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Printed Circuit Board Surface Finish Defects

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Printed Circuit Board Surface Finish Defects Guide to Printed Circuit Board Surface Finish Defects Contents Page SMART Group Introduction 3 PCB Surface Finish Reference Images 4 PCB Surface Finish Introduction 6 PCB Surface Finish Defects 9 Text Books and Standards on Surface Finishes 30 PCB Surface Finish Training Products 31 PCB Surface Finish & Solderability Training Workshop/Webinars 32 Author’s Profile 33 Copyright and Disclaimer Text and images remain copyright of Bob Willis unless stated in the text and should not be copied or transmitted through any medium without prior agreement in writing from the author Although the author has made every effort to achieve accuracy of the content, no responsibility is assumed for errors or omissions to any of the text or references to other publications and documents This book includes hyperlinks to other websites owned and operated by third parties. These links are not recommendations. We have no control over the contents of third party websites, and we accept no responsibility for them or for any loss or damage that may arise from your use of them © Copyright Bob Willis 2017 V1 Page 2 Guide to Printed Circuit Board Surface Finish Defects Introduction Welcome to this SMART Group Defect Guide Our latest SMART Group Defect Guide, is aimed at final PCB surface finish. These finishes are provided to protect exposed copper and to maintain solderability of the pads and through hole termination points to allow successful soldering in reflow, wave, selective and manual processes The choice of solderable coatings in the industry has grown over the years with multiple vendors offering alternatives for all main coating types. Originally, we used fused tin/lead, solder level, copper OSP or flux lacquer coating. With the move to smaller pitch and the introduction of lead- free we have many choices which can make selection difficult and an issue for the supply chain to provide the options a customer demands. Most suppliers focus on just two or three coatings so have options but can control their process. Our latest survey of 100 engineers shows that surface finish is still a common cause of process issues for users assembling bare PCBs With information to help the newcomer and expert user we hope you find this a useful guide and ask you to pass on some SMART Group knowledge to your team, your customers and suppliers. If you have any other defect types relating to surface finishes why not add them to the NPL Defect Database http://defectsdatabase.npl.co.uk or send them to [email protected] Many thanks again for support to SMART Group Bob Willis SMART Group Technical Committee © Copyright Bob Willis 2017 V1 Page 3 Guide to Printed Circuit Board Surface Finish Defects PCB Surface Finish Reference Images The following example images are typical of printed circuit boards with different surface finishes. A surface finish is the final coating on surface mount pads and through holes when supplied by the manufacturer. The surface finish is specified by the assembler based on the process for assembly and environment the final product may be exposed to. Using the correct surface finish is very important to obtain good yields during assembly and soldering, the introduction of fine pitch & lead-free assembly increased the options Copper Organic Solderable Protector (OSP) (50 – 200x) Immersion Silver (50 – 200x) Immersion Tin (50 – 200x) © Copyright Bob Willis 2017 V1 Page 4 Guide to Printed Circuit Board Surface Finish Defects Solder Levelled Lead-Free (50 – 100x) Nickel Gold (50 – 200x) Whichever surface finish is specified by the designer or assembly engineer it is very important that they understand the material, its application in PCB manufacture and how to assess the quality of the coating. They should also know the specific type and supplier of the finish chemistry used and the method used by the manufacture to test the solderability of the boards prior to despatch. This is important to avoid the debate about what is or is not acceptable. Solder float and dip testing is not reflective of modern reflow technology Engineers should be fully aware of the packaging of the boards and the length of the supply chain prior to assembly so they can correctly judge changes in solderability that may occur prior to the first assembly steps. Often prototypes and NPI boards are produced from boards made yesterday and delivered by aeroplane. Volume manufacture is often based on batches of boards that are much older and have spent the first part of the life on a ship. It is recommended that goods receipt and technicians on the assembly line know how to recognise different surface finishes and if they are acceptable when first unpackaged for use. They should have some reference like the images in this guide but, better still, examples from their own supplier It is recommended that the quality department take low and high