Tom
10-27-2002, 11:56 AM
IPC Advanced Study Guide Page Reference: Pages 23 & 24, Section 1.3
There are numerous decisions and design options to be considered all of which start with the type of assembly being produced. Types of assemblies include; Through-hole, Surface Mount Technology, Ball Grid Array, Chip Scale Packaging/HDI, and Mixed Technologies. Each of these technologies can carry the requirement for the use of specialized surface treatments. The surface finish issues become critical when there are a mixture of technologies on the same assembly the designer wants the best condition for each component attachment.
As component geometries shrink the effect of solder crowning on lands becomes a greater assembly challenge. The crowning effect creates an unleveled surface making even application of solder paste more difficult. The result is less uniformity in the final solder joints. When dealing with peripherally pinned devices and ultra-fine pitch TSOP/TSSOP devices the planarity of the surface or surface topology becomes a critical design factor. Immersion/plated finishes offer a flatter base for attachment of these components than conventional surface finishes.
There are various types of surface finishes put on the lands intended for surface mount solder applications. One of these is Hot Air Solder Leveling (HASL). This is the current predominant surface finish where the board is dipped into molten solder after the final copper plating process. In this process, the board is either dipped vertically or horizontally into the solder bath at approximately 260 ºC and excess solder is blown away with hot air. The key problem with HASL processes is that solder thickness varies widely from 0.75*m to 35 m.
It was generally believed that the lower thicknesses were not acceptable because of the very thin layer of solder and the feeling was that this thin layer would not provide good protection against copper oxidation. However, many studies, based on soldering results, have been done to show that there is no correlation to the solder thickness or the solder coverage on the lands. Thus, acceptance criteria for solderability of printed boards should include or be entirely based on functional testing (solderability) of sample product.
The wide variety of thicknesses seen in the solder from the HASL process also adds to the coplanarity problem for components on the board. This additional uneven surface compounds the paste printing problem because it gets more difficult to assure good gasketing between the stencil and the board. This lack of gasketing means leakage of paste underneath the stencil and thus there are more needs to clean the stencil or remove all paste from the board and reapply.
There are many other metallic protective coatings that can be used to cover the copper circuitry. It should be noted that fused tin/lead was used by the industry for many years until it became a practice to put solder mask over the melting metal. The solder mask would deteriorate when the board was subjected to heat thus it became popular to remove this etchant resist so that solder mask could be applied over the bare copper (SMOBC). Some manufacturers have found that making the tin/lead plating etch resist very thin allows the solder mask to be applied directly over the plating.
Because there was not sufficient thickness to reflow during the assembly temperatures, the mask adhered relatively well, especially to the laminate on either side of small conductors. This practice however, is only accomplished in a few designs and the surface copper must have relatively small features so that the major solder mask adhesion is provided by the laminate. Nevertheless, fused tin/lead, and solder coated copper are still viable solutions for surface finishes since "nothing solders like solder".
There are many other surfaces that also apply. All of these metallic finishes have their pros and cons, and it depends on the board manufacturer's capability to provide a solderable surface that work in conjunction with the solder mask requirements.
A gray tin can be applied to the board. Here the board is plated with tin only and the solder mask is applied over the tin plating. There is also an immersion organic tin (immersion "white" tin). This process has fine grain tin co-deposited on the board then covered with an inorganic protectant.
Other metallic coatings include immersion silver; boards are plated with silver over the copper. Sometimes a tarnish inhibitor is also applied.
In the nickel/gold finish, the lands of the board are over-plated with a layer of nickel and a thin coating of gold is then applied to protect the nickel's solderability. With nickel/palladium gold, first nickel is applied then a layer of palladium to enhance solderability of the nickel followed by gold for tarnish protection.
With the widespread use of ball grid arrays (BGAs) and finer pitch devices, the need for flatter board surfaces has become more critical. Thus, the popularity of an alternative surface finish that is not metallic has become useful in some applications. This organic solderability preservative (OSP) is particularly of value if the boards are to be assembled in a reasonable time frame (less than one year). OSP is an anti-tarnish coating of an organic compound. This coating is applied over the copper to prevent the copper from oxidizing.
OSP is a water-based organic compound that selectively bonds with the copper to provide an organic metallic layer that protects the copper. Various chemistries are available. Some of the common ones are benzotriazol, benzimidazol, imidazol. Coatings are applied by dipping the board into the OSP bath or by spraying methods. Either method works well as long as the process is controlled to achieve a uniform coating. Applying OSP is the final process in the fabrication of the board. It occurs after testing so that no test probe pins pierce the coating and let the copper to oxidize.
