Abstract
This application note discusses Analog Devices' wafer-level package (WLP) and provides the PCB design and SMT assembly guidelines for the WLP.
Introduction
The WLP offers advantages of small size and low inductance. Analog Devices' WLP are manufactured on wafers. Backside lamination is applied to enhance the mechanical strength of WLP body. Pb-free solder balls are used as die to PCB interconnects. An example package outline is shown in Figure 1.
Figure 1. SEM photo of a WLP.
Additional T&R information
Analog Devices ships WLPs in tape-and-reel (T&R) format. T&R requirements are based on the EIA-481 standard.
Figure 2. Package outline drawing of a 0.4mm pitch WLP.
PCB Design
Two types of land patterns are used for surface-mount packages (Figure 3):
- Non-solder Mask Defined (NSMD) pads – The solder mask opening is larger than the metal pads. The NSMD pad size is the same as the metal pad.
- Solder mask Defined (SMD) pads – The solder mask opening is smaller than the metal pad. The SMD pad size refers to the size of solder mask opening.
Figure 3. Illustration of NSMD and SMD land pad patterns.
Although both NSMD and SMD pads are used in applications, NSMD pads are recommended. NSMD pads have the advantages of more precise pad dimension and better solder joint reliability at the board side. Only one type of a pad (NSMD or SMD) and one type of pad surface finish should be used at a given footprint. The recommended pad sizes are listed in Table 1. The width of the trace should not be more than 100µm to avoid excessive wetting of the solder onto the trace at entry to a NSMD pad, which can change the solder joint shape. A teardrop is recommended at the trace entry to lower the risk of a trace crack.
SMD pads can be used when wide trace entry to a pad is required for reasons such as high current carrying. Larger metal pads and wider metal traces can be used for SMD pads.
Via in pad (VIP) is acceptable. The dimple at VIP can cause solder voiding at assembly. Small voids at VIP do not significantly degrade the solder joint reliability. The user can assess the acceptable via quality. Complete flat VIP can be achieved via capping although it is not required. It is recommended to put VIP at the corner ball location to improve the PCB reliability.
Ball Pitch (mm) | Acceptable PCB Pad Diameter (µm) | Recommended PCB Pad Diameter (µm) |
0.5 | 220 to 280 | 250 |
0.4 | 200 to 260 | 250 |
0.35 | 190 to 220 | 200 |
0.3 | 160 to 190 | 180 |
PCB Surface Finish
Organic Solderability Preservative (OSP), Electroless Nickel/Immersion Gold (ENIG), Electrolytic Nickel/Gold, Electroless Nickel Electroless Palladium/Immersion Gold (ENEPIG), Immersion Silver and Immersion Tin finishes are used in the industry. OSP is recommended for applications that require drop test reliability.
SMT Assembly
Standard SMT equipment and process are used for WLP assembly. The process flow is as follows:
Incoming WLP inspection
↓
Paste deposition
↓
WLP pick and placement
↓
Solder reflow
↓
Flux cleaning (optional)
↓
Inspection
Both solders paste or flux printing and dipping approaches provide acceptable assembly quality and reliability. Analog Devices WLP meets Joint Electron Device Engineering Council (JEDEC) level 1 moisture sensitivity classification. No baking is needed before assembly.
Stencil Design
Solder paste, paste flux, or liquid flux can be printed on the PCB with a stencil prior to the assembly. High-quality laser-cut stainless-steel stencil with nano coating improves transfer efficiency and consistency. It is recommended for WLPs, especially those with pitch smaller than 0.4mm. The solder paste inspection (SPI) is also recommended for such fine-pitch WLP assembly. The optimum stencil aperture size depends on stencil manufacturing technology, printing equipment, solder paste type, and process parameters. Recommended stencil thickness and reference aperture sizes are listed in Table 2.
WLP Ball Pitch | 0.5mm Pitch | 0.4mm Pitch | 0.35mm Pitch |
Recommended stencil thickness | 4 mils | 4 mils | 4 mils |
Reference stencil aperture size | 250µm | 250µm | 200µm |
Paste printing for 0.3mm pitch WLP is challenging. Users should determine stencil design based on the equipment capability, solder paste choice, and stencil technology. Alternatively, flux dipping can be used.
