2. SCOPE This document will review the board design requirements, printing process, reflow process, rework considerations, and troubleshooting.

Transcription

1 Solder Reflow Guidelines for 0.65 mm Pitch BGA Packages Application Note 1. PURPOSE This application note provides basic guidelines for 0.65 mm pitch lead-free solder BGA reflow process and design requirements for high assembly yield and reliability. 2. SCOPE This document will review the board design requirements, printing process, reflow process, rework considerations, and troubleshooting. 3. REFERENCE DOCUMENTS JEDEC STD-020D-01 Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices. IPC-7351 Generic requirements for Surface Mount Design and Land Pattern IPC-A-610D Acceptability of Electronic Assemblies IPC JEDEC 9702 Monotonic Bend Characterization of Board-Level Interconnects IPC JEDEC 9704 Printed Wiring Board Strain Gage Test Guideline 4. DEFINITIONS ENIG: Electroless Nickel Immersion Gold OSP: Organic Solder Preservative BGA: Ball Grid Array NSMD: Non-Solder Mask Defined SMD: Solder Mask Defined AWG: American Wire Gauge HASL: Hot Air Solder Level 5. BOARD DESIGN REQUIREMENTS 5.1. Incoming Inspection The IPC-A-600 Acceptability of Printed Circuit Board shall be used as the minimum quality requirement for board acceptance criteria. Class 2 or 3 is recommended PCB Material Higher lead-free reflow temperatures require the use of better thermally resistant laminate preferably with a Tg >170 C Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 1 of 10

2 5.3. Surface Finish A compatible surface finish needs to be considered during board design. Commonly used finishes are shown in Table 1. Table 1: Commonly used surface finishes Surface Finish Immersion Tin Immersion Silver ENIG: Electroless Ni, Immersion Au OSP: Organic Solderability Preservative Pb-free HASL: Hot air solder level Potential Risks Shelf life, etchant entrapment in small vias Shelf life, sulphur creep corrosion Cost, black pad Limited thermal resistance, weak PTH fill Planarity, extra thermal cycle The surface finish shall be selected based on the process, product and environment. The surface finish shall meet the ANSI/ J-STD-003 solderability requirements Design The board design shall meet the IPC-D-275 design requirements Landing Pad NSMD vs. SMD NSMD: Non-Solder Mask Defined A BGA Non-Solder Mask Defined pad (Figure 1) is a solder pad where the solderable surface is not limited by the solder mask. This type of pad offers better reliability than the SMD. The typical solder mask opening is at least 100 µm greater than the Cu pad. Solder Mask Pad Figure 1: Non-Solder Mask Defined SMD: Solder Mask Defined A BGA Solder Mask Defined pad (Figure 2) is a solder pad where the solderable surface is limited by the solder mask. Typically the Cu pad size is at least 100 µm greater than the solder mask opening. SMD pad definition introduces stress concentration points in the solder that may result in solder cracking under extreme conditions. This pad type also offers less control over final dimensions. Solder Mask Pad Figure 2: Solder Mask Defined Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 2 of 10

3 Solderable Surface Diameter For best reliability, the PCB solderable surface shall have a ratio of 1:1 to the component BGA solderable surface. Table 2: BGA Pad Solderable Diameter Part # BGA Pad Solderable Surface INSSTE32882-GS µm INSSTE32882LV-GS µm INSSTE32882UV-GS µm imb02-gs02/imb02-gs02a 350 µm Trace Width & Via Location The trace width (Figure 3) that connects to the BGA pad shall not be larger than half of the pad size. For smaller pad size less than 300 µm, the trace width shall not be smaller than 100 µm. Large unmasked traces thieve solder, reducing the amount of solder available for the joint, skewing the solder and potentially reducing reliability. Also, the trace shall never be in a corner for a rectangular pad as it introduces a stress on the solder joint. Pad Solder Mask Figure 3: Trace Width Clearance Around the BGA A 2 mm depopulated area all around the BGA is recommended for removal & rework purposes. This gap allows the rework tools to reflow the solder as needed for removal and minimizes impact to adjacent devices Thermocouple Location & Profile Thermocouples shall be placed under the Inphi BGA package for optimal reflow profile temperature measurement (Figure 4). This aids in reducing the effect of thermal mass variation and component location on board. (a) (b) Figure 4: Thermocouple Location: a) wire-bond, b) flip chip Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 3 of 10