There are numerous decisions and design options to be considered all of which start with the type of assembly being produced. Types of assemblies include; Through-hole, Surface Mount Technology, Ball Grid Array, Chip Scale Packaging/HDI, and Mixed Technologies. Each of these technologies can carry the requirement for the use of specialized surface treatments. The surface finish issues become critical when there are a mixture of technologies on the same assembly the designer wants the best condition for each component attachment.
As component geometries shrink the effect of solder crowning on lands becomes a greater assembly challenge. The crowning effect creates an unleveled surface making even application of solder paste more difficult. The result is less uniformity in the final solder joints. When dealing with peripherally pinned devices and ultra-fine pitch TSOP/TSSOP devices the planarity of the surface or surface topology becomes a critical design factor. Immersion/plated finishes offer a flatter base for attachment of these components than conventional surface finishes.
There are various types of surface finishes put on the lands intended for surface mount solder applications. One of these is Hot Air Solder Leveling (HASL). This is the current predominant surface finish where the board is dipped into molten solder after the final copper plating process. In this process, the board is either dipped vertically or horizontally into the solder bath at approximately 260 ºC and excess solder is blown away with hot air. The key problem with HASL processes is that solder thickness varies widely from 0.75*m to 35 m.
It was generally believed that the lower thicknesses were not acceptable because of the very thin layer of solder and the feeling was that this thin layer would not provide good protection against copper oxidation. However, many studies, based on soldering results, have been done to show that there is no correlation to the solder thickness or the solder coverage on the lands. Thus, acceptance criteria for solderability of printed boards should include or be entirely based on functional testing (solderability) of sample product.
The wide variety of thicknesses seen in the solder from the HASL process also adds to the coplanarity problem for components on the board. This additional uneven surface compounds the paste printing problem because it gets more difficult to assure good gasketing between the stencil and the board. This lack of gasketing means leakage of paste underneath the stencil and thus there are more needs to clean the stencil or remove all paste from the board and reapply.
There are many other metallic protective coatings that can be used to cover the copper circuitry. It should be noted that fused tin/lead was used by the industry for many years until it became a practice to put solder mask over the melting metal. The solder mask would deteriorate when the board was subjected to heat thus it became popular to remove this etchant resist so that solder mask could be applied over the bare copper (SMOBC). Some manufacturers have found that making the tin/lead plating etch resist very thin allows the solder mask to be applied directly over the plating.
Because there was not sufficient thickness to reflow during the assembly temperatures, the mask adhered relatively well, especially to the laminate on either side of small conductors. This practice however, is only accomplished in a few designs and the surface copper must have relatively small features so that the major solder mask adhesion is provided by the laminate. Nevertheless, fused tin/lead, and solder coated copper are still viable solutions for surface finishes since "nothing solders like solder".
There are many other surfaces that also apply. All of these metallic finishes have their pros and cons, and it depends on the board manufacturer's capability to provide a solderable surface that work in conjunction with the solder mask requirements.
A gray tin can be applied to the board. Here the board is plated with tin only and the solder mask is applied over the tin plating. There is also an immersion organic tin (immersion "white" tin). This process has fine grain tin co-deposited on the board then covered with an inorganic protectant.
Other metallic coatings include immersion silver; boards are plated with silver over the copper. Sometimes a tarnish inhibitor is also applied.
In the nickel/gold finish, the lands of the board are over-plated with a layer of nickel and a thin coating of gold is then applied to protect the nickel's solderability. With nickel/palladium gold, first nickel is applied then a layer of palladium to enhance solderability of the nickel followed by gold for tarnish protection.
With the widespread use of ball grid arrays (BGAs) and finer pitch devices, the need for flatter board surfaces has become more critical. Thus, the popularity of an alternative surface finish that is not metallic has become useful in some applications. This organic solderability preservative (OSP) is particularly of value if the boards are to be assembled in a reasonable time frame (less than one year). OSP is an anti-tarnish coating of an organic compound. This coating is applied over the copper to prevent the copper from oxidizing.
OSP is a water-based organic compound that selectively bonds with the copper to provide an organic metallic layer that protects the copper. Various chemistries are available. Some of the common ones are benzotriazol, benzimidazol, imidazol. Coatings are applied by dipping the board into the OSP bath or by spraying methods. Either method works well as long as the process is controlled to achieve a uniform coating. Applying OSP is the final process in the fabrication of the board. It occurs after testing so that no test probe pins pierce the coating and let the copper to oxidize.