Although I am not a professional PCB designer, I would like to share what made in additive process means to PCB designers, especially the relationship with solder resist. Here are some important factors that I think designers need to consider.
1. Definition of solder resist
Essentially, any definition provided by EDA tools is incorrect; The vector file specifies the deviation of the assumed continuous surface. Up to now, solder resist technology has been in the form of subtractive method. SUSS MicroTec Company developed the front-end JETxSMFE, which can run on CAM workstation to simplify the manufacturing process. The software can interpret all relevant information and correctly control the details of incoming files.
2. Advantages of inkjet solder resist process
One of the advantages of inkjet solder resist process is that it can avoid filling any holes or vias. Some people in the industry may not be able to completely “mask”. However, it is generally believed that the through hole of solderless film improves the reliability of PCB. According to the manufacturing practice, the unmarked holes may still be covered with solder mask, which will lead to ink on the printing table. This is not a big problem, because the scraper can easily remove the excess solder resist ink, and can also replace the workbench or its substrate (if it exists). However, both solutions will cause short downtime. To make a long story short, if you want to make the production workshop work smoothly, you must specify all drilling requirements in the design.
3. Rules of Soldering Bridge
The solder mask traces on the laminate separate the two nearby copper pads. This graph represents the pad design choice of non-solder mask defined (NSMD for short). However, this choice has several limiting factors, resulting in the artificial demand for narrow solder dam. Start with numbers: Imagine two pads, 200µm apart, nothing special. What is the maximum size of the solder bridge between them? Assuming that LDI process is used, its size comes from the latest constraint of applying this technology. The 200µm pitch decreases (down to 100µm) with the laser beam width plus twice the alignment accuracy. Any more challenging pad distance will also make it more difficult to define the solder bridge rules.
According to this reasoning, many requirements of using traditional techniques to build solder-resistant bridges with a length of 50µm or less seem reasonable. So how to deal with inkjet printing? Did it join the challenge of the last 1 micron? No, on the contrary, it challenged the original design choice and caused controversy: solder mask defined (SMD) pad or NSMD pad? It’s neither. On the contrary, it is a pad-defined solder mask (PDSM for short). However, this acronym has not been confirmed by the industry, so it can’t be searched by Google. Ink-jet printing is a coating technology driven by volume, which means that it is possible to fill gaps, and even encourage the use of this method.
4. Define the thickness
Controllable thickness means the relationship between the conformality of the coating and the flatness independent of the surface morphology. Which thickness design scheme is more suitable for solder resist? PCB designers should know exactly the function of solder resist. “Ink jet upgrade” summarizes the basic common sense: although copper without solder resist will be oxidized, laminate will not; High-voltage copper traces without solder mask may cause short circuit, but bare laminate does not, so laminate does not need solder mask. At present, the reason of solder resist coating is convenience (less use of developer) and providing support for any template frame. The first problem is no longer a problem, because inkjet does not use developing chemicals; On the second issue, ink jet is more attractive and can be shaped into a grid or array, which significantly reduces the coating material and provides the same support.
Combining the concept of constant coating thickness with PDSM can build a new density for ball grid array (BGA) structure.
5. Check the design rules
There are a series of considerations here, which belong to the category of design rule check, and the final check is completed by CAM software. However, the following description can avoid misunderstanding of manufacturers. Designers must understand:
The solder mask has the smallest radius, which largely depends on the selected inkjet equipment.
The minimum feasible gap between solder resist features is smaller than the minimum feasible printed solder resist features.
A thinner solder mask layer allows a smaller feature size, whereas a thicker copper layer or a thicker solder mask layer will increase the minimum feature size.
The cross section of the narrow solder mask is similar to a dome. The traditional rectangular section no longer exists, but is replaced by a contour without edges and cavities, thus improving the mechanical and chemical stability.
The solder resist structure is based on layers. This means that other geometric features can also be added on the top of the solder resist for mechanical support of components or limitation of spot coating.
As mentioned earlier, the current CAM solution can deal with details such as solder resist thickness, copper thickness and drilling hole, but can’t define its correct position. The ideal solution should be integrated in CAM software or design tools, which can define the required solder resist thickness of components at each position. The designer will add all information at the component level to prevent the tombstone effect.
