMT magazine for the last six years. He was responsible for co-ordination and introduction of the First series of hands-on lead-free training workshops in Europe for Cookson Electronics during These events were run in France, Italy and the UK and involved lead-free theory, hands-on paste printing, reflow, wave and hand soldering exercises. Each non commercial event provided the first opportunity for engineers to get first hand experience in the use of lead-free production processes and money raised from the events was presented to local charity. More recently he co-ordinated the SMART Group Lead-Free Hands On Experience at Nepcon Electronics This gave the opportunity for over 150 engineers to process four different PCB solder finishes, with two different lead-free pastes through convection and vapour phase reflow. He also organised Lead-Free Experience 2, in He has also run training workshops with research groups like ITTF, SINTEF, NPL & IVF in Europe. Bob has organised and run three lead-free production lines at international exhibitions Productronica, Hanover Fair and Nepcon Electronics in Germany and England to provide an insight to the practical use of lead-free soldering on BGA Ball Grid Array, CSP Chip Scale Package, 0210 chip and through hole intrusive reflow connectors. This resulted in many technical papers being published in Germany, USA and the United Kingdom. Bob also defined the process and assisted with the set-up and running of the first Simultaneous Double Sided Lead-Free Reflow process using tin/silver/copper for reflow of through hole and surface mount products. Bob also had the pleasure of contributing a small section to the first Lead-Free Soldering text book Environment - Friendly Electronics: Lead-Free Technology written by Jennie Hwang in The section provided examples of the type of lead-free defects companies may experience in production. Further illustrations of lead-free joints have been featured in here most recent publication Implementing Lead-Free Electronics He has helped produce booklets on x-ray inspection and lead-free defects with DAGE Industries, Balver Zinn and SMART Group Mr Willis led the SMART Group Lead-Free Mission to Japan and with this team produced a report and organised several conference presentations on their findings. The mission was supported by the DTI and visited many companies in Japan as well as presenting a seminar in Tokyo at the British Embassy to over 60 technologists and senior managers of many of Japans leading producers. Bob was responsible for the Lead-Free Assembly & Soldering "CookBook" CD-ROM concept in 1999, the world s first interactive training resource. He implemented the concept and produced the interactive CD in partnership with the National Physical Laboratory (NPL), drawing on the many resources available in the industry including valuable work from NPL and the DTI. This incorporated many interviews with leading engineers involved with lead-free research and process introduction; the CD-ROM is now in its 3rd edition. Bob Willis currently operates a training and consultancy business based in England. Bob is a member of the SMART Technical Committee. Although a specialist for companies implementing Surface Mount Technology Mr Willis provides training and consultancy in most areas of electronic manufacture. He has worked with the GEC Technical Directorate as Surface Mount Co- Coordinator for both the Marconi and GEC group of companies and prior to that he was Senior Process Control Engineer with Marconi Communication Systems, where he had worked since his apprenticeship. Following his time with GEC he became Technical Director of an electronics contract manufacturing company where he formed a successful training and consultancy division. As a process engineer, he was involved in all aspects of electronic production and assembly involved in setting up production processes and evaluating materials; this also involved obtaining company approval on a wide range of Marconi's processes and products including printed circuit board manufacture. During the period with Marconi, experience was gained in methods and equipment for environmental testing of components, printed boards and assemblies with an interest developed in many areas of defect analysis. Over the last 15 years he has been involved in all aspects of surface mounted assembly, both at production and quality level and during that time has been involved in training staff and other engineers in many aspects of modern production. Over the past few years Mr. Willis has travelled in the United States, Japan, China, New Zealand, Australia and the Far East looking at areas of electronics and lecturing on electronic assembly. Mr. Willis was presented with the Paul Eisler award by the IMF (Institute of Metal Finishing) for the best technical paper during their technical programmes. He has conducted SMT Training programs for Texas Instruments and is currently course leader for Reflow and Wave Soldering Workshops in the United Kingdom. Mr Willis is an IEE Registered Trainer and has been responsible for training courses run by the PCIF originally one of Europe's largest printed circuit associations. Bob has conducted workshops with all the major organisations and exhibition organisers World Wide and is known for being an entertaining presenter and the only presenter to use unique process video clips during his workshops to demonstrate each point made. Find out more at: ASKbobwillis.com leadfreesoldering.com Bobwillisonline.com PackageOnPackage.co.uk Bobwillis.co.uk Mr. Willis was Chairman of the SMART Group, European Surface Mount Trade Association from and has been elected Honorary Life President and currently holds the position of SMART Group Technical Director, he also works on BSI Standards Working Parties. He is a Fellow of the Institute Circuit Technology, an NVQ Assessor, Member of the Institute of Quality Assurance and Society of Environmental Test Engineers. Bob Willis currently writes regular features for AMT Ireland, Asian Electronics Engineer and Circuits Assembly the US magazine. He also is responsible for writing each of the SMART Group Charity Technology reports, which are sold in Europe and America by the SMTA to raise money for worthy causes. Bob ran the SMART Group PPM Monitoring Project in the United Kingdom supported by the Department of Trade and Industry. He was coordinator of the LEADOUT Project for SMART Group. LEADOUT was one of the largest EU funded projects, currently he is coordinating European projects TestPEP, ubga and ChipCheck August 2010

