Solder Paste Printing Fundamentals The stencil printing of solder paste is the most important stage in the modern assembly process. If the process is not set up or operated correctly then the final assembly yield will not meet the needs of the industry. There are many excellent articles on printing from the industry leaders like DEK and MPM so this months issue provides a guide to process set up, inspection standards and typical process defects. DEFECT GUIDE Unfortunately during solder paste printing defects do occur and it is important that they are recognised quickly before soldering defects occur. Many of the defects are not directly associated with the printing process so it is often necessary to re examine printed board specifications, stencil design and the paste materials specifications. Skipped Pads (72.tif) The "halo" effect as it is called is caused by uneven pads normally caused by tin/lead solder levelled pads. It is exaggerated by excessive squeegee pressure when rubber blades are used during printing. Many companies are moving away from tin/lead to gold, silver and copper as they are all flat finishes. Further information on alternative finishes and how to use them is on a video tape available from The SMART Group see Web Page www.smtuk.demond.co.uk Paste Slump (73.tif) Slumping of solder paste is a function of the material or the environment. Correct specification of the material with advice from the supplier can often eliminate the problem. A higher metal content will reduce slumping. Trials should be conducted on the material after the printing operation to measure the degree of spread. Slumping may occur due to the high ambient temperature, particularly on fine pitch where the ratio of height to width is large. Poor Wash Off (74.tif) Solder paste if poorly printed, is often removed and then reprinted. A proper procedure should be devised for washing. If not, paste will remain in the through holes and fuse in the holes during reflow. It will also be present around the resist windows and be seen after reflow. It is good practice to monitor the number of wash offs as a quality check and mark the boards on the edge with an indelible pens which helps to identify the boards during final inspection if problems are seen. Solder Balling (75.tif) Solder balls are small balls of tin/lead solder left after reflow of a paste deposit. In the example solder balls remain after testing a paste sample. Ideally all the paste should reflow forming one large ball. The problem may be caused by the quality of the paste, poor storage, age or incorrect use. During the solder ball test it is now uncommon for paste to fail like the example shown. Smudged Print (76.tif) Clearly something has come into contact with the print, either poor manual handling of the board after printing or the stencil contacting the print after separation. This may occur if there is a snap off set or the separation is uncontrolled. Uneven Print (77.tif) In this example the volume of paste is different on one side of the pattern. This is caused by the use of soft rubber blades and too much pressure. The blade tends to sink into the stencil aperture and scoop out the paste. Smudged Print (79.tif) Clearly something has come into contact with the print either poor manual handling of the board after printing or the stencil contacting the print after separation. This may occur if there is a snap off set or the separation is uncontrolled. Misaligned Print (80.tif) The fault shown is either caused by poor initial board alignment or board error. It is a common fault on printing when boards are located by their edges. Even with tooling hole location, error may occur with large board areas. If the fault is random on the boards the only way to overcome the problem is optical board recognition as the boards are just not consistent. Smudged Print (81.tif) This fault is caused by the movement of the stencil on the pad surface. The uneven tin/lead pads allow the stencil to move on the pads causing inconsistent paste deposits. Changing the surface finish on the boards will greatly enhance the printing process. The most common alternative finish to tin/lead is gold over nickel but there will be moves to copper with a protective finish due to potential cost reductions. The following specification is an example of a document produced for in house quality control and is a standard which is necessary when operating a ISO 9000 quality system. The document can also be a basis for operator training and a reference for process set up. PROCESS SPECIFICATION FOR STENCIL PRINT 1. INTRODUCTION This specification defines the procedures to be used in the stencil printing process. To achieve a good soldered joint it is essential to print accurately and with a consistent thickness of solder paste on the surface of each of the pads on the surface of the circuit board. 2. PROCEDURE 1) Check that the stencil design corresponds to that issued on the Kitting List. 2) Ensure that the stencil is clean with no blockages in the apertures. There should be no evidence of damage to the surface of the metal foil. Fit stencil to printer. 3) Fit PCB to tooling and check for alignment, the PCB should be held firm. 4) Raise bed until PCB just contacts stencil surface. Align the PCB image to the stencil image. 5) Adjust snap-off distance as specified in the engineering set up sheets. 6) Check squeegee blades are clean, flat and fit to printer. 7) Adjust downstops and squeegee pressure as required. 8) Open solder paste pot and stir lightly. A fresh pot of paste must be used at the start of each day. Check the Use-By date of each new pot. If this has been exceeded inform your Supervision. Do not use paste if out of date. 9) Enter the job reference on product Route Card. 10) Print first off PCB and check print quality and registration with the inspection criteria. 11) Sign first off on Route Card when satisfied with the print standard. 12) Print complete batch of PCBs inspecting each print either manually or automatically. 13) All paste print wash-offs should be done before batch is completed. i. Remove excess solder with an old unused squeegee blade. ii. Clean PCB in the cleaning system. iii. Reprint PCB. 14) Sign Route Card to show completion of batch to next operation. 15) At end of print run remove squeegees and clean thoroughly. Remove stencil and scrape off excess paste carefully. Clean stencil thoroughly. Inspect stencil and return to stores. 16) Printer should be cleaned thoroughly. INSPECTION CRITERIA The following inspection criteria may be used in production and ideally example photographs obtained of production board to assist production staff during inspection. Most modern printing systems have inspection systems but a basic criteria needs to be established and programmed in to the system. SOLDER PASTE PRINT FINE PITCH ILLUSTRATION OF SATISFACTORY, ACCEPTABLE AND UNACCEPTABLE SATISFACTORY Solder paste should be aligned with all pads. Solder paste conforms to size of pad aperture size in the stencil. Surface of solder paste print is smooth, flat and free from voids. ACCEPTABLE Solder paste may be slightly Misaligned but covers more than 50% of solder pad surface. UNACCEPTABLE Less than 50% of the pad is covered. Solder paste has slumped between pads. CHIP SOLDER PASTE PRINT DEPOSIT SATISFACTORY Solder paste is aligned with pads. Solder paste conforms to size of pad aperture in the stencil. Surface of solder paste is smooth, flat and free from voids. ACCEPTABLE Less than optimum amount of solder paste. Solder paste covers more than 75% of solder pad. UNACCEPTABLE Less than 75% of the pad is covered. Solder paste has slumped between pads. Examples of inspection criteria may be viewed on the Bob Willis Web Page along with examples which may be down loaded for examination. Engineers may also obtain examples of poor prints and a process defect guide to paste printing defects. Bob Willis is an independent process consultant. Further process advice and information is available on Bob Willis Internet Home Page:http://www.bobwillis.co.uk