PCBA's thermal design solutions and what to do when assembling

PCBA welding often produces large temperature differences during heating. Once this temperature difference exceeds the standard, it will cause poor soldering, so we must control this temperature difference during operation. The thermal design of pcba is constructed from many parts, each with different functions. If this temperature difference is relatively large, it may cause poor soldering, such as open soldering and rope suction of QFP pins; shifting of chip components; shrinkage fracture of BGA solder joints. By the same token, we can solve some problems by changing the heat capacity.

(1) Thermal design of the heat sink pad: In the soldering of the heat sink component, the phenomenon of less tin on the heat sink pad is encountered, which is a typical application that can be improved by the heat sink design.

For the above situation, the design can be designed by increasing the heat capacity of the heat sink. Connect the vents to the inner ground plane if the ground plane is less than 6 layers. A localized heat sink layer can be isolated from the signal layer while reducing the aperture to the smallest available aperture size.

(2) Thermal design of high-power grounding jacks: In some special product designs, the plug-in holes sometimes need to be connected to multiple ground/electric plane layers. Because the contact time between the pin and the tin wave during the wave soldering is very short, the soldering time is very short, often 2~3s. If the heat capacity of the jack is relatively large, the temperature of the lead may not meet the soldering requirements, forming a cold solder joint.

PCBA's thermal design solutions and what to do when assembling

In order to avoid this, a design called a star-moon hole is often used to separate the solder hole from the ground/electric layer, and a large current is passed through the power hole.

(3) Thermal design of BGA solder joints: Under the mixed process conditions, the one-way solidification of solder joints will produce a unique phenomenon of "shrinkage fracture". The root cause of this defect is the mixing process itself. Features, but can be improved by slowly cooling the BGA corners. According to the experience provided by the case, the solder joints that usually have shrinkage fractures are located at the corners of the BGA. By increasing the heat capacity of the BGA corner solder joints or reducing the heat transfer speed, the shrinkage joints can be cooled synchronously with other solder joints, or other Post-weld cooling， therefore， prevents cracking under BGA warpage stress due to the first cooling.

(4) Design of chip component pads: As the size of the chip components becomes smaller and smaller, there are more and more phenomena such as shifting, and flipping. The occurrence of these phenomena is related to many factors, but the thermal design of the pads is one of the more influential aspects.

If one end of the pad is connected to a relatively wide wire and the other end is connected to a relatively narrow wire, the heating conditions on both sides are different. Generally, the pad connected to the wide wire will melt first (this is contrary to the general expectation). It is generally believed that the pad connected to the wide wire is melted due to the large heat capacity. In fact, the wide wire becomes a heat source, which is related to the heating mode of the PCBA. The surface tension generated by the first melted end may also shift the component or even Flip

(5) The effect of wave soldering on the surface of components

1. BGAO: Most BGA pins with a center-to-center distance of 0.8 mm and above are connected to the circuit layer through vias. When soldering waves, heat is transferred through the vias to the BGA pads on the surface of the component. Depending on the heat capacity, some are not melted, some are semi-melted, and are easily broken and broken under thermal stress.

2. Chip Capacitors: Chip capacitors are very sensitive to stress and are prone to cracking due to mechanical and thermal stresses. With the widespread use of tray selection wave soldering, chip components at the edge of the tray window are susceptible to cracking due to thermal stress.