3 Text Books On Cleaning Selection of books on cleaning printed board assemblies. There are others that feature chapters on cleaning but are not dedicated to cleaning. The books range from New two volume publication released in Text books cover all aspects of the cleaning processes and materials not just electronic applications. Handbook for Critical Cleaning, Second Edition - 2 Volume Set Barbara Kanegsberg & Edward Kanegsberg (Editors). Cleaning & Contamination Defect Guide The guide can be downloaded or viewed online along with supporting video clips on common process defects seen during cleaning go to

4 Specification Related to Cleaning There is also an IPC document IPC AC 62 Post Solder Aqueous Cleaning Handbook Product reliability Conformal coating Customer specification It looks much better!!! Other production requirements In circuit testing Rework and repair areas Use of more activated fluxes Soluble masking materials Solder paste stencils Solder paste wash off Wave soldering fixtures Why do we Clean?

5 Potential Process Failures Copper dendrites can cause intermittent product failures during product operation. Copper ferns grow from one electrode to another in the presence of a moisture layer on the surface of the board White residues caused by the use of an incorrect cleaning process or chemistry. The flux is not soluble in the chosen cleaning process or some other reaction has caused the flux to become insoluble Printed board failure due to a conductive short formed on inner layers of a multilayer boards generally referred to as a CAF failure. CAF stands for Conductive Anodic Filamentation and related to the materials and conditions inside the board Design for Cleaning Does your design require cleaning? Mechanical and electrical performance Component Compatibility Define a test procedure (Ref IPC 9501 & IPC TM 650) Component stand off Component location Printed board specification (ref IPC 600 & IPC 5702) Final product mounting position

6 Design for Cleaning Component Stand off Height Design for Cleaning Component Standoff Height Typical Component Standoff Heights (Updated from Les Hymes Cleaning Text Book listing) Component Type Lead Count Standoff Height Planar Area in. sq in. Through hole Mounted Components Plastic DIP Ceramic DIP Surface Mounted Components* Rectangular Chip Components RC RC Leadless Chip Carriers (0.050 in. pitch) SOIC SOT Square Plastic J Lead PLCC (0.050 in. pitch) Quad Flatpack (0.030 in. pitch) Plastic BGA (0.050 ) LGA/QFN PoP Device (0.018 ) Standoff can vary depending on the solder mask used and whether mounted in solder paste for retlow or attached with adhesive for wave soldering. The stand off height can also vary depending on the supplier of the parts

7 Testing Component Compatibility Some component issues are very obvious but care needs to be taken on the assessment to avoid short or long term reliability problems Component Test Method Test 5 or more components Measure the weight of the parts Check the component markings Clean for a minimum of 3min or cycle time Reweigh the parts Inspect the markings after wiping once Check part dimensions with reference Has supplier confirmed parts Inspect sealing, moulding & operation

