9 SMT Board Assembly Process Recommendations

9 SMT Board Assembly Process 
Recommendations

9 SMT Board Assembly Process Recommendations

9 SMT Board Assembly Process Recommendations .......................................................................9-1 9.1 Introduction............................................................................................................................9-2 9.2 Solder Paste ...Printing..............................................................................................................9-2 9.3 Component Placement...........................................................................................................9-2 9.4 Reflow Soldering...................................................................................................................9-3 9.4.1 Pb-free vs SnPb Reflow Soldering ................................................................................9-3 9.4.2 Reflow Profile Development Considerations ................................................................9-3 9.4.2.1 Reflow Profile Board Preparation .............................................................................9-4 9.4.2.2 Minimum Solder Joint Peak Temperature.................................................................9-5 9.4.2.3 Maximum Solder Joint Peak Temperature ................................................................9-5 9.4.2.4 Time Above (Initial) Melting Point...........................................................................9-5 9.4.2.5 Rising and Falling Ramp Rate...................................................................................9-6 9.4.2.6 Reflow Equipment.....................................................................................................9-6 9.4.2.7 Reflow Atmosphere...................................................................................................9-6 9.4.2.8 Board Warpage ..........................................................................................................9-7 9.4.2.9 Double Sided SMT Board Assembly ........................................................................9-7 9.4.3 Sample Reflow Parameters............................................................................................9-7 9.4.4 Sample Reflow Profiles .................................................................................................9-8 9.4.4.1 Sample Desktop Reflow Profile ................................................................................9-9 9.4.4.2 Sample Mobile Reflow Profile ..................................................................................9-9 9.4.4.3 Sample Server Reflow Profile .................................................................................9-10 9.5 Rework.................................................................................................................................9-10 9.5.1 Pb-free vs SnPb Rework..............................................................................................9-10 9.5.2 Risk of SnPb and Pb-free Mixing................................................................................9-10 9.5.3 Rework Profile Board Preparation...............................................................................9-10 9.5.4 Pad Cleanup After Component Removal ....................................................................9-11 9.5.5 Paste Printing Methods at Rework ..............................................................................9-12 9.5.6 Re-balling BGAs Not Recommended .........................................................................9-13 9.5.7 Sample Rework Parameters.........................................................................................9-14 9.5.8 Rework Profile Development ......................................................................................9-15 9.5.9 Sample Rework Profiles ..............................................................................................9-16 9.5.9.1 Sample BGA Rework Profile ..................................................................................9-16 9.5.9.2 Sample Socket Rework Profile................................................................................9-17 Intel Packaging Databook Revised 12-2007 9-1 Board Reflow Process Recommendations 9.1 Introduction This chapter addresses the surface mount technology (SMT) board assembly process for reflow soldering SMT components to boards, as well as rework soldering for removing and replacing individual components on already-assembled boards. The information in this document is for reference only. Manufacturing processes are unique, and may require unique solutions to ensure acceptable levels of quality, reliability, and manufacturing yield. Due to differences in equipment and materials, customer-specific process parameter development and validation is required. 9.2 Solder Paste Printing Standard tin-lead (SnPb) solder paste alloy is composed of 63% tin (by weight) and 37% lead, which is commonly expressed as 63Sn/37Pb or Sn/37Pb, a eutectic composition that melts at 183C. Although there are a number of lead free (Pb-free) alloys, the most commonly used compositions contain tin, silver, and copper, commonly expressed as SAC, for SnAgCu. Within SAC solders, by far the most common usage is Sn/3Ag/0.5Cu, a near-eutectic which melts between 217°C and 220C. Apertures sizes can be 1:1 with pad size, but certain parts may require reduced apertures to reduce solder ball defects. Larger pads may benefit from crosshatched openings, to reduce the amount of paste applied, and to control scavenging. Pb-free solder paste may spread less during reflow than SnPb paste, potentially leaving extremities of pads unsoldered. Although full pad coverage by solder is not a requirement of IPC-A-610, some customers prefer to enlarge apertures to ensure that pads are covered. Using a metal squeegee reduces scavenging and provides more consistent printed paste volume. Equipment used to print SnPb paste can be used, without modification, to print Pb-free paste. Process parameters (such as squeegee speed, pressure, and separation speed) need to be optimized for the specific solder paste used. 9.3 Component Placement Pick and place machines used for SnPb boards can be used for Pb-free boards as well. Adjustments to lighting and vision algorithms may be required because of the slightly different appearance of Read the full 9 SMT Board Assembly Process Recommendations.

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