In last month’s column, I presented various etching defect causes related to equipment parameters. In this month’s column, I will discuss various other causes that lead to etching defects.
Surface Preparation and Photoresist Adhesion
Final etching is often one of the least understood processes where downstream root-cause defects are concerned. While it is easy to point the finger at the etching process itself, skilled troubleshooters must be wary of downstream processes and materials. Case in point is surface preparation before photoresist lamination, as well as the condition of the copper foil. Figure 1 is a schematic showing a deep depression in the copper foil.
Now, while indentations and scratches in the foil and the glass weave can negatively impact resist adhesion, in a previous column I discussed the importance of surface preparation and overall cleanliness of the copper surface. If there are issues with neck downs, resist lifting, blistering, etc., it is easy to understand how etching solutions (even when under optimum process control) can seep under the lifted resist and etch the copper, creating an open. One can easily see how the depression in the copper foil surface can act as a conduit permitting etching solution to etch away copper (Figures 2 and 3).
While frustrating as this situation is, a process engineer and skilled troubleshooter can look downstream to find the root cause and take corrective actions. Case in point is the resist lamination process. Assuming one has achieved a clean copper foil surface with a reasonable degree of surface roughness, the mechanics of the resist lamination process takes center stage.
There are several critical variables in the resist lamination process that are keys to success. In hot roll lamination, heat is applied to the rolls and transferred through the polyester film coversheet and the resist to the resist/copper interface to achieve good resist-to-copper conformation and adhesion [1–2]. The rolls are heated by a variety of methods. Check with the equipment supplier in this case.
While temperature is important, pressure, as supplied by the rollers, is a significantly more important variable with respect to resist conformity. Regardless, temperature plays an important role in reducing the resist viscosity for improved flow. For the hot roll laminator, Table 1 shows what these values are.
While these parameters are critical to quality, one must not ignore the condition of the rollers that make the contact between the copper layers and resist. For optimum resist conformity, the rollers must be properly maintained and changed after so many uses. These rollers eventually can bow and receive scratches and gouges after repeated use. These conditions are deleterious and negatively impact resist adhesion (Figure 4).
As shown in Figure 4, the condition of the latter two sets of rollers requires attention. Otherwise, resist conformity and adhesion is compromised, including sporadic resist lifting, neckdowns, line width reduction, ragged traces, and opens caused by etch-outs. Note that the rollers are made of rubber. The pliability of the rubber helps to make intimate contact with the resist-to-board interface. Increased usage, time, and temperature will increase the hardness of the rubber coating, thus reducing the ability of the rolls to make intimate contact.
Finally, the actual temperature at the resist/copper interface is the key deliverable with respect to resist adhesion and conformity. This temperature depends on the contact time of the resist with the heat as supplied by the rollers, the temperature of the heat source, and the thermal mass and temperature of the board. The contact time, in turn, is a function of the lamination speed and of the roll/film "footprint" in the lamination roll nip. The lamination speed is set by the hot roll rpm and diameter of the rolls, while the "footprint" (i.e., the width of the hot roll/board contact zone in the nip) is determined by the durometer and thickness of the roll material, as well as the roll pressure and the “foot” that the rollers create.
- Karl H. Dietz, “Fine Lines in High Yields, (Part XXVIII): Advances in Hot Roll Lamination of Dry Film Photoresist (Part A),” CircuiTree Magazine, December 1997, p. 60.
- Karl H. Dietz, Fine Lines in High Yields, (Part XXIX): Advances in Hot Roll Lamination of Dry Film Photoresist (Part B),” CircuiTree Magazine, January 1998, p. 18.
This column originally appeared in the December 2020 issue of PCB007 Magazine.