8 Testing Component Compatibility Test board used by the author in 1980 s to asses the impact of ultrasonic energy used to aid cleaning in solvent based systems. The components were used in an open state wire bonds exposed to solvent and the vibration on the bond sites. Never any failures! GEC Hirst Research conducted a large study in the 90s on conventional and SMT components subjected to ultrasonic cleaning with different frequencies and differing solvent volumes. Testing on sample was conducted on parts for standard cleaning times++ Printed Board Orientation for Operation + BGA BGA CSP CSP + BGA BGA CSP CSP CSP + CSP BGA BGA Better drainage of any moisture that forms during product operation

9 Printed Board Orientation for Operation Cleaning + BGA BGA CSP CSP + BGA BGA CSP CSP CSP + CSP BGA BGA Better drainage of any cleaning material, no puddling on the surface of the PCB Flux Compatibility with Cleaning Solubility of flux residues in the solvent is the most important factor is successful cleaning. It is possible for flux and other material to be removed by the force of the spray or with the use of ultrasonic energy. However this is not really a cleaning process

10 Flux Activity Classification J-STD Flux Activity In order to illustrate an approximate analogy between L, M and H type fluxes with the traditional classes of rosin-based fluxes (R, RMA, RA and RSA) as well as other fluxes such as water soluble or synthetic activated fluxes, the following guideline is offered: L0 Type Fluxes -- L1 Type Fluxes -- M0 Type Fluxes -- M1 Type Fluxes -- H0 Type Fluxes -- H1 Type Fluxes -- All R, Some RMA, Some Low Solids no-clean Most RMA, Some RA Some RA, Some Low Solids no-clean Most RA, Some RSA Some Water soluble Some RSA, Most Water soluble and Synthetic activated Flux Materials of Composition 2 Flux activity Levels (% Halide) Flux Designator /Flux Type 1,3 Low (0%) L0 ROL0 Low (<0.5%) L1 ROL1 Rosin (RO) Moderate (0%) M0 ROM0 Moderate ( %) M1 ROM1 High (0%) H0 ROH0 High (.2.0%) H1 ROH1 Low (0%) L0 REL0 Low (<0.5%) L1 REL1 Resin (RE) Moderate (0%) M0 REM0 Moderate ( %) M1 REM1 High (0%) H0 REH0 High (.2.0%) H1 REH1 Low (0%) L0 ORL0 Low (<0.5%) L1 ORL1 Organic (OR) Moderate (0%) M0 ORM0 Moderate ( %) M1 ORM1 High (0%) H0 ORH0 High (.2.0%) H1 ORH1 Low (0%) L0 INL0 Low (<0.5%) L1 INL1 Inorganic (IN) Moderate (0%) M0 INM0 Moderate ( %) M1 INM1 High (0%) H0 INH0 High (.2.0%) H1 INH1

11 Solder Paste Residues after Reflow Two different suppliers products after reflow in air. The resides would also look different if reflowed in nitrogen which will impact the ease of cleaning. Residues that are left on a board assembly are generally more difficult to clean, this is true for paste or liquid fluxes used in wave soldering Cost of Cleaning Materials & Equipment Equipment Batch 35-45K Inline K DI water system Batch 10-12K Inline 20-30K Process Monitoring on site or laboratory (What about your contractors?) Contamination 12K SIR Measurement 22K Cleaning materials Running costs Disposal costs You must inform your local authority Energy costs

12 Dendrite Formation on Conductors 5-10 volts Copper dendrites can form between two conductors with a voltage, a moisture layer and contamination on the surface of a board. It can also occur if the contamination comes from the environment if moisture is allowed to form and stay on the surface. A simple test method based on this illustration has been used in the industry Dendrite Formation Test Method SIR test patterns were placed on scrap areas of the board for in process testing. The test could be conducted in a few minuets or a couple of hours on the shop floor for quick feedback rather than the traditional SIR test used in the industry for qualification testing

