Tips and Tricks

What is QFN soldering?

What is QFN soldering?

The perimeter connections (not the heat sink in the middle) on these can actually be soldered with a regular iron by applying lots of flux and touching each side and pad with a tinned iron. …

How thick should a solder stencil be?

Solder paste inspection is a key process step to ensuring a good consistent reliable joint. Paste volume is the best indicator for this. The use of stainless steel stencils and stainless steel blades is recommended to ensure consistent paste deposit. The recommended stencil thickness is 0.005” (0.125 mm).

What is QFN in PCB?

QFN stands for quad flat no-lead package. It is a leadless package that comes in small size and offers moderate heat dissipation in PCBs. Like any other IC package, the function of a QFN package is to connect the silicon die of the IC to the circuit board.

What are SMT stencils made of?

Typically, stencils are made of stainless steel or nickel and can be categorized based on the manufacturing process (process by which aperture is formed on the stencil foil), which follows. Electroforming: This is an additive process by which stencil foil is created by electroforming nickel.

How do I choose a stencil thickness?

Typically the stencil thickness would be between 4 thou and 8 thou thick. If the stencil is too thick for the size of apertures then it can lead to the paste being held by surface tension to the inner walls of the aperture.

How do you calculate stencil ratio?

The area ratio of a stencil aperture is the area of the aperture opening divided by the area of the side walls. It is interesting, as Ismail points out, that the area ratio of a circular aperture is the same as that of a square aperture.

Does QFN reach reflow temperature before the PCB?

Cross-Sectional Analysis No solder wicking into the Thermal Vias •Supposition is that the QFN reached reflow temperature before the PCB •Thermal via holes ~19mils (larger then desired) •No indication of any via filled material •No tenting at bottom or top 9 Power Matters.

Why do assemblers use stencils for design?

Once the design makes it to the point of assembly, the assembler has limited options to impact reliability, and these are dominated by stencil design (to affect standoff height and solder joint configuration). Standoff height can be controlled in several ways (bumping, use of preforms, stencil modification).

What are the trade-offs for adjusting solder paste?

The trade-offs for adjusting solder paste are also often mentioned; decreasing the volume of solder paste can reduce solder defects such as bridging but can negatively impact reliability, while increasing solder volume can do the opposite.

How important is die to package size ratio in QFN Assembly?

Careful assembly of QFNs and proper printed circuit board (PCB) design can result in acceptable reliability depending on the overall design. One area that has not been well addressed, however, is the impact of die to package size ratio, and how this factor should be considered in circuit card assembly.