Additive Reality: Let's Drop a Line About PCB Cross Section

My article in the April 2022 issue of Design007 Magazine, titled “Additive Manufacturing Requires Additive Design Techniques,” presented several cross sections of solder mask coated with an inkjet technology. However, the choice of the cut’s location, and therefore the highlight of the picture, is slightly different from the usual dam or copper edge coverage. This illustrated my point and most structures of solder mask showed in the figures would not be anything new to my readers. Several articles have explained lateral definition and stacking principles.

Still, when the knowledge used as a starting point to examine these cross sections is from traditional solder mask (which mostly resembles rectangles), it is difficult to trust right away that any other shape will be as reliable.  

The best way to highlight this difference is not necessarily with a cross section, but with a top view.

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Figure 1: Comparison of traditional vs solder mask. The tapering of the solder mask thickness towards the edge of the pattern is especially visible on copper. (Source: ACB-Atlantec SAS)

Figure 1 shows one of the concepts explained in my March 2022 column titled, “Drop-forging Solder Mask Thickness With Inkjet.”

The usual questions refer to the solder mask adhesion to copper or laminate, its inertness to chemical attacks of post-treatments at high processing temperatures, and temperature stability during further manufacturing or product ageing.

The wetting behavior of the surface, controllable by the pre-treatment choice, has a wide process window1. The contact angle imposed by the wetting behavior, when combined with the low viscosity of the ink, fills any intricate roughness created by pre-treatments. This ensures the mechanical bonding of the anchor effect. Additional to the safety of this mechanical bonding is the inherent tapering of the solder mask thickness towards its edge. This ensures an increasing ratio between bonding and volume to bond. In other terms, the anchoring is always the same for any point where the solder mask is present, though the volume of solder mask anchored is less, which means that any mechanical action to remove solder mask will not find a convenient spot to leverage. The thickness tapering of the solder mask can raise doubts of chemical inertness with respect to typical chemistry of post-treatments or environmental stress during the lifetime of the PCB product (for example, skin sweat for a smartwatch). However, the solder mask material counteracts this stress by plain chemical inertness, not by means of thickness. Material suppliers ensure a negligible chemical interaction with coated solder mask. Therefore, any thickness, as long as it’s continuous, is enough. Just to make the point with absurd logic, if thickness were the protecting mechanism against chemical attack, when would it ever be thick enough?

luca_fig2_0422.jpg
Figure 2: Comparison of edge definition after ENIG post-treatment. (Source: ACB-Atlantec SAS)

A pragmatic proof of such adhesion is visible on ENIG-treated boards. The chemistry used to apply such metal finishing relies on aggressive substances at high bath temperatures. Inkjet solder mask, thanks to its profile and the properties defined before, ensures that its edge is the real ENIG edge. Traditional solder mask, due to negative foot situation, might not be able to grant the same precision of definition. It is not necessarily bad, but it is one point where process control isn’t as strong.

The holy grail of perfect definition with traditional solder mask technology is the “zero foot.” A fine-tuning of the coating process and its several steps will result in such a situation. Still, a lot can change during production. In the end, few micrometers of positive or negative foot will most likely not negatively affect the overall design, unless the board is an HDI type. However, a negative foot situation might especially facilitate the trapping of chemistry. Despite heavy rinsing, the sheer length of solder mask contour in a board increases trapping chances. This situation will lead to failures, since an active chemistry is left where it’s not supposed to be.

Inkjet follows the pattern definition to establish its edges. These are not positive feet; the edge is exactly where it should be. The coated pattern therefore benefits from improved edge adhesion, accurate definition and improved control on reliability.

References

Additive Reality columns by Luca Gautero: “Drops of Technology” (July 2021) “Printhead Selection or ‘Shop ‘Til You Drop’” (August 2021).

Luca Gautero is product manager at SUSS MicroTec (Netherlands) B.V. 

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2022

Additive Reality: Let's Drop a Line About PCB Cross Section

05-04-2022

My article in the April 2022 issue of Design007 Magazine, titled “Additive Manufacturing Requires Additive Design Techniques,” presented several cross sections of solder mask coated with an inkjet technology. However, the choice of the cut’s location, and therefore the highlight of the picture, is slightly different from the usual dam or copper edge coverage. This illustrated my point and most structures of solder mask showed in the figures would not be anything new to my readers. Several articles have explained lateral definition and stacking principles.

