3.2 Solder paste volume

 

In order to get a good printing effect, fresh solder paste is used in the production, and it is stirred evenly before printing, and the printing position is accurate. These are the prerequisites for the formation of good solder joints. At the same time, in order to ensure the welding quality and long-term reliability of BGA solder joints, the amount of solder paste is also an important factor. However, the thickness of the template and the size of the opening of the template should meet the following ratio requirements to get a good demoulding effect.

Pin width opening size / template thickness ≥ 1.5

Opening area/side area≥0.66

3.3 Welding temperature curve

 

The heat split convection re-welding temperature curve consists of four parts: preheating, heat preservation, reflow and cooling. The function of the preheating zone is to preheat the printed board, and the function of the heat preservation zone is to further heat the printed board and activate the flux in the solder paste to remove chlorides, so that the solder paste can spread on the pad. At the same time, it is expected that the temperature difference of all points on the printed board will be minimized. The solder paste in the reflow zone will melt, and the SMD will be soldered, and a metallurgical reaction will occur to form a reliable connection. The cooling zone will quickly solidify the solder joints to form solder joints. Any unreasonable temperature setting will result in bad solder joints.

 

 

For BGA solder joints, the temperature difference △T between the edge and the center solder ball is a key factor affecting the quality of the solder joint. If the temperature difference is large, the internal solder balls will not be fully melted, and "faulty soldering" is likely to occur. Therefore, before entering the reflow zone, △T should be made as small as possible, preferably within 5°C, so as to ensure that all solder joints enter the fusing state at the same time; the time above the liquidus line is reasonable, not too long or too short, it is best There is a "flat top" to ensure that the ΔT of all solder balls is as small as possible while making the solder balls fully melted, so that a reliable metallurgical connection can be formed.

 

The cooling rate is also an important factor affecting the quality of solder joints. If the cooling rate is too fast, the solder joint will be disturbed and cracks will occur; if the cooling rate is too slow, the surface of the solder joint will be rough, the crystal grains will be coarse, and the reliability will be poor.

 

Different temperature curves should be formulated according to different printed board thickness, assembly density, and device characteristics.

 

3.4 Vapor phase welding

 

For some special products, especially in military products, with the improvement of integration, the increase of wiring density and the increase of the number of printed boards, this "powerful" printed board has a large heat capacity. At this time, the hot air convection reflow soldering seems a bit inadequate, because the temperature difference of the devices in different places on the same printed board is relatively large, and overheating or overcooling will occur. And when the temperature curve needs to be changed, the hot air convection system needs a long conversion time to reach a stable state. The vapor phase reflow welding does not have these problems. It does not need to adjust the temperature curve. The maximum temperature of the printed board is equivalent to the boiling point of the liquid used, which is more suitable for the welding of large heat capacity components.

 

3.5 Manufacturability analysis

 

Valor software can be used to analyze the manufacturability of the design drawings, so that the design and production are well connected, and at the same time, the process is optimized, making the production and manufacturing more scientific and rational, thereby increasing the yield rate.

 

4. BGA rework

 

There are two methods for repairing the "virtual welding" of BGA solder joints as follows.

 

4.1 Non-destructive repair

 

In view of the typical phenomenon of "the BG device has a signal when pressed by an external force, otherwise there is no signal", we believe that a small number of solder joints may be poorly formed and defective. So inject flux from around the device and then remelt it. This method sometimes makes the above-mentioned "virtual welding" phenomenon disappear, which can meet the requirements of electrical performance.

 

4.2 Destructive repair

 

The commonly used rework method is to heat and forcibly remove the BGA device with "virtual soldering", and then perform ball planting or replace with a new device for soldering

 

The above two rework processes are generally completed in the BGA rework station, but if the heating system of the rework station cannot be accurately and fully heated, a reflow oven is required to complete the repair, but the price paid is that all components on the entire board are reworked. A thermal shock may cause damage to some devices. In short, the quality of the product may not be fully guaranteed after repair.

 

5. Summary

 

In short, the soldering process of BGA devices is more complicated than that of general surface mount devices, and there are more factors that affect the quality of solder joints. This paper analyzes the causes of typical "virtual soldering" defects and believes that the soldering temperature profile, the amount of solder paste, the surface conditions of the device and printed board pads, and the printed board design have a greater impact on the occurrence of "virtual soldering".

 

As the circuit design becomes more integrated, the printed board assembly density will be higher, and more uBGA and CSP will be used. Therefore, in the actual production in the future, we must start with the above-mentioned improvement methods, comprehensively consider various factors, strictly control the assembly process, and pursue the ultimate goal of zero defects and no repairs.