PCBA Tech - Introduction to SMT soldering process-wave soldering

1. The history of wave soldering

Wave soldering has existed for decades, and as the main method of soldering components, it has played an important role in the growth of PCB utilization. Making electronic products smaller and more functional, PCB (the core of these devices) makes this possible, which is a huge driving force. This trend has also given birth to new soldering processes as an alternative to wave soldering.

2. The working principle of double wave soldering machine

The dual wave soldering machine is developed on the basis of the single wave soldering machine to adapt to the mixed installation characteristics of plug-in components and surface mount components. Since this invention, its structure has been basically fixed in the form of "turbulent wave + smooth wave" .

1) Ripple wave

The main function is to generate an upward impact turbulent wave, which will drive away the bubbles formed due to the "masking effect" (as shown in the figure), so that the tin wave can be in close contact with the pad to reduce the occurrence of solder leakage. The upward impact of the turbulent wave is also conducive to good tin filling of the mounting hole.

2) Smooth wave

As its name suggests, its main function is to produce a smooth tin wave without crests and troughs, which is used to modify the weld shape. The structure and width of the smooth wave have a great influence on the quality of wave soldering, which determines the straight-through rate of wave soldering to a certain extent, which is also the value of wave soldering machines of different brands.

(1) Smooth wave process analysis

The smooth wave can be divided into three process areas: PCB entry area (before point A), heat transfer area (area between A-B) and PCB exit area (after point B).

3. Process control

1) Flux spraying

Paste a piece of white paper on the PCB with double-sided adhesive tape, apply flux, and check if the flux is evenly sprayed, if it leaks, and if the flux enters the hole, especially the OSP hole.

Leaking spray is often a common cause of bridging and sharpening.

2) Warm up

Preheating has the following purposes:

(1) Volatilize most of the flux to avoid splashing during soldering and a drop in the tin wave temperature (because the flux needs to absorb heat to evaporate).

(2) Obtain proper viscosity. If the viscosity is too low, the flux is easily taken away by the tin wave prematurely, which will make the wetting worse;

(3) Obtain an appropriate temperature. Reduce the thermal shock and board deformation when PCBA enters the solder wave;

(4) Promote flux activation.

4. Judgment of suitable warm-up results

(1) For leaded soldering, the soldering surface is about 110°C. For a given PCBA, it can be judged by measuring the surface temperature of the component; it can also be touched by hand, and it is sticky. Too dry can easily cause soldering problems.

(2) For OSP boards, the preheating temperature needs to be increased appropriately, such as 130°C.

(3) ENIG board depends on whether single wave or double wave is used. Dual-wave requires a higher preheating temperature, and single-wave requires a lower preheating temperature to avoid dewetting at the edge of the pad.

5. Welding

(1) The turbulent wave should have a certain impact upwards, forming irregular valleys and peaks;

(2) The smooth wave tin wave surface must be flat, and the wave height is adjusted to achieve defect-free welding.

6. Common poor welding and countermeasures

1. Bridging

1) Types of bridging

There are many factors that affect bridging, such as design, flux activity, solder composition, process, etc., which require continuous improvement in many aspects.

According to the causes, bridging can be roughly divided into two types: insufficient flux type and vertical layout type.

(1) Insufficient flux type.

The characteristic is that there is no wetting or partial wetting of multi-leads with tin, pads, and lead heads (the most easily oxidized).

(2) Vertical layout type.

The characteristic is that the solder joints are full, the lead head is covered with tin, and the tin is suspended, as shown in the figure. This is a common type of bridging. As its classification name, it is mainly related to the thickness of the tin wall formed by the leads-the diameter, length and spacing of the leads.

Of course, it is also related to the layout of the components on the PCB, the activity of the flux, the height of the tin wave, the preheating temperature and the chain speed, etc. The influencing factors are many and complicated, and it is difficult to solve 100%. Generally, it occurs in connector components with relatively small lead spacing (≤2mm), relatively long extension (≥1.5mm), and relatively thick, such as European sockets.

2) Improvement measures:

(1) Design

a) The most effective measure is to use a short lead design. For 2.5mm pitch leads, the length should be controlled within 1.2mm; for 2mm pitch leads, the length should be controlled within 0.5mm. The simplest experience is the "1/3 principle", that is, the lead length should be 1/3 of its pitch. As long as this is done, the bridging phenomenon can basically be eliminated.

b) Components such as connectors. As far as possible, the length direction of the components should be arranged parallel to the transmission direction and the soldering process pads should be designed to provide continuous carrier capability.

c) Use a small pad design, because the strength of the PCB solder joints of the metallized hole basically does not depend on the size of the pad. In terms of reducing bridging defects, the smaller the pad ring width, the better, as long as it meets the minimum ring width required by PCB manufacturing.

(2) Craft

a) Use a narrow flat wave wave soldering machine for soldering.

b) Use an appropriate transmission speed (it is advisable that the lead can be detached continuously). Fast or slow chain speed is not conducive to the reduction of bridging phenomenon. This is because (traditional explanation) the chain speed is fast, and the time to open the bridge is not enough or the heating is insufficient; the slow chain speed may cause the temperature of the lead near the package end to drop. But the actual situation is far more complicated than this. Sometimes, a lead with a large heat capacity and a long lead should be fast, and vice versa. Therefore, try more in practice.