13 Dendrite Formation Test Method Select a printed board assembly and a location for test with two terminations approximately 0.010" apart. Apply one drop of distilled water across the two terminations. The distilled water should be confirmed at 1meg H 2 O. Connect a power supply to the two terminations with a 3-5 volt supply. A lower or higher voltage may be used if the products being assessed normally use a different voltage. All tests should be conducted at a single voltage to aid comparison. The circuit board cleanliness may be based on the growth of copper dendrites across the terminations or by monitoring the current and should be <300 micro amp's after 1 min. Using a 5-10X magnification the reaction if any may be observed between adjacent terminals. When the test is complete the water and any reaction products may be removed. The circuit board may then be used in an undamaged state. Repeat the test on another area of the board for confirmation of the results obtained. The test may be used on bare boards, soldered and cleaned boards, con formally coated boards or boards using a no clean process. The test is very sensitive and is an ideal method of assessment but it does not currently relate and can not be compared to existing cleanliness standards. The tests may be conducted and then compared with long term SIR tests or ionic contamination measurements. It is possible to conduct the different tests and compare the results and then set an internal reference criteria. Test method developed and used from material supplied from Motorola and GEC Cleaning Performance Assessment Glass slides mounted to printed boards with different stand off heights. Used to asses cleaning, trapping of cleaning solution and the ability of the cleanliness assessment system to accurately detect residues. Flux is placed under the glass slides prior to cleaning trails. The method is also used to asses the cleanliness assessment systems by using a calibration fluid under the slides and monitoring the removal rates Bob Willis GEC/Marconi technical paper 1986

14 Cleaning Performance Assessment Solder paste residues can be assessed in a similar way with glass slides to simulate component stand off heights to see the relative performance of cleaning system and material sets Cleaning under Area Array Packages Area array devices have a higher stand off than many parts and are not necessarily difficult to clean as shown during videoing. Drying is more of an issue with devices with a low stand off in a aqueous process provided the flux residues have been removed first Testing is conducted with devices bonded to a glass sheet to observe flux residues being removed

15 Cleaning Performance Assessment Check the performance of a cleaning process and your product design for trapping water. Clean a product using the defined process and then test the board by dropping it on a sheet of blotting paper laying on a table. Examine the amount of marks on the surface of the blotting paper. Cheap simple test for displacement of water from under components Cleaning (Drying) Performance Assessment It is possible to use the weight of the board assembly before and after cleaning in a water cleaning systems. This can determine the performance before and after drying of the boards. It does need an accurate set of analytical scales. This is not assessing cleaning more like the drying process

16 PCB Cleaning Options Water Cleaning batch or inline systems Semi Aqueous batch or inline systems Solvent batch systems (inline a thing of the past) Centrifugal batch system Batch Solvent Cleaning System Drain Rinse Rinse Drain Wash

17 Batch Cleaning System Rinse Rinse Rinse Drying Wash (Second Wash) Batch Cleaning System

18 Batch Cleaning System Batch Cleaning System

19 Batch Cleaning System Inline Cleaning System 1 st Wash 2 nd Wash 1 st Rinse Blower Final Rinse Blower Dryer BW Inline H2O In line cleaners can be configured with different stages of wash and rinse, blow off sections and drying steps. They can have bottom jets with topside board hold down chains. The number of nozzles, spray pattern and angle can be configured to meet the required applications. Throughput between 1 2 m/min normally set at the conveyor speed of the wave soldering system or just above, limited process changes

20 Inline Cleaning System Change the nozzle type to increase of decrease the width of the spray fan Change the nozzle type to increase the angle of the spray fan Batch Cleaner BGA 361 BGA 361 BGA 361 BW Batch H2O Often called dish washer cleaners but there is far more sophistication to the units and they are becoming the most popular method of water or semi aqueous cleaning board assemblies. In medium volume multiple chamber or

21 Cleaning Lead-Free Boards Wash time: 8 minutes Wash temperature: 145 F (62 o C) Rinse Cycles: 8 Resistivity cleanliness setting: 1000k/ohms Dry time: 12 minutes Dry temperature: 150 F (66 o C Cleaning solution: Aquanox A Kyzen CP5120 Solution concentration: 20% + 5% Desired cleanliness setting reached after 5 rinses Manual Cleaning Procedure Manual cleaning will always leave some residues on the surface of the board or under devices. If consideration is not given to selecting components that are compatible with the cleaning process second stage assembly will be necessary after cleaning.

