PCB Tech - Can baking before PCB go online increase solderability?

Some surface mount soldering (SMT) engineers or managers have a persistent love for "PCB baking". It should be said that they may not have a thorough understanding of the concept of "PCB baking"! If you have the opportunity to browse some forums related to PCB and SMT, you should often find that some people will regard "PCB baking" as a panacea for solving PCB quality problems, thinking that PCB boards are baked before SMT goes online. Whether the baking is good or not, I think baking can also increase the solderability, wettability, and tin climbing height of the board, but is this really the case?

I have to remind friends with this point of view that the main purpose and function of "PCB baking" is only to dehumidify/dehumidify to prevent the board from exploding. In fact, if the PCB is baked for too long, it is easy to make its surface treatment (finished) ahead of time. Oxidation occurs and causes adverse effects such as poor solder wetting.

Especially for OSP ((Organic Solderability Preservative, Organic Solderability Preservative)) surface treated boards, it is not recommended to bake at high temperature before tinning, because the OSP film is organic and it will be destroyed and appear in a high temperature environment.

Phenomena such as shrinkage and curling will easily break after high temperature and expose the originally protected copper layer. Once exposed to the atmosphere, the copper layer will begin to oxidize, which will have a very serious impact on solderability.

As for the ENIG surface treatment board, if the gold-immersion layer is plated thick enough, it should basically protect the nickel layer under the gold layer from rapid oxidation. Unfortunately, because the gold is now expensive, the current gold-immersion layer is It is made very thin. Although ENIG's board baking before tin is not necessarily speeding up the oxidation of the nickel layer, it will definitely accelerate the diffusion between the nickel layer and the gold layer, that is, the "gold" will go to " The "nickel" layer penetrates, and the "nickel" penetrates into the "gold" layer. Although this diffusion effect works at room temperature, but the speed is very slow, but heating will accelerate its diffusion rate. Once the nickel diffuses to the gold layer If the position that cannot be covered is in contact with air, the nickel will be oxidized, which is unfavorable for subsequent soldering operations. Fortunately, as far as Shenzhen Honglijie understands, the general ENIG board baking does not have a very large effect on the solderability, which means that the diffusion effect of baking is not significant, but it is not recommended to bake at high temperature for a long time.

Other HASL (spray tin) or ImSn (chemical tin, immersion tin plating) surface treatment boards, because the IMC layer (copper tin compound) has been generated as early as the PCB bare board stage, that is, it has been generated before the PCB soldering If the high-temperature baking is used again, it will increase the thickness of the IMC that has been formed in this layer, and it may also catalyze the conversion of its IMC from high-quality Cu6Sn5 compounds to inferior Cu3Sn. Although the IMC layer is an important factor in the completion of soldering, the IMC layer If it is too thick, it is actually not a good thing for the welding strength. Once a PCB factory compared the IMC layer to cement that is similar to bonding between two bricks. The thickness of the IMC layer only needs to be formed and grow uniformly, but If the IMC layer is too thick, it will become fragile and break easily.

So, having said that, where does the pre-baking of the PCB before the SMT go online can increase the solder wettability? Because according to the above description, in addition to the benefits of "PCB baking" which can dehumidify and prevent delamination of the board, "PCB baking" absolutely cannot help improve the solderability of the PCB. On the contrary, baking the PCB is harmful to its solderability.