Peelable solder masks in modern PCB fabrication: potential fields of application and processing abound as this handy device nears 30.

Peelable solder masks have been around since 1977. Since that time, it has been inconceivable to fabricate PCBs or flat packs without using peelable solder masks to protect selected areas from direct contact with solder or process chemicals. Reliable, time-saving and cost-saving in their application, they have long surpassed manual masking with heat-resistant tapes. Their economic and register-true application by means of semiautomatic or fully automatic screen printing, as well as their simple removal without leaving stubborn adhesive residues were, and are, the convincing advantages of peelable solder masks.

Even if peeling must be done manually (FIGURE 1), cost-saving aids can be realized with little effort. Printing of a lifting flap or of connecting strips between adjacent areas of peelable solder mask considerably simplifies the subsequent peeling of the mask (FIGURE 2). Thus larger areas as well as gold contacts, gold-plated turning contacts, multipoint connectors and carbon-conductive touch-key contacts can be both easily protected and uncovered.

[FIGURE 2 OMITTED]

Multiple and partly contradictory requirements in the field have led to a large variety of products from which suitable materials have to be selected for different--particularly also lead-free--soldering processes (wave soldering, reflow soldering and hot-air levelling) or chemical/electroplating finishing processes.

Leaded wave soldering is a relatively uncritical process since the temperature stresses are comparatively short. Therefore the selection of the peelable solder mask depends less on the required temperature resistance during processing than on the surrounding conditions: The need to tent plated through-holes or achieve a high definition requires the use of special thixotropic or highly thixotropic inks. A turbulent solder wave is critical--in such cases an ink with high mechanical stability should be selected.

Due to the higher soldering temperatures, particularly temperature-resistant systems should be preferred for lead-free wave soldering. A high temperature resistance is the most important ink selection criterion for reflow soldering and, in particular, for lead-free reflow soldering, since in these cases the PCB is not only exposed to high soldering temperatures but also to extremely long soldering times (FIGURE 3). In contrast, mechanical stress does not play a role so that weakly adhesive peelable solder masks can be used.

[FIGURE 3 OMITTED]

Hot-air levelling is the most critical process for peelable solder masks since it requires a high temperature resistance and strong adhesion as well as high mechanical stability all at the same time. Generally, peelable solder masks can only be used with acceptable process safety in vertical hot-air levelling equipment.

In general, peelable solder masks are not suitable for horizontal HAL procedures. While there may have been exceptions when using leaded solders despite the thermal and, in particular, the extreme mechanical load (due to the rollers at the in- and outlets of the tinning station and the air knives) with lead-free solders, the combined thermal and mechanical load is too high.

Independent of the applied ink system the thermoplasticity of the inks inevitably leads to mechanical damage and ripping of the ink coating. A stronger cross-linking by means of higher temperatures and/or longer curing times improves the adhesion, but negatively influences the residue-free peelability. A lesser cross-linking increases the sensitivity to damage.

Therefore, especially in this case, we must select a system with high mechanical stability that exhibits adequate adhesion and temperature resistance after optimizing the soldering temperature and the pressure of the air knives.

Multiple Soldering

The temperature resistance of peelable solder masks is solely of a temporary nature, i.e., apart from the temperature the exposition time has a decisive influence on the temperature resistance of such a system.

In the case of advanced requirements the total temperature load to which the peelable solder mask is subjected must be considered up until the point when it is peeled off. The curing parameters must be adapted to the follow-up process(es): By reducing the curing time and/or temperature the temperature resistance of the system in the follow-up processes is increased. The temperature resistance can also be increased by applying a greater coating thickness.

As a rule, only particularly temperature-resistant peelable solder masks are suitable for multiple soldering, as in SMD technology, although the suitability of these inks must be warranted for each soldering process. Curing should be as mild as possible while still ensuring sufficient adhesion and mechanical stability to handle the PCBs.