22 Examination of Dendrite Formation Looking for corrosion under surface mount devices without removing the component. Use of back grinding to expose the pads and allow probing of the pads Assessment of the Total Process SIR Surface Insulation Resistance - IPC 9201A Also used for in process assessment Contamination Testing Traditional ionic testing Ion Chromatography Visual Inspection - IPC610 UV Inspection Cleaning traps and drying marks

23 Traditional Cleanliness Monitoring 0.1 ug/cm 2 Traditional Cleanliness Monitoring Now often referred to as ROSE testing. Equipment can be purchased to run either 75/25 or 50/50 IPA and distilled water for testing. The test units range in price from 11-20K 0.1 ug/cm 2

24 Traditional Cleanliness Monitoring 0.5 ug/cm 2 Traditional Cleanliness Monitoring 1.1 ug/cm ug/cm 2 of equivalent sodium salt solution

25 Ionic Contamination Measurement Measurement of contamination on a board is also a matter of considering the total result, the total time and the rate at which the contamination rises over time. Remember the ionic contamination can come from soldering materials, the printed board surfaces, fabrication materials and other sources. Many people are happy that the result shows it to be less than the specification limit Ionic Contamination Measurement Measurement of contamination on a board is also a matter of considering the total result, the total time and the rate at which the contamination rises over time. Remember the ionic contamination can come from soldering materials, the printed board surfaces, fabrication materials and other sources. Many people are happy that the result shows it to be less than the specification limit

26 Ionic Contamination Measurement Results of tests on lead-free boards before and after reflow soldering with lead-free solder paste Ionic Contamination Measurement A method used to check the capability of contamination measuring systems to monitor under small stand off components has been used successfully with calibration test fluid. A glass slide is placed with a fixed standoff height. A known quantity of test solution is placed under the glass slide. The test for PCB cleanliness is conducted in the normal way, the result should equal to the test fluid applied

27 Surface Insulation Resistance Testing Measuring changes in surface resistance is a standard way of testing cleanliness and long term reliability of a test board or a complete process assembly based on industry standards. A test pattern will show a change in surface resistance when exposed to high temperature and humidity based on the level of contamination on the test vehicle. Test systems cost between 22-28K, test racks for direct mounting of test boards to avoid manual connection and the associated errors are 6-12K Flux & Resin Test Kit The kits were developed by Zestron as a shop floor test to detect either flux or resin residues on the surface of joints or surrounding areas. The examples above are part of a trial on conventional through hole test boards and surface mount reflowed joints

28 Water Break Test Water break test is used to asses the impact or performance of materials on a surface. Tape residue, oxide formation, handling, cleaning etc. A perfectly clean copper surface should support a perfect coating of water. Any contamination or surface oxide will case the water to separate on the surface IPC 610 Inspection Standard Cleanliness Acceptability Requirements This section covers acceptability requirements for cleanliness of assemblies. The following are examples of the more common contaminants found on printed board assemblies. Others may appear, however, and all abnormal conditions should be evaluated. The conditions represented in this section apply to both primary and secondary sides of the assemblies. See IPC- CH-65 for additional cleaning information. Contaminant is not only to be judged on cosmetic or functional attributes, but as a warning that something in the cleaning system is not working properly. Testing a contaminant for functional effects is to be performed under conditions of the expected working environment for the equipment. Every production facility should have a standard based on how much of each type of contaminant can be tolerated. The more cleaning that has to be done, the more expensive the assembly. Testing with ionic extract devices based on J-STD-001, insulation resistance tests under environmental conditions and other electrical parameter tests as described in IPC-TM-650 are recommended for setting a facility standard.

