What is the IMC? We always heard the manufacturing engineer talking about this keyword when discussing to the soldering quality. What is the role of IMC layer in the soldering? Does IMC layer will affect the soldering strength and solderability? What is the best IMC layer thickness?
The IMC is the abbreviation of InterMetallic Compound in the electronic manufacturing industry. It grows between the solder alloy and Cu substrate for the Printed Circuit Board Assembly (PCBA). The IMC is neither a pure metal element nor alloy (Some expert think IMC is a kind of alloy). The IMC is a chemical compound that chemically combined by two or more elements. So, you may see the chemical formula of IMC like Cu6Sn5、Ni3Sn4、AuSn4…etc.
Is that possible both solder joint bridging and open solder joint defect happen at same BGA (Ball Grid Array) or CSP (Chip Scale Package) device?
It is hard to see both solder joint bridging and open solder joint defect happen at same BGA device in the general situation, but it is still possible. There is a case that BGA substrate and PCB has a big deviation of CTE (Coefficient of Thermal Expansion) then make the BGA substrate with smile-up curve (concave) deformation and PCB with cry-down curve (convex) due to longer TAL (Time Above Liquidus) or big temperature deviation between above and below heater in the reflow oven.
Printing the solder paste on the PCB (Printed Circuit Board) and go through reflow oven to re-melt the solder paste and solder the electronic components on PCB is most common process in today’s electronics manufacturing industry. Printing the solder paste on the PCB just like painting on the wall. The only different is solder paste printing ask more accurately position and must control the solder paste volume on each pad. It requires a certain jig call “stencil” is a sheet-metal made and have designate aperture on the position that needs to print solder paste.
Almost of current PCB assembly manufacturer apply the “stencil” jig to enhance the solder paste printing process to get more accurately printing position of solder paste and control the solder volume. Once the solder paste print shift from its pad too much then the solder will not joint the electronic component and PCB pad together. The solder paste volume over or less quantity will make solder short or open defect.
Recently WorkingBear got complaint from R&D and ask increase the solder paste volume on the MLCC (Multi-Layer Ceramic Capacitor) component pads to improve the capacitor broken issue. R&D said he had verified that the capacitor’s footprint in the board design is correct and followed manufacturer’s design guidelines. However, looks like there is not enough solder paste to form proper heel fillet on the terminal of capacitor. This insufficient solder paste volume make the solder cracking?
Recently one of my top manager ask to mount the shielding-can or call shielding-cover directly on the Printed Circuit Board to instead of mount the shielding-clip or call shielding-frame on the board first then cover the shieling-can. He said this “new process” is more cheap and can reach better RF (Radiation Frequency) performance. Also most of current popular cell phone board apply this solution.
Well, I must say yes. Directly mount the shielding-can on the board is cheaper and save labor hours as compare it with mount one component (frame or clip) on the board first then install another component (can or cover) to cover it. But this new requirement also got lots of negative feedback from SMT factory and repair center. Because of directly mount shielding-can on board means that repair guy must de-solder the shielding can first before repair the component under the shielding can. It make the repair job more difficult and may give more trouble and damage the board during repair.
Related article: The benefit to use shielding clips to instead of shielding frame