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​SMT processing QFN and LGA void defects and solutions
PCBA Tech
​SMT processing QFN and LGA void defects and solutions

​SMT processing QFN and LGA void defects and solutions

2021-11-06
View:85
Author:Downs

I. Introduction

The impact of voids...reliability issues-heat dissipation issues-various failures-customer complaints; do you want to solve the voids?

SMT processing QFN and LGA void defects and solutions

Causes of voids in QFN components

>Rapid growth, the bottom heat dissipation pad is too large, and the cavity is >25%

QFN Hollow Solution

>Steel mesh design>Furnace temperature adjustment>Solder paste adjustment

>Another simple and convenient solution-one-how to apply solder lugs to QFN components LGA component cavities?

 2. Reasons for QFN holes

QFN structure diagram

>Four-side flat package without leads

>The grounding PCB pad is under the component body, usually the size is 4mm*4mm

>The ground pad is in direct contact with the solder paste

 >Solder Paste and Steel Mesh

>The volume of flux in the solder paste occupies 50%. The more the amount of tin, the larger the amount of flux. >For stencil openings, more air outlet channels are needed, but too many channels means less tin

pcb board

 Excessive holes will cause short-term product failure or long-term reliability risks

>The hazard of a single large cavity during PCBA assembly

>LED, automotive electronics, mobile phone products, and many industrial products are very sensitive to voids and require reduction of voids

2.1 The influence of steel mesh design on voids

Through X-ray inspection, it is found that most of the time the shape of the QFN cavity is one or several larger cavities

In the experiment, the size of the QFN ground pad is 4.1mm*4.1mm. In the steel mesh design, we use the following methods

SMT processing QFN and LGA void defects and solutions

 2.2 The influence of furnace temperature design on voids

SMT processing QFN and LGA void defects and solutions

 3.3 solder paste adjustment

The flux is difficult to volatilize in the molten solder joints to reduce outgassing

-Appropriate high boiling point solvent

-The volatility of the solvent

Increase the activity of flux

A better solderability, which helps to squeeze out the flux gas

 Three, another solution-soldering piece

What is a solder lug?

>The same properties as solder paste, the same alloy of solder SnPb, SAC305, etc.

>Solid, different shapes, square, round, irregular shapes

>Volume can be accurately calculated

>1%~3% flux or no flux

 Why do solder tabs also need flux?

>Flux plating on the surface of the soldering pad can help the QFN pad and PCB PAD to remove oxidation and help soldering

 1%~3% Flux will not form a large outgassing and cause the cavity to be too large.

4. How to apply solder lugs to QFN components?

The thickness of the solder lug?

>In the experiment, the size of the grounding pad is 4.1mm*4.1mm, the size of the soldering pad is 3.67mm*3.67mm*0.05mm, and the surface is plated with 1% flux

>Generally speaking, the size of the solder pad accounts for 80%-90% of the size of the pad

 Welding sheet/steel mesh thickness-50~70%

In the experiment, the steel mesh is 4 mils thick, and the thickness of the solder pads is 2 mils. On the QFN pad opening, the ground pad does not need solder paste soldering, just open a 0.4mm round hole at each of the four corners to fix the solder lug

5. How to mount?

>Material tape loading, machine automatic placement

>Can also choose box, tray, or bulk, manual patch

 SMT furnace temperature adjustment?

>No need, go through the furnace with other components

>The same alloy, the same temperature

>Only 1%~3% flux, no requirement for outgassing

Welding effect

> Compared with solder paste, 1% flux in the soldering chip not only reduces the proportion of flux, but also the flux in the soldering chip is mainly solid, which reduces the content of volatiles.

>1% of flux can remove the oxidation of the pad surface and help form a good soldering.

>The void ratio is 3~6%, and the single largest void is about 0.7%

6. What is an LGA void?

lGA pads-58 round pads with a diameter of 2mm and 76 round pads with a diameter of 1.6mm, with via holes on the PCB pads. The void ratio is between 25%-45%.

Solution 1---Using solder lugs, the voids are reduced to 6-14%

Solution 2---Indium10.1HF

 Solder tab and solder paste compatibility issues

No-clean solder paste and no-clean flux are used in the experiment.

If the solder paste is water-washed, the surface of the soldering piece can be used without flux, but whether the soldering effect reaches the ideal value needs to be reconfirmed

The solder paste only needs to print the four corners of the QFN grounding pad, and the requirement for the amount of tin is as small as possible. It only serves as a fixed pad.

The size of the solder lug is generally 80% of the ground pad

The thickness of the solder sheet is generally 50% to 70% of the printing thickness of the stencil solder paste

The weight ratio of no-clean flux is generally 1.5%

Need to consider no-clean flux compatibility

Also pay attention to the pressure not to be too high when mounting, so as not to cause the welding piece to be squeezed and deformed, no need to adjust the furnace temperature curve.

Seven, summary

The influence of different solder pastes on the voids of QFN is very large. The opening of the steel mesh and the adjustment of the soldering furnace have certain help to reduce the voids. The use of solder tabs in the process:

>Various shapes of solder tabs, with flux on the surface, and very low flux residues after the furnace;

>It can be packaged by tape, SMT placement equipment can be quickly and accurately placed;

>During reflow, no need to modify the furnace temperature;

>Very low void rate, whether it is a large pad or a small pad;

>In addition, when SMT can't provide sufficient amount of solder by printing solder paste alone, mounting solder can provide accurate and repeatable amount of solder to increase the amount of solder.