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Additive Reality: Drop-forging Solder Mask Thickness With Inkjet

03-28-2022

Traditional coating processes for solder mask handle thickness as a single parameter for the whole board. There will be differences between the amount of material on the laminate or on the copper, and these will depend on the height of the copper and in some cases on the section of the copper feature. Overall, it is reproducible: It looks homogeneously green and is easy to estimate its amount of material consumption. Therefore, everybody is happy.

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Additive Reality: Isolated Raindrops Announce the Storm

02-16-2022

It is an early January evening after office hours. Despite my best New Year’s intentions of striking a good balance between work and personal life, when I hear the ding of an incoming email, I decide to check it anyway. It’s a message from the U.S., where Don Monn of Taiyo has just given his view on the inkjet market. Something he says immediately strikes a chord, “What I can tell you is … there are well over 20 installations … between North America and Europe. There are another five or six already committed to be installed in the first quarter of 2022.” But his numbers don't immediately add up for me.

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Additive Reality: Green Drops, White Drops or Both: Do Solder Mask and Legend Make a Good Team?

01-19-2022

The combination of solder mask printing and legend printing seems an obvious and attractive solution, like bringing chocolate and vanilla together. Still, the gain would rely on obtaining both functionalities without adding complexity. However, from the summary consideration of my last column on the equipment construction, two separate printhead arrays would match best the different requirements of legend and solder mask. In this column the story continues.

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2021

Additive Reality: A Report From 2021 (Drop)tronica

12-08-2021

After months of social distancing, productronica brought people and ideas together in a single place. Its most pragmatic demonstrations were the tools on the floor of the B3 Hall at the München Messe. The fair had representations for almost all equipment needed for PCB manufacturing and inspection. Three of these tools were solder mask inkjet printers from different brands, as was the case in 2019—a sign that business has continued, albeit a slow pace, through the pandemic period. However, this year there was another vibe, this time about additive manufacturing. At the entrance, an animation on a giant screen showed how a company is delivering a bottom-up approach for PCB prototyping—an inspiring manufacturing frontier at the doors.

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Additive Reality: Your Company Drops Open the Inkjet Printer Box, Now What?

11-29-2021

The moment will come when some of you readers will advance from interest to complete involvement with the technology. This will be a fun ride as you will experience first-hand the concepts seen so far in this column. However, we all know that any reliable technology relies on one healthy, not so exciting, good habit: preventive maintenance. Preventive maintenance and incidental repairs are not the same, though they might follow the same instructions. Still, this does not mean that these are interchangeable.

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Additive Reality: Drop Your (Solder) Mask, It's Sampling Time

10-17-2021

Inkjet printing equipment around the world is printing solder mask on PCB half fabricates. This effort goes under the name of “sampling.” Major PCB manufacturers have been asking either equipment or material suppliers (or both) to provide a solder mask coated sample with inkjet technology to make cross comparisons.

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Additive Reality: Solder Mask Patterning at the Edge Between Drops and Bricks

09-26-2021

The digital form of the inkjet printing technology goes through files containing a rasterized image; these bitmaps, in their simplest form, contain information about presence (or absence) of drops. Additionally, the resolution brings in the drops pitch.

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Additive Reality: Printhead Selection or ‘Shop ‘Til You Drop’

08-17-2021

If inkjet tools could be found on an e-commerce site, there would be several product specifications of which many would specify the jetting properties; these would basically detail the printhead(s) in the system.

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Additive Reality: Drops of Technology

07-16-2021

The solder mask application revolves around solder mask material. Still, not all the attention should focus on the material alone. The inkjet printed solder mask layer will be made of a collection of drops and next to the solder mask ink a few other materials play a role in the shaping of these drops.

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Additive Reality: Drop Placement Accuracy

06-20-2021

New columnist Luca Gautero takes a clever engineering spin on William Tell and the apple and how it relates to inkjet equipment for solder mask coating.

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