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[分享]CSP and BGA process guidelines[文章] [复制链接]

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2003-06-23
只看楼主 倒序阅读 使用道具 楼主  发表于: 2003-09-11
1.0      PURPOSE                                                                                                                                                                                                                                                                                             The CSP and BGA process guideline establishes a methodology to ensure suppliers’ meet Nvidia’s criteria for Proper CSP and BGA processing for high yields and the greatest long-term reliability.

2.0      SCOPE
This procedure is applicable to all suppliers and sub-contractors that deliver products and services to Nvidia.

3.0      REFERENCE DOCUMENTS
3.1      The issues of the following documents on the date of use form a part of this specification to the extent shown here in.
3.2      ISO9002 Quality Systems - Model for Quality Assurance in Production, Installation and Servicing.
3.3      630-0011-001 PCB Assembly guidelines (Available from Agile)
3.4      OPSQA70009 - Electrostatic Discharge (ESD) Damage Control Specification. (Available from Agile)
3.5      IPC610C-Acceptabiliy and Inspection criteria for Printed Circuit Assembly’s
 
4.0      MATERIALS AND/OR EQUIPMENT
4.1      Industry standard SMT production equipment.
4.2      Minimum 2D paste inspection equipment to inspect solder deposition before placement and reflow.

5.0      DESIGN IMPLEMENTATION
5.1      Land pattern design and board finish requirements are the most important considerations for proper implementation of BGA’s and CSP devices.    

6.0      PCB BOARD FINISHES
6.1      Choices of board finish will have direct impacts on process yields and long-term reliability. Nvidia recommends the use of immersion gold, immersion silver, or white tin as a choice of board finish. The use of OSP’s is not recommended due to board handling issues and rework concerns. HASL is not recommended due to coplanarity issues. See the attached application notes from Tessera for more information.
7.0      GENERAL REQUIREMENTS / COMMENTS
7.1      If a company utilizing industry best standards for SMT assembly are currently achieving acceptable assembly yields than the implementation of CSP and BGA’s into the process should not require any additional resources or any new methodology’s to achieve the same yields.
7.2      Industry standard contract assembly shops have shown the implementation and assembly of BGA and CSP package’s have shown                     to have fewer process related defects than the use of fine pitch QFP and TSOP packages.
7.3      BGA”S and CSP’s have higher yields because of there tendency to self align if placements is less than 50% off unlike there leaded counterparts that would require rework to remove opens and bridges. The BGA and CSP packages also are a more robust package and do not have the coplanarity or bent leads issues that QFP”S and TSOP”S can sometimes have.

8.0      ASSEMBLY PROCESS ISSUES
8.1      The SMT assembly process has many factors that must be closely controlled to achieve good process yields and long term reliability of the finished product. The key focus areas that will affect yields will be covered in this section.
8.2      Since BGA’s and CSP’s will self-center and reflow process parameters are the same as standard SMT components it has been found that proper process controls at the solder paste printing process along with the proper design of the stencil are primary in the success of BGA and CSP implementation.
8.3      Selection of solder paste is critical to the process. For most a no-clean paste containing 63Sn/37Pb (Tin/Lead) is selected. The mesh size is important also. A mesh size type 3 can be used for most types of BGA’s however for CSP a type 4 of finer mesh may give you better printability and release of the solder paste from the stencil.
8.4      Solder paste should be stored in a temperature controlled refrigerator and removed at least 4 hours before use. It must be mixed thoroughly before applying to the stencil. It must have a uniform viscosity and be free from any foreign material. Also important to good printing results is to control the environment in the screen printer or the total factory so as to prevent the paste from drying out which can cause clogging of the apertures and poor paste release. Recommended temperature should be controlled at 70F (21C) to 74F (23C).
8.5      Automatic stencil printing machines with the ability to automatically clean the stencil are required. You may find that the cleaning frequency will need to be used every 2nd or 3rd print in order to maintain good process controls and good printing results.  
8.6      In-Line inspection machine should be used to inspect all BGA and CSP lands for any skips and voids of solder paste. Defective boards shall be cleaned and washed before reprinting of solder paste.

        9.0     STENCIL DESIGN REQUIRMENTS
9.1      Stencil design is critical for proper CSP implementation. See the enclosed recommendations from Tessera.
9.2      Nvidia’s recommendations are for a .005” thick laser cut stencil with electropolishing and a .014” square opening with .006”R (max) in the corners. Also spec out the use of trapezoidal openings of .001” (.015”) overall taper from solder side to board side.      

10         PART PLACEMENT
10.1 Typical pick and place equipment may be used. For the CSP memory devices with a 0.80mm ball pitch placement accuracy should be within 0.1mm or (.004”). Although perfect placement accuracy is the goal it is usually not a critical process requirement due to the fact that even with up to 50% misalignment BGA’s and CSP’s can self-center during the reflow process due to the surface tension of liquidus solder. In fact unless there are gross misalignments of the device do not try to manually align the component before reflow, as you will induce more process defects. If the component is misaligned by more than 50% you should remove the component and rework the board. Nvidia has located a silkscreen outline around the perimeter of the BGA and CSP devices to aid in visual alignment inspection.

11      SOLDER REFLOW
11.1      Reflow profiles for CSP/BGA devices are similar to other SMT packages. Forced air convection reflow ovens are the preferred type of ovens to use. Profiles used should meet the solderpaste manufacture’s recommended profile parameters. For most paste suppliers using a 63/37 tin/lead eutectic solder alloy paste the slope of the profile should not exceed 3 degrees C per second with 2 degrees C per second being the goal. Eutectic solder becomes liquidious at 183C but the temperature should continue to rise by 30C-40C to ensure that proper wetting occurs. The dwell time above liquidious should extend to 60 seconds to ensure that all the flux materials separate from the alloy to help eliminate voids within the solder joints. Max temp should be kept under 230C with the goal being at 225C for no more than 20 seconds. Once again check with your paste supplier for their recommended profile. Each PCA is to have it’s own Thermal Profile. The profile for the center and an outer edge ball must be part of the thermocouple setup. Five minimum points are to be measured. Run Data Pac (M.O.L.E.) (KIC) at each PCA model change over, if a different model oven is used, and once a day. It has been found that for proper soldering process control it is critical to inspect the paste coverage before placement of the CSP/BGA devices.

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只看该作者 沙发  发表于: 2003-09-11
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