Why need to partially increase the solder paste volume?
This is in order to meet current smaller and thinner electric component design, the stencil thickness is made as thin as possible to accurately control the tiny solder paste which print on the PCB.
Unfortunately, there is not only small component be placed on the board but also includes big components. So how can this big electronic SMD component get enough solder paste volume to mount on the board? The stencil thickness is too thin to provide more volume and stencil aperture also can’t be opened too much over pad size. There shall be another solution to provide extra solder paste volume to this kind of big component and also for the DIP components will apply PIH (Paste in Hole) process.
Here WorkingBear will provide some solutions that EMS already did it:
Some production line uses the semi-auto dispenser to add extra solder paste on the land pattern of board. The solution is ok for the solder volume which doesn’t need accurate control. For example, some the grounding tab of connectors, smart card leads, SIM card leads. Disadvantage, however, is the needs to add extra operator on the SMT line, and hard to control the solder paste volume and location. Sometimes the operator may touch other components already in position due to human error.
Auto solder paste dispenser can easily control the solder paste volume, location and less human error risk. However, disadvantage is that investment is needed to setup extra dispenser machine in the SMT production line.
The step-up or step-down stencil is made by using the laser solution to remove the certain area thickness in the stencil to partially increase or reduce the solder paste thickness volume for the designed components. However, the amount of thickness to step-up or step-down is limited. It can’t be increased or reduce too much from regular stencil thickness. For example, if uses a 0.10mm thickness of stencil then the certain area usually be increased to 0.127mm thickness only and can’t over 0.15mm. This is because most of component that needs to increase solder paste volume had small components locate on the adjacent area. The solder paste volume will be reduced gradually from certain thicker area of stencil to regular area. It is hard to control the gradually solder volume change. Too much of the solder volume will cause solder short for the small components and fine pitch IC.
The solder preforms is a solid solder paste and form as standard shapes such as squares, rectangles, washers and discs. Typical sizes range from .010″ (.254mm) up to 2″ (50.8mm). Different sizes, as well as customized shape are available, but more expensive.
The tape & Reel packing are also ready for the standard performs size to meet pick and place machine to improve the yield rate and save labor hours. It is usually placed on the printed solder paste during SMT process like a chip capacitor is placed using standard pick and place component libraries and equipment. It just like a solder paste needs go through the high temperature reflow oven then melt and joint the electric component with PCB pad together. It is recommended to use preforms with the solder paste together. Because preform is a solid shape and the paste will fix it on PCB without moving during transit in the SMT equipment path.