As a consequence of the temperature stress, the cross-linking degree and the tear resistance and adhesion of the ink increase; if the temperature stress is too high a very strong adhesion results, leading to a loss in peelability and in extreme cases even to a kind of charring of the ink as seen in FIGURE 4.

Electroplating Processes

When used in electroplating processes, a sufficient adhesion is of highest relevance to keep process chemicals from migrating under the peelable solder mask. Thus the peelable solder masks must be cured considerably longer and at higher temperatures than during soldering processes--but in the case of previous temperature stress from soldering, the curing times and/or temperatures must be reduced.

The substrate has a decisive influence upon the adhesion so that it must be checked individually to determine whether the adhesion is sufficient. Moreover, an adequate "leaching" resistance (resistance against bleeding/dissolving of lacquer components) is mandatory. The chemicals of the used baths, bath handling and degree of the potentially tolerable leaching (influence upon bondability, durability of the baths) must be considered.

Peelable solder masks that contain insoluble pigments instead of soluble dyestuffs should be favoured for electroplating processes since leaching of such chromophoric substances is impossible.

Besides a good process resistance, a good peelability and high tear strength are required for manual removal of the peelable solder masks at a later stage. These are considerably influenced by the application of a sufficiently high layer during screen printing. When printing on areas without plated through-holes, a layer thickness of 250 [micro]m to 300 [micro]m should be the aim, while for tenting plated-through holes as well as in the case of advanced requirements (during hot-air levelling, reflow soldering or multiple soldering), the layer thickness should even be 300 [micro]m to 400 [micro]m.

Optimum conditions during screen printing, such as the use of a rounded squeegee, low squeegee pressure and printing speed as well as push-stroke flooding of the screen by means of a right-angled elastomer squeegee, contribute to the application of high, bubble-free layers. Special attention should also be paid to the screen selection as well as the stencil fabrication.

To print peelable solder masks, only wide-meshed fabrics, such as 12 T polyester fabrics for large-area or blanket printing, or 17 T and 18 T polyester fabrics to print smaller areas, are suitable. Steel fabrics or fabrics with thinner polyester threads in comparison to the T fabric can also be used. They can be more easily removed from the lacquer and enable a smoother surface.

Depending on the application and screen fabric a stencil build-up of 200-700 [micro]m should be arranged. For a long period of time, such a high stencil build-up could only be realized with highly viscous copy layers or thick film layers. However, both processes possess serious disadvantages: Coating the fabric with highly viscous copy layers is very time-consuming since they are difficult to process and require a drying time of up to 24 hours. While a stencil build-up using a thick film stencil (thick film) is faster, it is significantly more expensive and the bond with the screen fabric is poorer than with an emulsion, which in turn affects the printing stability.

Lately a simple, practice-oriented and low-cost solution has been launched in this respect, consolidating the advantages of the aforementioned processes: an optimally flowable liquid emulsion with very short drying times and high stability as well as definition during printing on account of a good bonding with the screen fabric: The high stencil build-ups required for the printing of peelable solder masks are achieved without difficulty by means of multiple wet-in-wet coating (FIGURE 5).

[FIGURE 5 OMITTED]

Peelable solder masks are very reliable in their application and processing. Nevertheless, in unfavorable cases different influential factors, such as layer thickness, PCB layout, substrate conditions, curing and soldering conditions or process chemicals, can lead to failures that--since they occur mainly on the completed PCB--result in high costs. In such instances it is recommended to involve the ink manufacturer in the troubleshooting and corrective actions or, ideally, to have already engaged him in the selection of a suitable peelable solder mask for a specific application purpose.

REFERENCES

Application information sheet AI 2/29, "Selection criteria and processing instructions for the peelable solder resists (solder masks) of the series SD 2950," by Lackwerke Peters GmbH + Co KG.

TANJA TOURE is a technical writer at Lackwerke Peters, responsible for the creation of technical documentation. She can be reached at [e-mail].