By Mark Thompson | Published on: December 09, 2013
Greetings! In today's post I will be talking about various surface finishes and solder mask issues that can arise due to those surface finishes...
In today's RoHS environment, immersion silver and gold replaces tin-lead. What does this mean to your mask artwork files? Interestingly, when tin-lead finishes were the norm, most solder mask clearances associated with surface mount devices were "gang relieved," which is to say there were no webs of solder mask material between the mounts. This tended to create situations where solder wicking occurred, shorting mounts together.
Today, not only do we have the capability to deal with solder mask webs between surface mounts as little as 0.004 to prevent wicking, the wicking itself no longer occurs with the alternative surface finishes of today, such as immersion gold and immersion silver, or even the lead-free solders.
That's the good news.
The bad news, if there really is any at all, would be that certain solder mask configurations are better suited to the alternative surface finishes of today. Today's fabricators sometimes have to come up with creative process alternatives for solder mask configurations versus surface finishes such as electroless nickel immersion gold (ENIG) and immersion silver.
Let me give you an example: Some customers will provide a solder mask clearance for a given feature on one side and no clearance on the other (say, for a connection/probe point or via-in-pad situation). If left as is, the solder mask feature that is tented or covered on one side creates a cup that does not allow solutions through the barrel of the hole during the ENIG process, trapping the solutions and potentially resulting in blackened or oxidized holes.
This problem can be approached a number of different ways at the fabrication level, and you may be contacted by your fabricator to consult about acceptable process deviation such as:
In other cases where the customer's desire is to plug a via adjacent to a surface mount but still clear the mount itself, this forces a situation in which the mask material bleeds through onto the mount from the opposing side, depositing mask material on the surface mount. Here fabricators will often ask if these specific vias can be cleared of mask and not be plugged.
Solder Mask Defined Features
A solder mask defined feature is created over an underlying copper land area where the mask clearance itself defines the feature. Avoid the use of solder mask defined features whenever possible. Sometimes the mask edges are not as well defined over metal, or worse, they leave small deposits of mask material that prevent the surface finish from adhering in these locations. In addition, without an underlying pad, the surface finish does not get full encapsulation of the pad, making it potentially more difficult at assembly.
A way around this is to have a thermal tie under the mask clearance on the circuit layer so the edges of the mask clearance are over material, not copper, and you are still tied electrically to the metal by way of the thermal spokes. When this is not possible, for adhesion reasons, stay with a green LPI type solder mask, because the green color holds up better that the other colors for "mask defined" features.
The green LPI solder masks can hold a 0.004 web between surface mounts on a surface mount device, while the other colors - red, blue, etc. - generally require a minimum of 0.006 web between mounts. For jobs without solder mask, "deep" or hard gold finishes that feature impedance controlled traces create a situation called "skin effect."
Skin effect is defined by Wikipedia as "the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the ‘skin' of the conductor. This skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current."
Polar Instruments explains on its Web site: “This means that most current at high frequencies flows in the outside of the conductor in a very narrow skin on the conductor (hence the name) which in the case of nickel plated copper concentrates most of the current in the relatively lossy nickel.”
Over the length of the trace or even the TDR test coupon, this can produce erroneous TDR readings if not approached correctly by the fabricator. The POLAR Impedance Test System provides compensation for this effect.
A better explanation of this phenomenon can be found at www.polarinstruments.com. I'll get more into alternative surface finishes in future posts.
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