The design of the component pads of the PCB is a key point, and the quality of the final product lies in the quality of the solder joints. Therefore, whether the pad design is scientific and reasonable is very important.
For the same component, all symmetrically used pads (such as chip resistors, capacitors, SOIC, QFP, etc.) should be designed to strictly maintain their overall symmetry, that is, the shape and size of the pad pattern should be exactly the same. In order to ensure that when the solder is melted, the surface tension (also known as wetting force) acting on all solder joints on the component can be balanced (that is, the resultant force is zero), so as to facilitate the formation of ideal solder joints.
What are the basic requirements for PCB pad design that can be underfilled? 1. Basic requirements for PCB pad design
1. PCB design: the spacing between the underfill device and the square device is more than 200Um.
2. Appropriately reduce the pad area, increase the pad spacing, and increase the filling gap.
3. The minimum distance between the underfill device and the surrounding components should be greater than the outer diameter of the dispensing needle (0.7mm).
4. All semi-through holes need to be filled and covered with solder mask. Open half-vias can create voids.
5. The solder mask must cover all metal substrates outside the pads.
6. Reduce bending and ensure the flatness of the substrate.
7. PCB board processing should eliminate the trench-shaped solder mask openings as much as possible to ensure consistent fluidity, ensure the consistency and smoothness of the solder mask, and ensure that there are no small gaps to accommodate air or flux residues. These are all later SMT. The cause of voids in chip processing.
8. Reduce the exposure of base materials around the solder balls, and coordinate with the dimensional tolerance of the solder mask to avoid inconsistent wetting effects.
2. Preparation before bottom filling
Mainly cleaning and baking chips. Baking the chip before the filling process removes voids created by moisture. For a 1mm thick substrate, it needs to be baked at 125°C for 2H. Different packaging forms require different time. If the temperature of the baking is increased, the time can be shortened accordingly.
No-flow underfill material is a mixture of flux, solder, and filler material. The no-flow underfill process involves dispensing a no-flow underfill material onto the pad locations prior to device placement. When the assembly board is re-sweat, the underfill material can act as a flux to activate the pads, form the solder joint interconnection, and complete the solidification of the filler material in the reflow soldering colleague, combining the two processes of soldering and glue curing. for one.