We use cookies to provide our visitors with an optimal site experience. View our privacy notice and cookie notice to learn more about how we use cookies and how to manage your settings. By proceeding on our website you consent to the use of cookies.
Proper pad design is critical to solder your components onto your board efficiently. There are 2 common types of soldering methods for an exposed pad package - Solder Mask Defined (SMD) and Non-Solder Mask Defined (NSMD). Each has its own characteristics and advantages.
Solder Mask Defined (SMD)
SMD is to define the pad area to which the solder ball will be soldered. This method reduces the likelihood of the pad lifting during the soldering or desoldering process. The disadvantage, however, is that this method reduces the amount of copper surface area available for the solder ball connection, and reduces the space between adjacent pads. This limits the thickness of the traces between pads and may affect the use of vias.
Non-Solder Mask Definied (NSMD)
NSMD is to use copper to define the pad area to which the solder bump will be soldered. This method provides a larger surface area for the solder ball connection and provides more clearance (compared to SMD) between pads, allowing for wider trace widths and more flexibility in the use of vias, but NSMD is more susceptible to pad lift during soldering and desoldering. In this way, NSMD helps for better solder connections and allows solder joints to package pads.
I’ve never heard from process engineers something good about SMD type. It is more complicated from a printing point of view especially when we are talking about small components.
However some manufacturers of small LGA and uBGA packages strongly recommend SMD pad design. There are two arguments I saw:
lower inductance (why?)
better alignment (perhaps yes, but taking into account possible printing issues…)
Lower inductance is really important when it comes to high frequency applications, this reduces potential cross talk or worse impedance problems (gets very complicated in that case). As for alignment, it is true that it does help with SMD layouts. SMD also stands for surface mount device, and anything that can be soldered directly on a pad without going through the board can be classified as such. SMD designs also drastically decrease footprints compared to other methods of mounting. There are always caveats to each design, good ones and bad ones. It is somewhat difficult to print such tiny spaces considering certain SMD designs though, the traces can get so tiny that scratching them would cause damage to the circuit path.
Regarding the lower inductance it is absolutely clear. It is not only about high speed interfaces, but also about EMI of DC/DC converters indeed. The one thing is not clear for me, normally to reach the same solder joint area I have to make a copper pad bigger in case of SMD pad design. How it may help me to reduce an inductance?
I’m using pads of 0.23mm for uBGAs im my design and in fact NSMD works for me better due to significantly less printing issues like low deposition and bad gasketing. LGAs are even more painful than BGAs. The mask has a thickness it causes worse wetting of the pad during the printing process and worse self-positioning during reflow just because a quite small package when the mask thickness becomes comparable with a stencil thickness.
So in general, the theory is more copper for a pad or trace will increase some level of self-inductance in the trace. Even though there is no “coil” per-say, it can still produce some level of inductance that could potentially change impedance values thus affecting EMI radiation or absorption. A through hole component has much higher levels of inductive coupling and inductance in general. Other forms of SMD devices with much larger pad area will produce more inductance in theory. So smaller pads (as small as you can go) is generally a good idea, but you are potentially looking at more complex issues like certain grounding practices and whatnot. There are a lot of variables here, but BGAs in general I can imagine have certain problems because the multiple pins are so close to each other. I think people are saying in general, compared to through hole or larger devices, SMD devices have much smaller levels of inductance considering pad size and trace size. I don’t have super detailed info on the whole process, so it is definitely more of an Engineering issue here.
Sure, larger copper larger inductance with the same pitch.
However how SMD allows to reduce the inductance if the copper cannot be reduced? That blows my mind.
Yeah, for solder masked defined, I am not quite sure why it would have lower inductance…at least from a logical standpoint. I was making comparison between surface mount and through hole, not solder mask defined and non-solder mask defined. If I had to guess, maybe it has to do that the whole thing is covered by this mask instead of having a separate layer. Separate layers can cause inductive coupling, which is usually worse.
Sorry for the confusion, I was specifically talking about surface mount devices (SMD) not solder mask defined (SMD also). I hate when things in electronics have the same acronym but different meanings.
We are on the same page.
Could you please take a look at this?
It is about modern GaN transistors. And everything is fine there except SMD/NSMD comparison that contradicts with my previous knowledge on this matter. That’ why I’m trying to find an answer why they prefer SMD.
This is a great discussion thread. Might I suggest you pose these questions through the “Ask a GaN Expert” feature on the www.epc-co.com website? All questions posed through this channel are immediately routed to an EPC engineer who typically responds within 48 hours if not sooner.
In the spirit of full disclosure, I am an EPC employee.
I will follow your advice. But since you are from EPC may be you know the real reason why you guys insist on SMD type of pads.
I know well that flex PCBs or cell phone PCBs often uses such type of pads just because delamination is one of the most frequent issues on them during drom tests.
However, it requires abnormally high precision of mask applying and mask to copper registration that causes higher price and limits number of manufacturers.
If SMD reduces the inductance, i would venture to guess that it is because the existance of soldermask around the edges of the pad prevent the solder from adhering to the sides, resulting in the solder occupying a physically larger volume. inducance/crosstalk is a square relationship with distance, and I would imaging that in fine pitch applications, if neighboring connections are now closer in linear distnace (because of more physical solder), then the mutual inductance/crosstalk between the two gets the 4th power increase (because both contribute a square). idk
Solder Mask is helpful but not necessarily required.
It’s especially handy for fine pitch/small devices to keep from the solder creating natural bridges/dead shorts on a pcb, but during the assembly process for some designs it’s not that hard to come in with a solder braid/wick after the fact and clean up shorts/bridges.
It’s not that big a deal for hand soldering a PCB but if the assembly process is surface mount with a reflow oven it helps greatly to prevent those shorts and have the solder gravitate to where you want it.
Solder Mask is a thin lacquer like layer of polymer that covers the pcb and copper traces you don’t want solder to get on and also prevents shorts from happening if conductive material inadvertently touches the circuit board, it’s an electrical insulator.
Paste Mask in PCB design is the physical locations the designer wants to deposit solder during the assembly process.
If this is a manual process often the paste mask is cut out with a laser cutter into a stencil and solder paste is squeegee’d onto the circuit board
To give an example, I laid out a circuit board that I want to do reflow oven solder process, I sent the Paste Mask file to the good people at OHS Stencils. https://www.oshstencils.com/
They created this stencil for me, now I can use this stencil to put solder on my circuit board where I need it, and then use a tweezers to put my parts on the board after I’ve deposited solder paste.
For examples of mask material that can be DIY applied, there are UV curable solder mask out there otherwise there are temporarly/peelable materials out there using things like latex rubber < Some can be found by clicking here >
