How to make QFN solder climb height of 50% or more?

In order to make QFN soldering climb tin height of more than 50%, it is necessary to optimise the stencil design, solder paste selection, soldering environment control, pretreatment process in four aspects.The following are specific programmes and validation cases：

I. Optimisation of stencil design (adjustment of key parameters)

Hole opening method:

Adopt the inner cut outer pull design: inner cut to reduce the size of the pad cross-cut (to avoid even tin), the outer expansion to increase the amount of solder paste.

Case: a customer will be stencil external expansion from 0.15mm to 0.30mm, the thickness of 0.12mm adjusted from 0.13mm, climb the height of tin from 25% to more than 50%.

Around the small pad opening width is recommended 0.22 ~ 0.24mm (to meet the 5-ball principle), the length of the flared 0.15 ~ 0.25mm.

Heat dissipation pad processing:

Open hole area according to the pad size of 40% ~ 60% of the design, and bridging (field type) to avoid solder paste overflow short circuit.

II choose high activity solder paste

Solder paste type: Use QFN special solder paste, which has strong wetting properties and can climb anti-gravity.

Comparison: Ordinary solder paste climbing tin height less than 25%, dedicated solder paste up to 85% ~ 99%.

Flux paste assistance:

In the QFN side or PCB pad coated with flux paste after the second reflow, the use of flux to remove oxides and guide the tin liquid climb.

III control the soldering environment

Nitrogen protection:

Oxygen content in the furnace chamber is controlled below 1000ppm to reduce copper surface oxidation.

Note: Nitrogen flow needs to be balanced to avoid small devices (such as 0201) standing pieces.

IV, pretreatment and process monitoring

Pre-tinning treatment:

Pre-tinning of QFN devices after aging screening to repair solder end wettability.

SPI Inspection:

Ensure that the solder paste coverage area is ≥ 70% of the stencil opening, and the deviation of the solder paste amount is within the range of 30%~220%.

Process Validation:

Determine the optimal parameter combination (e.g. stencil outreach, solder paste type, reflow temperature profile) through orthogonal tests.

V. Case Validation

Example of successful improvement:

A customer optimised the stencil opening (by increasing the flare by 1) and replaced the special solder paste, so that the height of tin climbing was increased from 25% to more than 50%, and some of them reached 100%.

The flux paste is applied and reflowed twice, and the effect of tin crawling is significantly improved.

Summary

Need to comprehensively adjust the stencil design, solder paste selection, soldering environment and pretreatment process, and through the test to verify the combination of parameters.Focus on ensuring:

Adequate amount of solder paste (expanded stencil + special solder paste);

Reduction of oxidation (nitrogen protection + pre-tinning);

Process monitoring (SPI testing + process validation).

The above solutions can systematically solve the problem of insufficient QFN tin crawling and meet the height requirement of more than 50%.