Solder Paste
Conventional Pb-free solder pastes can be used for WLP assembly. Type 3 pastes can be used for 0.5mm and 0.4mm pitch WLP assembly, while a type 4 paste is preferred for 0.35mm and 0.3mm pitch WLP. SnPb solder paste should not be used for Pb-free WLP assembly.
Automated Component Pick and Place
Standard pick and place equipment can be used for placing Analog Devices WLP. A fine-pitch IC packaging placement equipment is preferred for better accuracy. Plastic pick-up nozzles are recommended. Pick and place force should not exceed 2N.
WLP body outline can be used for component recognition. For better placement accuracy, solder balls can be used for alignment. With this approach, a look-up camera is used to recognize the solder balls. The equipment enters the ball array itself to the footprint for better alignment accuracy.
Marking Inspection for Thin WLP and Ultra-Thin WLP
More and more demands on thinner packages are driven by mobile or wearable applications. For WLP, a special process is needed after wafer thinning to certain thickness to handle large warpage. Marking through tape (on 2-in-1 backside laminate) is adopted in Thin-WLP or Ultra-Thin WLP, and the marks are shallower than normal.
Scattered light (not straight) is highly recommended for marking inspection. The equipment set up with either light-emitting diode (LED) ring light or side lamp or dark field function can achieve scattered light effect easily.
Reflow
All Analog Devices WLPs are compatible with industry-standard solder reflow processes. An optimized reflow profile considers of flux type and all components to be soldered to the board. The use of nitrogen inert atmosphere reflow is optional. It has demonstrated a better centering of the Pb-free WLP on the pads and less solder oxidation compared to air reflow.
Flux Cleaning
Post reflow cleaning is not recommended, especially when a no-clean type solder paste is used. If cleaning is required, a spray-under-immersion or ultrasonic-immersion cleaning method is recommended. A thorough study of flux, paste, and cleaning solvent compatibility must be evaluated.
Underfill
In general, underfill material is not required for WLP. In certain applications, underfill can enhance WLP mechanical robustness when proper underfill material is selected.
Rework
Rework is not recommended. It should only be performed using a controlled and qualified process that prevents mechanical and electrostatic discharge (ESD) damage.
Reliability
Reliability requirements are listed in Table 3.
Stress | Specification | Abbv | Condition | No. of Lots/SS per Lot | Duration/Acceptance |
MSL Preconditioning | JESTD20 | PC | MSL1 | 3 lots/150 units | Visual and Electrical test |
High Temperature Storage | JESD22-A103 | HTS | 150°C | 3 lots/77 units | 1000hrs/0 Fail |
Temperature Cycling | JESD22-A104 | TC | -40°C to +125°C, 1 cycle/hr | 3 lots/77 units | 1000 cycles for array size = 6×6/note 500 cycles for array size > 6×6/note |
Operating Life Test | JESD22-A108 | HTOL | TJ = 135°C | 3 lots/77 units | 1000hrs/0 Fail |
Drop test | JESD22-B111 | DT | Cond B | 1 lot/60 units | 150 drops/note |
Note: Meet less than 5% failure rate at 90% confidence level at the number of cycles specified for the reliability stress. |
Thermal Performance
Thermal modeling is performed with JEDEC still air conditions. The junction-to-ambient thermal resistance values for 0.5mm and 0.4mm pitch WLP are listed in Table 4 and Table 5, respectively.
Array Size | Pitch (mm) | ΘJA (°C/W) for 1S0P Board | ΘJA (°C/W) for 2S2P Board |
2×2 | 0.5 | 329.2 | 87.4 |
4×4 | 0.5 | 154.8 | 49.1 |
6×6 | 0.5 | 110.4 | 37.7 |
8×8 | 0.5 | 87.8 | 31.6 |
10×10 | 0.5 | 73.4 | 27.7 |
12×12 | 0.5 | 63.4 | 24.8 |
14×14 | 0.5 | 55.9 | 22.5 |
Array Size | Pitch (mm) | ΘJA (°C/W) for 1S0P Board | ΘJA (°C/W) for 2S2P Board |
2×2 | 0.4 | 434.5 | 102.6 |
4×4 | 0.4 | 209.7 | 57.9 |
6×6 | 0.4 | 151.3 | 45.7 |
8×8 | 0.4 | 121.2 | 38.2 |
10×10 | 0.4 | 101.9 | 33.6 |
12×12 | 0.4 | 88.4 | 30.2 |
14×14 | 0.4 | 78.1 | 27.5 |
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