There seems no direct correlation occurring between PCB (printed circuit board) vias plugged with solder mask and via copper. However, badly-performed solder mask plugging possibly leads to destructive results on PCBs. As a type of special technology for stencil printing, solder mask plugging technology for PCB fabrication develops with the application and constant progress of SMT (surface mount technology). Via plugging features the following characteristics:
• Among all the vias on PCB boards, majority of them don't need to be exposed except component plugging vias, thermal-dissipation vias and test vias. Solder mask plugging stops flux or solder paste from being exposed on component side through vias in the later component assembly stage since it will possibly lead to short circuits. In addition, solder paste can be saved through the application of solder mask plugging technology.
• Solder mask plugging vias are compatible with the requirement called by SMT, stopping adhesive stuck on the surface of components such as ICs (integrated circuits) from flowing off through vias.
• Solder mask plugging technology prohibits flux, chemicals or humidity from entering the narrow space between BGA components and circuit boards, reducing reliability risk due to difficulty of cleaning.
• Sometimes, to meet the demands of automated assembly line, vacuum should be leveraged to absorb PCB for transportation or inspection. Therefore, vias are required to be plugged with solder mask to stop vacuum leakage which possibly causes loose holding.
Causes for Badly-Performed Solder Mask Plugging
One of causes for badly-performed solder mask plugging lies in incomplete or insufficient solder mask plugging.
Incomplete or insufficient solder mask plugging refers to one situation when there's no solder mask oil at the top part of vias while there's only a little solder mask oil being left at the bottom part.
Another example for incomplete or insufficient solder mask plugging indicates that there's solder mask on the left side of via while a so called air hole expands downwards along hole wall from via aperture on the right of via. It then expands towards left side of via wall when it comes close to the middle part of via with a cross section generated. Via copper almost breaks at the intersection between cross section and via wall copper.
Causes for Via Copper Breaking or Thinness
Once incomplete or insufficient solder mask plugging takes place, micro etching solution or acid solution might flow into via in later fabrication process of PCB. The vias are usually small with diameter less than 0.35mm. With solder mask plugging occurring, no or little solder mask oil is left for tenting at via opening while there's solder mask in the middle of via or at the bottom of via so that there's no passing ways for solutions within vias. Solutions can only be hidden at the intersection of solder mask and via wall, incapable of being eliminated, which will finally cause via copper breaking or thinness.
Damage Resulting from Via Copper Breaking or Thinness due to Badly-Performed Solder Mask Plugging
a. When copper becomes so thin at the internal side of via, resistance will arrive at milliohm level. It fails to be tested out with two-wire measurement method applied so that defective products fail to be exposed.
If electrical test fails to make via copper thinness issue exposed, thin copper will be broken away due to high-temperature operation and Z-axis expansion in PCBA (printed circuit board assembly) phase containing soldering. As a result, electronic products will suffer from insufficient functions implementation or they will possibly become unstable in functions when customers are using them in long term. When it comes to thin via copper without being totally broken, that fails to be found out through the application of ordinary inspection methods including AOI, AXI and visual inspection. Once it is found out, all the products belonging to the same production batch will be required to be scraped, causing large loss.
b. As far as via copper breakage or circular breakage is concerned, PCB manufacturers are able to find it through electrical testing. Nevertheless, an issue is available that copper etching through micro etching solution is such a long process that it won't be generated until it arrives at customer's stage. It means that via copper breaking can only be found by customers who only feel the unstable working state of electronic products. For example, when customers find electronic products suffering from black screen or being stuck, it will possibly result from via copper breakage.
Solutions
a. From the Perspective of Engineering Design
After engineering department of a PCB fabrication factory receives PCB design files from customers, aperture of plugging via and its requirement should be focused on. Generally speaking, the aperture of a plugging via should be below 0.35mm and it shouldn't be too large since too large aperture tends to make plugging incomplete or insufficient more easily. Although customers bring forward requirement on plugging via, they usually don't set specific regulations on fullness of plugging vias. In accordance with the regulations of IPC, plugging via fullness hasn't been specifically defined as well. Based on the requirement set by the widest range of PCB manufacturers and my years' engineering experience, I believe it best for plugging vias to feature fullness of more than 75%.
b. From the Perspective of Solder Mask Plugging Technology Improvement
Up to now, PCB industry masters the following type of solder mask plugging via technologies:
Technology#1 Via plugging → Solder mask printing (aluminum sheet participates in via plugging and exhaust plate is used)
Technology#2 Via plugging occurs with solder mask oil printing at an equivalent time.
Technology#3 Resin plugging → Solder mask printing
Technology#4 Surface finish → Via plugging
As far as via plugging fullness is concerned, the first and the third via plugging technology is suggested being applied because both methods contribute to high fullness. However, they suffer from complicated manufacturing processes with aluminum sheet and exhaust plate required. Moreover, two or more printers are needed for synchronous printing and board baking takes more time.
Technology#2 features a high manufacturing efficiency but it's quite difficult to control via fullness. This type of technology isn't suggested because low via fullness will arouse thin via copper or via copper breaking according to the discussion in previous part of this article.
Technology#4 isn't usually applied so it won't be discussed in later part of this article.
c. Via Opening Issue Exposed in Electrical Testing
Via opening inspection indicates whether incomplete or insufficient via plugging does occur with thin via copper or copper breakage generated.
As is discussed previously, electrical testing is seldom capable of getting thin via copper but is capable of exploring circular copper breakage issue. If open vias are explored during electrical test, it can be applied to verify whether it results from electroless plating copper, plating or badly-performed solder mask via plugging. With issue cause explored, corresponding measures can be thereafter listed.
d. From the Perspective of Solder Mask or Resin Quality
Technological test has to be implemented on new via plugging solder mask oil and via plugging resin so that their quality can be guaranteed. Then, they should be used to participate in small batch test to further verify their performance and quality. As is depicted in previous part of this article, low quality of via plugging solder mask or via plugging resin will lead to some problems, air hole for example. As micro etch solution enters air hole, via copper will be slowly etched away with thin via copper or via copper breakage. Their quality can never be compromised by their low cost.
Solder mask application plays a key role in PCB fabrication and via plugging fullness is significantly important because it is concerned with products' appearance and correlated with via copper quality issue due to incomplete or insufficient via plugging. As a result, extraordinary attention should be paid to practical administration. Specifically, regulated procedures should be conformed to; manufacturing administration should be refined; inspection standards should be clarified so that fullness of via plugging will be fully guaranteed.