29 IPC 610 Inspection Standard IPC 610 Inspection Standard

30 IPC 610 Inspection Standard Successful Process Introduction Start checking components/products compatibility today Plan and define your process introduction Select flux, cleaning process and chemistry as a package Chemistry suppliers may have already tested Consider and monitor running costs Keep the process as simple as possible Set up a suitable monitoring process Review the process totally every 6 months

31 To Clean or Not to Clean -The Practical Reality Bob Willis bobwillisonline.com Surface corrosion due to trapped flux after wave soldering. The flux was trapped under the body of an axial mounted component. The board failed during testing in the manufacturing facility.

32 Excessive flux residues were trapped under this coil assembly. Due to the location flux capilaried between the base of the device and solder mask coating leading to poor cleaning Surface dendrites under the solder mask coating probably due to contamination on board surface. The copper dendrites have formed between one track and a via hole pad. The board failed during operation in the field.

33 During cleaning a coating on the surface of the component has been partly removed, the lost of coating did not have any impact on the device or it operation, however the customer did not like it!!. Combination of solvent and aqueous cleaning processes caused the coating of the resistor to lift. Some of the solvent may have leached into the coating and the drying stage of the water cleaning step caused it to swell.

34 Surface corrosion and dendrites due to trapped flux after wave soldering. The flux had reached the top surface of the connector due to the excessive spray pressure used. The board failed during testing in the manufacturing facility.

35 Surface corrosion of the stranded wire due to trapped flux after hand soldering. The flux had wicked up the stranded copper wire inside the insulation. The board failed in the field after 12 months of operation. Copper dendrites forming across the surface of the solder mask due to high moisture layer and some form of contamination. The board failed in the field after 3 months of operation.

36 Corrosion between two conductors under the solder mask. The mask was designed with no separation gap around the pin. During wave soldering the flux wicked up the hole around the pin and could not be cleaned leading to corrosion. Surface corrosion on the laminate surface of the previous example after the mask had been removed. The boards failed after just a couple of days in operation, due to pure flux forming between conductors

37 BGA ball terminations after reflow in a no clean process. The sample board assembly had been through cleanliness testing in a 75%/25% IPA and water. The result was these wonderful white resin rings around the terminations. Poor wetting on the surface of the board is a direct result of the board being washed off after a poor solder paste printing. Its is important to make sure that any cleaning process used for this application does not effect wetting.

38 Poor wetting on the surface of the board is a direct result of the board being washed off after a poor solder paste printing. Its is important to make sure that any cleaning process used for this application does not effect wetting. Poor wetting on the surface of the board is a direct result of the board being washed off after a poor solder paste printing. Its is important to make sure that any cleaning process used for this application does not effect wetting.

39 Solder balling left on the surface of the printed board after wash off. Solder paste particles have become trapped in the resist opening around the pad and when reflowed caused excessive balling. Solder paste particles are visible in the via hole and the paste has not fully reflowed on this resistor network. It is probable that the solder mask has retained some of the solvent cleaner used to clean the board resulting in poor reflow.

40 Excessive low residue flux on the surface of a board. In this case incorrect preheat allowed this frosty coating form which was considered a cosmetic issue. Considerable dendrite formation under the surface of a connector after rework and poor cleaning. The main cause is the amount of flux used during the rework operation.

41 White residues were left on the surface of the board after incorrect cleaning procedures where used. It is not uncommon for some engineers to allow white residue as a cosmetic defect indicator, care needs to be taken as this image shows. Unknown surface contamination under the conformal coating has caused blistering of the coating between these two surface mount devices. Copper dendrites were found during careful removal of the coating.

42 NPL Process Defect Database NPL Process Defect Database

43 NPL Process Defect Database NPL Process Defect Database

44 NPL Process Defect Database NPL Process Defect Database

45 Cleaning & Contamination Defect Guide The guide can be downloaded or viewed online along with supporting video clips on common process defects seen during cleaning go to Do you have any questions? bobwillisonline.com