Application Note 5026

Transcription

1 Surface Laminar Circuit (SLC) Ball Grid Array (BGA) Eutectic Surface Mount Assembly Application Note 5026 Introduction This document outlines the design and assembly guidelines for surface laminar circuitry (SLC) ball grid array (BGA) packages. Package Description SLC BGA is a custom platform supporting a wide variety of performance applications. Flip-chip is the first level interconnection, and solder ball is the second level interconnection to a printed circuit board (PCB). SLC BGA packages are qualified to JEDEC level 4 moisture sensitivity. The SLC BGA package consists of (see Figure 1): Core up to 6 layers Build up layers up to 4 on each side of the core Body size up to 45 mm square Body thickness not including the stiffener ring and lid will vary depending on the core dielectric and prepeg thickness Bumped die up to 16 mm square Coplanarity at the tip of the balls: 0.20 mm for packages larger than 35 mm 0.15 mm for packages smaller than 35 mm Heat spreader is 0.5 mm thick Nickel-plated copper Stiffener ring is 0.6 mm copper black oxide Bypass Capacitors (0508 and 0306) 1 mm pitch grid array Solder ball size mm diameter Printed Circuit Design SLC BGA packages are qualified for PCBs with a coefficient of thermal expansion (CTE) of approximately 16 to 20 ppm/ C. Avago Technologies used Entek Cu-56 surface for all evaluations. Pad Design The package is soldered to the printed circuit board using a dog bone pad design (Figure 2). The solder mask opening is larger than the copper pad to prevent the creation of high stress points in the eutectic solder at the solder mask interface. Since the via is not tented, it is important to include a solder mask dam between the pad and the via to prevent loss of the solder paste. During evaluation, vias were not tented on either side to prevent the entrapment of flux and to allow for testing during troubleshooting. Tenting a via on only one side can lead to the entrapment of flux or contaminants in the via. The pad diameter is 0.50 mm ( inches). A clearance area of inches (5.08 mm) is required for stepdown if the SLC BGA package is near a fine pitch component or needs rework. The interconnection is a low melting temperature 63/37 Sn/Pb round solder ball that is joined to the printed circuit board using eutectic solder. The standoff height is about mm, but under the die the standoff is less than mm Figure 1. SLC BGA Cross Section * SLC is the registered trademark of Kyocera Corporation.

2 Printed Circuit Board Design Simply selecting the correct pad will not ensure good solderability. Local board warpage is a major contributor to local Z axis tolerances. PCB warpage can be minimized by proper design/layout of the circuitry. The assembly thermal mass should be as uniform as possible across the printed circuit board. The form factor of the board is critical; the larger the board the more critical the thickness and fixturing during reflow. Layout of the printed circuit board layers should be as symmetrical as possible to reduce warpage during reflow. Solder Paste Deposition on the Topside Solder Paste Volume Measurement Component Placement: SLC BGA and Other Others Reflow Clean if Required Inspect Figure 3. Single-Sided SMT Process Flow Solder Paste Deposition on the Topside Solder Paste Volume Measurement Figure 2. Dog Bone Design Component Placement: SLC BGA and Others Surface Mount Process Flow The SLC BGA packages were assembled to printed circuit boards using standard surface mount processes and equipments. The SLC BGA package can be attached on the bottom side of the printed circuit boards. The SLC BGA package is compatible with a single-sided surface mount and hybrid process where a wave solder is being used to attach through-hole components. The two process flows are described in Figures 3 and 4. Reflow Clean if Required Backside Adhesive Dispense SMT Component and Through-hole Placement Wave Solder Clean Inspect Figure 4. Hybrid SMT/TH Process Flow 2

3 Solder Paste Deposition Solder paste deposition is arguably the most critical step to assure good reliability of the SLC BGA package. Attention to solder paste, stencil and printing parameters will produce a consistent process. a. Solder Paste SLC BGA does not require a special paste, but considerations should be given to the consistency of the paste from lot to lot and the recommended solder paste supplier reflow profile. SLC BGA packages have a high thermal mass. A typical solder paste is eutectic (63/37 Sn/Pb), 90% solder by weight, about 50% solids per volume, type 3 particle size. Avago Technologies used Indium NC-SMQ 92 J. b. Solder Paste Requirements Nominal paste volume of 1450 cubic mils is recommended for reliable solder joints. A typical good print will have a standard deviation of 7-10%. Print registration should be inches to assure reliable solder joints. c. Stencil Design A stencil of inch thickness with a round aperture of inches will deliver the required volume of solder paste. d. Print Inspection Avago Technologies strongly recommends 100% visual inspection for clogged stencil opening and misregistration. Avago also recommends solder paste volume measurements. Placement a. Placement Force The solder balls are not easily damaged during placement if the part is picked directly from the trays and placed on the boards without excess handling. A placement force of 150 grams will reduce the paste displacement from the balls. b. Placement Accuracy SLC BGA packages are forgiving during placement. The SLC BGA packages will self align during reflow. The SLC BGA package can be placed 50% off the pad. Vision recognition provides the fastest placement and the most accurate placement. This technique uses a look up camera to locate the tips of the balls and a look down camera to locate the fiducials on the printed circuit board. For better accuracy, a camera with a resolution of 2.6 mils per pixel was used in conjunction with an algorithm that took four shots, one at each corner of the package. Solder Reflow a. Reflow requirements It is critical to maintain the solder reflow profile recommended by the paste supplier. Avago Technologies used a no clean paste but a water soluble paste can be used. The peak temperature of the lead should not exceed 220 C. Supplier recommendations for that solder paste: Ramp Rate: 0.5 to 1 C/sec Reflow: 210 C for seconds Total heating dwell time may be four to six minutes depending on thermal inertia and component sensitivity. Maximum ball temperature: 220 C Maximum board temperature: 240 C Minimum ball temperature: 200 C b. Thermal profiling Proper thermal profiling is required to establish the right reflow profile and ensure that all joints meet the profile specifications. Placing the thermocouple in the solder joint leads to the most accurate thermal profiling. The best way to achieve this is to drill from the backside into the joint and to plug the hole with epoxy. The recommended locations for thermocouples is: Board surface Top of SLC BGA Joint in the center of the SLC BGA Joint at the edge of the SLC BGA The printed circuit board should be fully populated to reflect the true thermal mass. One profile does not fit all printed circuit boards. c. Reflow Solder reflow can be achieved in various ways, and Avago Technologies has experience only with convection ovens. Nitrogen atmosphere results in good wetting and fillet formation. 3

