6 Surface-Mount Technology (SMT) Guidelines and PCB Best Practices
The WLCSP is a surface-mountable package with bottom ball termination of its external connections. The land pattern design for all WLCSP is based on the IPC-7351 and IPC-7095 standards. A Non Solder Mask Defined (NSMD) pad design is recommended for all board pads, as shown in the following figure.
The NSMD board pad configuration provides a more robust solder joint than the SMD pads because the solder in the NSMD configuration wets the sides of the Cu pads, improving the strength of the solder joint between the WLCSP package and the PCB pads. In the SMD pads, the edge of the solder mask can be a stress initiator at the base of the solder ball, which can result in solder joint cracking. NSMD requires the solder-wetted area of the pad to be determined by the defined Cu area, not by the solder mask. This is an advantage because of the tighter control on the Cu etch process than on the solder mask development operation. Furthermore, the smaller pad size in the NSMD design also provides more room for tight pitch routing on the PCB.
The size of the WLCSP part requires care in the PCB design process for optimum reliability, processing, and performance. An example of typical land pattern information is shown in the following figure. Additionally, the user must consider these rules while designing the PCB board:
- The best reliability results are achieved when the PCB laminate glass transition temperature is above the operating range of the intended application.
- Ni/Au surface finishes are not recommended for Pb-free solder devices. The Organic Solderability Preservative (OSP) surface finish will deliver superior reliability performance.
- Cu traces routed away from the PCB land pads should be less than 100 µm wide (preferably = 75 µm) in the exposed area inside the solder mask opening (for NSMD pads). Wider trace widths will reduce the part stand-off and impact the reliability of the solder joints.
- Trace routing away from the WLCSP device should be balanced in ‘X’ and ‘Y’ directions to avoid unintentional component movement as a result of unbalanced solder wetting forces.
- No-clean solder paste.
| Symbol | Min. | Nom | Max. |
|---|---|---|---|
| D1’/E1’ | 1.600 | ||
| e’ | 0.400 | ||
| ØPAD | 0.200 | 0.230 | — |
| ØSM | 0.300 | 0.330 | — |
| Trace Width (L) | — | 0.075 | — |
| SPACE (Cu-to-Cu) | — | 0.170 | — |
| SM-Space | — | 0.070 | — |
| ØVIA PAD | 0.200 | 0.230 | — |
| ØDRILL | 0.100 | 0.125 | — |
- To obtain the PCB land pattern guidelines and dimensions for each WLCSP, refer to the www.microchip.com/Packaging.
- The information in this PCB land pattern should only be used as a guideline. Other factors, such as end-user layout and design, product-specific application, and actual experience must be considered to define the final PCB land pattern for the optimum component mounting process.
Reflow Soldering and Profiling
As with all SMT components, it is important that furnace profiles be monitored on all new board designs. In addition, if there are multiple package types on the board, the thermal profile must be measured at multiple locations. The component temperature may vary because of surrounding components, the location of the device on the board, and the package densities. To maximize the self-alignment effect of the WLCSP component, it is recommended that the maximum reflow temperature specified for the solder paste not be exceeded.
Microchip recommends that the user follows the guidelines of industry specifications IPC-7095 and J-STD-020 in developing the optimum reflow profile for the Pb-free WLCSP components on a given board.