Aluminum Skin Thickness -- the Key Quality AlSiC Product Data
While all statistics about AlSiC products are important, but no other data has so narrow an effective range. For example, though CTE 175W/mK does not reach the industrial standard of 180W/mK, the product still works. But once the skin thickness exceeded the limit of criteria even a little bit, the product does not function correctly at all.
The qualities of AlSiC products currently on China market vary greatly, some do not have Al skin, some skins exceed thickness limit, some even have fake Al skin. The thickness problem is the main reason to stop China AlSiC products from exporting. Let us have a look on the consequences brought by Al skin flaws.
1. AlSiC products without Al skin
AlSiC products could still be plated with Nickel or other metal even without Al skin. The firmness of the solder can not be guaranteed when chips are soldered on the AlSiC. Plating layer is a relative loose structure, solder need to penetrate through the plating and weld to material surface to form the firmness required. An AlSiC product without Al skin, 60% of its surface is ceramic, consequently decreases the solder firmness more than half.
AlSiC products without Al skin are not accepted by international market.
In addition, the skinless AlSiC has lower figure on its air-tightness.
2. Al skin exceeds limit
What problem is brought about by too thick Al skin?
The typical CTE ( the coefficient of thermal expansion ) of aluminum is 23ppm/℃, which is considerably larger than 7～8 ppm/℃ of AlSiC composite. For a base plate, its size could be as small as 100mm, or large over 200mm. the thick Al skin brings extraordinary stress inside the product when heated, the resulting distortion can be observed visually. And, by the way, if the skin was that thick, it is hard to say whether chips are soldered to AlSiC or a piece of aluminum, the module's working stability is unsure.
The following has been tested: for an AlSiC IGBT base plate, if the Al skin exceeded 0.2mm, the performance of the AlSiC could not be sured; when exceeded 0.3mm, the plate would distort apparently.
3. fake Al skin
Referring fake skin, Fadi here reminds customers, some AlSiC intentionally produce Al skin to skinless AlSiC product with thermal spraying machine. These products are unacceptable. These products, when break with hammer and put under microscope, impurity substance layer could be observed on the interface.
Currently, the western customer's criteria to Al skin thickness are less than 0.15mm. American products show excellent performance due to its capability of keeping the thickness less than 0.08mm. In this respect, Fadi Technology, owing to its specialized equipments and designated process, is a hundred percent sure about the integrity of its Al skin and thickness range not exceeding 0.08mm.
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