4 Type-k thermocouples with a wire gauge of 36 AWG should be used. Thicker thermocouple wires add measurable heat sink. Never use high-temperature tapes, such as polyimide (kapton) or aluminum tapes, because they tend to come loose during reflow. Loose thermcouples may result in measurement of the temperature of the air in the oven rather than the temperature of the solder joints. Always use high-temperature solder or thermally conductive adhesive to attach thermocouples to the solder joints. It is recommended to perform the oven profile using a typical manufacturing load. It is also important to make sure that the leading and exiting assemblies are within the profile requirements. Profiling should encompass all areas of the board and components of varying size and thermal mass. 6. PROCESS FLOW The BGA s surface mount assembly process flow includes: Floor life monitoring as described in JEDEC J-STD Solder paste printing on the board Solder paste visual inspection and volume control Package placement. Pre-reflow inspection Reflow Profile Cleaning (If required per the solder paste manufacturer recommendation) Post reflow inspection Rework (If required) 6.1. Floor Life The assembly house shall have an inventory monitoring system for floor life control to ensure that the product shelf life is not exceeded during assembly as per the JEDEC Specification J-STD- 020D-01. If the time limit is exceeded, a 24-hour bake at 125 C is recommended. The number of bake cycles shall be minimized and performed in a nitrogen atmosphere. An accelerated bake at 150 C is not qualified and is not supported by Inphi. Table 3: Maximum Floor Life Level Time Maximum Floor Life Conditions Level 1 Unlimited 30 C / 85% RH Level 2 1 year 30 C / 60% RH Level 2a 4 weeks 30 C / 60% RH Level hours 30 C / 60% RH Level 4 72 hours 30 C / 60% RH Level 5 48 hours 30 C / 60% RH Level 5a 24 hours 30 C / 60% RH Level 6 Time on Label (TOL) 30 C / 60% RH Note: Current J-STD-020D-01 Requirements Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 4 of 10

5 6.2. Solder Paste Solder Paste Requirement A Type 4 solder with a minimum of 90% metal by weight is recommended Dispense The solder paste printing process involves the transfer of solder paste through pre defined stencil apertures. Stainless steel stencils are recommended for solder paste application. Stencil manufacturing method, coating, thickness, aperture shape, aspect and area ratio, and solder paste metal mesh size are some of the factors that affect the solder deposition on the board and therefore the quality of the solder joint A 5 tapered aperture is preferred over a straight aperture. A smooth finished surface achieved by chemical polish is recommended over a rough surface. A plated surface finish is also recommended. In any case, the aperture size shall be slightly larger (50 µm) than the solderable surface All solder deposits should be inspected 100% for missing solder paste, uniformity, volume, and misregistration (maximum of 75 µm) During stencil design, special attention should be paid to pad coverage due to the higher surface tension of lead-free solder. This solder does not spread or wet the pad surface as well as eutectic solder The assembly house shall have a monitoring system in place for post print solder shelf life and shall follow the solder paste supplier recommendations Solder Paste Volume For a 0.65 mm pitch product the solder paste volume shall vary between: 350 µm pad minimum recommended solder volume = µm µm pad maximum recommended solder volume = µm Placement Sufficient placement force shall be used to place the component solder balls into the solder paste. Components shall never be just floating over the solder paste. Apply sufficient force (over travel) but do not exceed 300 g. Placement speed and height must be adjusted so that the vacuum tip does not leave any damage to the top surface of the package. Rubber tips should be avoided since they may leave residue that could later prevent good heatsink adhesion. Placement vacuum tip location over the package shall be centered within 0.5 mm at (X and Y) of the package center since off-center picking could damage the component. Placement vacuum tip must provide ESD protection as required by the product data sheet. Placement speed must be controlled to avoid solder spattering. Solder after placement shall surround the BGA balls uniformly. Minimum placement accuracy as measured between the BGA tip and pad center shall be less than 100 µm at 99.8% of the time, preferably 50 µm for a 0.65 mm pitch BGA package Reflow Oven A reflow oven equipped with a minimum of 8 heating zones is recommended to allow flexibility for profile optimization. Lead-free solder typically requires more profile adjustment. A nitrogen atmosphere is recommended since it improves wettability and self-alignment and reduces the temperature gradient across the oven. It also improves BGA joint visual inspection Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 5 of 10

