Thermal design solution

PcbA welding heating process often produces a large temperature difference, once the temperature difference exceeds the standard will cause bad welding, so we must control the temperature difference in the operation. The thermal design of a pcba is constructed from many parts, each with different functional characteristics. If the temperature difference is relatively large, it may cause poor welding, such as QFP pin opening welding, rope suction; Tablet element stele, shift; Shrinkage fracture of BGA solder joint. Similarly, we can solve some problems by changing the heat capacity.


(1) thermal design of heat sink pad: in the welding of heat sink elements, there will be less tin in the heat sink pad, which is a typical application situation that can be improved by heat sink design.


For the above situation, we can increase the heat capacity of the cooling hole to design. Connect the heat sink hole to the inner ground plane, if the ground plane is less than 6 layers. A local cooling layer can be isolated from the signal layer and the aperture can be reduced to the minimum available aperture size.


thermal design of high-power grounding jack: in some special product designs, the mounting hole sometimes needs to be connected with multiple ground/level surface layers. Since the contact time between the pin and the tin wave during wave soldering is very short, that is, the soldering time is usually 2~3s, if the heat capacity of the jack is relatively large, the temperature of the lead may not meet the requirements of soldering, forming a cold soldering point.

PCBA solution

To avoid this, a design called a star-and-moon hole is often used to separate the soldering hole from the ground/electrical layer, with a large current flowing through the power hole.


(3) thermal design of BGA solder joints: under the condition of mixed assembly process, there will be a special phenomenon of "shrinkage fracture" caused by unidirectional solidification of the solder joints. The root cause of this defect is the characteristics of the mixed assembly process itself, but it can be improved by optimizing the wiring design at the corner of BGA to slow down the cooling.


According to the experience provided in the case, the solder joints with shrinkage fracture are generally located at the corner of BGA, which can be synchronized with other solder joints or cooled after increasing the heat capacity of the solder joints at the corner of BGA or reducing the heat conduction velocity. In order to avoid the phenomenon that it is broken under BGA warping stress due to cooling first.


(4) design of chip element soldering pad: as the size of chip element becomes smaller and smaller, more and more phenomena such as displacement, stele setting and flip occur. The occurrence of these phenomena is related to many factors, but the thermal design of the solder pad is one of the most influential factors.

If bonding pad with quite wide at the end of the wire connection, on the other side with the narrow wire connection, so the heat on both sides of the conditions are different, generally with wide wire connection pad will melt (that, in contrast to the general thought, always thought and wide wire connection pad because of the large heat capacity and melting, wide wire became a heat source, actually this is associated with the heating way of PCBA), one end of the melt to shift element, the surface tension could even reverse it


(5) influence of wave soldering on element surface


1. BGAO: most of the BGA pins with 0.8mm and above pin center distance are connected with the line layer through the conduction hole. During wave soldering, heat is transferred through the conduction hole to the BGA solder joint on the element surface. Depending on the heat capacity, some are not melted and some are semi - melted, and are liable to fracture under thermal stress.


2. Chip capacitors: chip capacitors are very sensitive to stress, easy to mechanical and thermal stress and cracking. With the wide use of tray-selective wave soldering, the sheet elements at the tray-window boundary are easily broken by thermal stress.


What problems should be considered in PCBA assembly:

Solder paste printing process, the main solution is the consistency of solder paste printing (filling and transfer), rather than the demand for solder paste per solder joint. In other words, solder paste printing process solves the problem of fluctuation of welding pass-through rate, rather than the problem of high and low pass-through rate. To solve the problem of pass-through rate, the key lies in the solder paste distribution, which is to distribute the solder paste amount according to the demand for each solder spot through the optimization and matching design of the solder pad, solder resistance and stencil windowing. Of course, consistency of solder paste quantity is also related to the design. Different designs of PCB solder resistance provide different process capability indexes.


1. The area ratio

Area ratio refers to the ratio of the steel mesh fenestration area to the fenestration wall area.


2. Transfer rate

Transfer rate is the rate at which the solder paste is deposited on the pad during printing in the stencil window, expressed as the ratio of the actual amount of solder paste transferred to the stencil window volume.

3. Influence of area ratio on transfer rate 

Area ratio is an important factor affecting the transfer of solder paste. Generally, the area ratio required in engineering is greater than 0.66.

Under these conditions, the transfer rate of over 70% can be obtained.


4. Requirements of area ratio design

The area ratio of steel mesh design requirements, mainly affect the fine spacing elements. In order to ensure the area ratio of the window opening of the fine welded wire mesh, the thickness of the wire mesh must meet the requirement of the area ratio. This increases the amount of solder paste required for components that require a large amount of solder paste by increasing the area of the stencil window -- this requires a space around the pad to deform, which is a major consideration in element spacing design.