magnification images of known good surface finishes from the supplier that can be used as a reference by staff. The images should accurately show the colour, the surface texture and evenness of the coating. In the case of thin coating the surface of the copper topography. There should be no evidence of solder mask residues on the surface of the pads causing lack of proper surface coating of the pads or through holes. The images provided here are satisfactory, but may not represent the colour or surface preparation of your supplier © Copyright Bob Willis 2017 V1 Page 5 Guide to Printed Circuit Board Surface Finish Defects Printed Circuit Board Surface Finish Introduction The following is a brief description of the most common coatings used in the industry and generally referred to as surface finishes or solderable coating. Immersion Tin Immersion processes like silver and tin have an organic deposit as part of the process that reduces the oxidation that would be expected with a pure tin or silver surface. Tin and silver dissolve into the solder and become part of the solder joint and with such a thin coating it’s relatively undetectable. Both tin and silver are dissolved during soldering so the solder joint forms directly with the copper surface just like copper OSP. On gold boards the solder forms a joint with the nickel after dissolving the gold. Just like silver tin is mainly defined by the control of the printed board process although the assembly operation can have a major impact on the solderability. The key to the process is eliminating the copper/tin intermetallic forming at the surface of the coating. Now with 1um coating and a less porous finish this provides a more stable coating for assembly. It is being used for boards that also incorporate press fit connectors as this does benefit the insertion forces with a degree of lubricity. Having extended factory hold times before second side reflow or selective soldering will affect wetting just like delay in second stage assembly operations See IPC-4554 Specification for Immersion Tin Plating for Printed Circuit Boards Immersion Silver The coating is an immersion silver coating of between 0.08-0.1um which also incorporates an organic layer as part of the process. The silver "Alpha Level" coating is maintained in a highly solderable state by the organic coating. Although Cookson were the first to offer this coating there are other suppliers also providing silver solutions like MacDermid. The surface coating has all the benefits of any alternative finish and resembles the tin coating when soldered. If during soldering the solder dose not fully wet the pad surface it is not as obvious as with copper and gold pads. In the case of unsoldered holes or test pads there is no visible gold or copper, which to some engineers is an emotive subject. The coating cost in medium to high volume is equal to or less than nickel/gold. Like any alternative coating, provided it is processed correctly by the circuit board manufacturer, the surface will remain solderable even after multiple heating cycles. Hence it is compatible with double sided reflow. Issues have been seen with washed off boards and the use of temp solder mask coatings. The basic coating process must be reviewed with the supplier to make sure he is using guidelines from the chemical supplier. A key issue is the control of the process and the final rinse and drying stages. See IPC-4553 Specification for Immersion Silver Plating for Printed Boards Copper OSP The protective coatings are generally defined as organic coatings referred to as OSP, (Organic Solderable Protector or Organic Solderability Preservative). The most common coatings are benzotriazole and imidazole; both are organic nitrogen compounds. Benzotriazole has long been recognised as an anti-tarnish coating used in the general metal finishing industry. Inhibitor coatings are extremely thin and essentially invisible on the copper surface. © Copyright Bob Willis 2017 V1 Page 6 Guide to Printed Circuit Board Surface Finish Defects The coatings protect the copper by chemically bonding to the surface and prevent the reaction between the copper and oxygen. The coating may be applied by dip or spray coating and followed by a rinse operation to remove any residues remaining on the solder mask surface. If required, the coating may be removed and re-applied to rejuvenate a surface which has become un-solderable. If required, the surfaces would need to be re-cleaned with an acid etch and rinse prior to re-treatment. See IPC-4555 Specification for Organic Solderability Preservative (OSP) for Printed Circuit Boards (Committee Draft Never Released) Nickel Gold Gold over nickel has become very popular finish for surface mount boards particularly for fine pitch and mobile communication products like mobile phones. They have provided an ideal assembly surface, highly solderable and an aid to inspection due to the contrasting colour between component leads, solder and the solder paste.
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