6 Reflow Profile Table 4: Reflow Profile Phase JEDEC JTD-20D-01 Inphi Recommendation Ramp-up C/sec Soak Minimum Temperature (Ts min) Maximum Temperature (Ts max) Time (min to max) (ts) Maximum 3 C/sec 150 C 200 C 60 to 120 seconds Maximum 1 C/sec before Ts max. Maximum 3 C/sec after Ts max. Dwell time above liquidus (217 C) (Tl) 60 to 150 seconds 60 sec +/- 15 sec Peak Temperature (Tp) 260 C + 0 C /-5 C 250 C + 0 C /-10 C Dwell time within 5 C of Tp 10 to 30 seconds 10 sec to 20 sec max. Ramp down -6 C /second maximum -4 C / sec maximum Time from 25 C to Tp 8 minutes maximum 6 minutes maximum Follow solder paste manufacturer recommendation Figure 5: Reflow temperature profile 7. BOARD REWORK The BGA package rework/removal process flow is as follows: Board bake Component removal profile Component removal Site cleaning Inspection Solder paste dispense New component placement Component reflow profile Post reflow inspection Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 6 of 10

7 7.1. Bake Baking the board before component removal is recommended to avoid board damage and/or component damage. Preserving the removed component allows for electrical test or failure analysis to be performed, if required. In most cases, a safe bake is 125 C for 24 hours Board BGA Removal Profile A removal profile must be qualified for every BGA and BGA location on a board. Not all BGAs need the same amount of heat and location on the board affects the amount of heat needed to reach the solder liquidus stage. The profile must be within the component recommended profile (See section 6.4.1). The temperature shall be recorded at the center of the BGA array under the part. It is recommended to preheat the board to a temperature ranging between 100 C to 130 C to avoid thermal stress Component Removal No force shall be applied until the BGA temperature has reached the liquidus temperature. Applying force before liquidus may lead to pad damage and irrecoverable damage to the board or component. Once at liquidus temperature (217 C 218 C for lead-free SAC solder), vacuum pull can be applied to the part. Discard the removed component. Inphi does not support reuse of components Site Cleaning It is important to remove excess solder left on the pad to have a level and clean surface for subsequent processing. Flux can be used to help clean the site and must be chemically or water cleaned depending on the type of flux used. A standard manual soldering iron coupled with solder wick or desoldering tools are adequate for cleaning. Special care must be taken to not damage the solderable surface. If any damage to the pad or surrounding solder mask is noted, the recommendation is to stop rework and discard the board. After cleaning, the site shall be level and be free of a solder dome or cone with no visible thermal damage to the pad area and adjacent components, no pad lift and no damage to the solder mask Solder Paste Dispense Solder paste is normally dispensed on the redressed/cleaned site using a mini-stencil or any internal qualified dispense technique. The same solder paste as used in reflow is recommended. Additional solder paste is not mandatory for the replaced BGA component, but is highly recommended for reliability and wettability. If no solder paste can be reprinted or dispensed, a tacky flux is recommended to hold the component in place during reflow Placement The component shall be placed on the site with the same accuracy as the original component placement Reflow The original reflow profile shall be used for component replacement. Attach a thermocouple located at the BGA. To reduce board stress, it is recommended to pre-heat the back side of the board between 125 C to 150 C. A bake may be necessary if the component replacement is not done immediately after the removal of the defective one to prevent board damage Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 7 of 10