4 Cleaning No cleaning was performed since a no clean paste was used. Optimization of the cleaning and drying parameters are specific to each manufacturing house. Inspection Visual inspection of 100% of the solder joint is not feasible. Excellent process control is required to ensure good joint and high yields. An acceptable SLC BGA solder joint is one that exhibits a shiny solder ball with good wetting on the pad. (See Figure 5). Solder Joint Reliability The reliability of the solder connection is driven by the CTE mismatch between the laminate with a CTE of about 16 ppm/ C and the epoxy printed circuit board ppm/ C, since the PCB and the substrate are matched, the first failure was observed at 1642 cycles. a. Test Vehicle Design One SLC BGA package size was used for the test. The package was a 45 mm square. 45 mm square package 3 build up layers 6 core layers 45 by 45 by mm thick 1837 I/O (4 corners with depopulation of 3), 1 mm pitch Heatspreader 45 by 45 by 0.5 mm Daisy chain die 16 by 16 mm 3 outer ring daisy chain 2 inner ring daisy chain Level 1 was monitored during the test b. Printed Circuit Board Packages were assembled on an 18 by 12 inch panel that was singulated into a single image, 4.1 by inches. The 3.1mm thick FR4 board had 8 layers including ground and power planes. The board technology was OSP with a liquid photo-imageable solder mask. The pad size on the printed circuit board was inch circular, non-solder mask defined. c. Test Condition All the samples were subjected to an accelerated thermal cycling test in a single zone thermal chamber. The temperature cycled between 0 and 100 C within a 54-minute cycle. The cycle was as follows: C in 17 minutes Soak at 100 C for 10 minutes C in 17 minutes Soak at 0 C for 10 minutes All the daisy chains were connected to an HP data acquisition system. The resistance of each daisy chain as well as the four thermocouples was continuously scanned and recorded every 60 to 80 seconds on the hard drive. A thermocouple was mounted on the lid of several SLC BGA packages; all the thermocouples were connected to the data acquisition system. d. Weibull Plots for Virgin Parts The characteristic life (63.2% accumulated failures) for the virgin package is 2360 cycles and the slope is Figure 5. Acceptable SLC BGA Solder Joint 4

5 SLC BGA Rework The objective of the rework process is to create solder joints that are equivalent in reliability to the first pass assembly. It is important to note that the critical requirements for the first pass assembly still apply to the rework process, consistent solder paste deposition, and thermal profile for the reflow. The process flow for reworking SLC BGA package is similar to the first pass assembly. (See Figure 6). Thermal Profile SLC BGA Site Remove SLC BGA Package Site Preparation Solder Paste Deposition SLC BGA Package Placement Solder Reflow Clean if Needed Inspection Figure 6. SLC BGA Rework Process Flow a. Equipment needed: Hot gas tool for packagen removal, virgin package placement and reflow Tool to remove solder from the printed circuit board pads Stereomicroscope In-line aqueous cleaner Mini stencil to apply the solder paste to the board b. Preheat In order to minimize thermal shock to the printed circuit board, the entire board should be preheated between 75 and 100 C. c. SLC BGA removal Once the printed circuit board preheat temperature is achieved, a hot inert atmosphere is applied to the package until all the joints are reflowed and the package can be removed with a vacuum pick-up tool. d. Site Preparation Site preparation is needed to remove excess solder; usually most of the balls do stay with the package. The most common method is to use a solder vacuum tool. Caution should be exercised when using the tool not to damage the printed circuit board pad. After flux application, the tool is swept across each pad to remove most of the remaining solder. e. Solder Paste Deposition Replacing the eutectic solder is required to create a reliable SLC BGA solder joint. Solder paste must be applied to the printed circuit board. f. Package Placement After the solder paste deposition, the package needs to be placed in the solder paste. Various methods can be used to accurately place the package on the solder paste. g. Solder Reflow Once the package is placed on the printed circuit board, the eutectic solder paste must be reflowed to form a reliable joint. To get a good joint, the joint temperature should be kept between 200 and 220 C. For product information and a complete list of distributors, please go to our web site: Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies in the United States and other countries. Data subject to change. Copyright Avago Technologies. All rights reserved. AV EN - July 20, 2010