8 8. TROUBLESHOOTING 8.1. Non-Wet Non-wets are due mainly to process inability to break the molten solder surface tension during reflow Loss of Activator in Flux Losing the activator in the flux is one of the main sources of non-wet Insufficient Solder Paste Volume Released After Printing Adequate solder paste volume must be dispensed to avoid flux dry-out before the liquidus temperature is reached. Dry-out can be observed by a dry crust of flux between BGA and solder paste, normally at the BGA on the periphery Height Solder paste height is an important factor to consider as it is needed to compensate for the component coplanarity and warpage during reflow. Minimum solder height shall follow the component coplanarity provided in the outline drawing Uniformity Solder paste inspection after printing is critical to ensure that the printed solder is uniform and present in the expected volume on each pad Reflow Profile Not Matched to the Solder Paste It is important to adjust the reflow profile parameters such as activation time and duration and ramp rate before and after activation to the solder paste. Each solder paste has a unique chemistry and each factor needs optimization. There is no one size that fits all reflow profiles Board Solderable Surface Different pad size, shape, and copper trace width can require varying amounts of solder to compensate for coplanarity. Consider this aspect of assembly during board design Solder Spatter/Splash Solder spatter is caused by improper component placement speed. For lead-free solder, since it has a tendency to coalesce less than eutectic solder, it is preferable to use a thicker stencil with smaller openings rather than a thinner stencil with larger openings Board Temperature Uniformity (Hot Spots / Cold Spots) Board thickness, component density, and local copper density affect the board temperature during reflow creating areas with lower temperature for high density and higher temperature for lower density. Board temperature must be monitored to meet both board and component maximum recommended temperatures. Hot spots may create reliability issues, while cold spots may lead to poor wetting and opens Component Placement (Floating) If not enough force or over-travel is used during component placement, the component can float on top of the solder paste, resulting in inadequate contact and coverage all around the BGA. Due to solder paste viscosity and rheology, a slow steady placement is recommended to allow solder paste displacement all around the BGA. This will also help to reduce bridging risk and minimize solder ball formation Equipment / Conveyor Vibration During reflow, equipment vibration can be a source of solder displacement. This is a potential cause of non-wets if adequate contact between the solder paste and BGA is lost. Loss of contact reduces the effectiveness of the flux activator for reducing BGA oxidation Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 8 of 10

9 8.3. Shorts/Bridges Placement Speed Placement speed shall be slow enough to allow solder paste material to deform and move around the BGA and not be pushed out abruptly and cause solder to bridge with adjacent solder paste Solder Volume Solder volume and wider stencil openings shall be controlled such that there is a minimum distance between each BGA ball before reflow. During setup, components can be X-rayed before reflow to inspect and optimize the gap between BGA balls Placement Accuracy BGA balls shall be placed within 50 µm of the board pad true center. If a BGA ball comes in contact with solder paste on two adjacent pads, the self centering effect may drag some of the solder and cause a bridge Stencil Smearing Solder paste may smear between the stencil and the board. Once the paste begins to smear, more solder will run between the stencil because the solder spheres prevent good gasketing of the stencil to board. Visual inspection for smearing before component placement should be performed Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 9 of 10

10 Contact Information Inphi Corporation 112 S. Lakeview Canyon Rd., Suite 100 Westlake Village, CA Phone: (805) Fax: (805) Visit us on the Internet at: For each customer application, customer s technical experts must validate all parameters. Inphi Corporation reserves the right to change product specifications contained herein without prior notice. No liability is assumed as a result of the use or application of this product. No circuit patent licenses are implied. Contact Inphi Corporation s marketing department for the latest information regarding this product. Version Updates Version 1.0 (dated ) Initial Release From Version 1.0 to 1.1 (dated ) 1. Added imb02-gs02 / imb02-gs02a part to Table 2 (page 3). 2. Added diagram to Figure 4 for Thermocouple Location: b) flip chip (page 3) Solder Reflow Guidelines_AN_Ver1.1 INPHI PROPRIETARY Page 10 of 10