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Improved organic solderability preservatives for mixed metal finishes |
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| Authors: |
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| Michael Carano Electrochemicals, Inc., Maple Plain, MN. |
John Hunt Innovex, Inc., Chandler, AZ. |
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The recent IPC Technology Roadmap has identified several significant developments with respect to future packaging requirements. One such trend is the requirement to achieve very high-density features on the surface of the printed wiring board. This requirement is catalyzed by an increased packaging density, driven by higher I/O’s, and smaller, faster and cheaper packages. What we see then is alternative package designs, such as Ball Grid Array (BGA), Tape Automated Bonding (TAB), Chip on Board (COB) and Flip Chip (FP). When such components are mounted onto the surface of the board, it is commonplace today to require a bare board with multiple metal finishes. In addition to an OSP coated copper, gold and tin-lead can be selectively deposited onto the pwb. Treating a mixed metal finish board with a standard OSP leads to significant discoloration and film formation on the gold and tin-lead surfaces. In addition to the negative cosmetic impact, the excess film on the gold and tin-lead can interfere with subsequent operations, such as wire bonding and soldering. A solution to this problem is to develop a highly selective OSP that will not deposit on metal surfaces such as gold and tin-lead, but provide significant levels of solderability protection for bare copper surfaces. This paper describes such a development and its implementation in a fabrication and assembly facility. Back to top INTRODUCTION The trend toward alternative surface finishes to promote the solderability of bare copper surfaces has been well defined. While it is true that hot air solder leveling, (HASL) is still the predominant surface finish; alternative technologies are being implemented for a variety of reasons:
Driven by the technological shift to the SMOBC Process, HASL provided a quick and simple way to provide solderability protection for the exposed copper by depositing a thin layer of eutectic solder on the exposed copper surfaces. Certainly, this process has served the fabrication and assembly industry well for many years. Nevertheless, fine-pitch surface mount and the use of BGA and Flip Chip technologies will continue to increase, and the very tight tolerances of the mounting pads for these technologies will preclude the future use of HASL. On the other hand, OSP Technology (Organic Solderability Preservatives) is environmentally friendly, provides a coplanar surface, and requires very low equipment maintenance. The process is designed for horizontal conveyorized processing. However, vertical immersion systems are also easily integrated into the printed wiring board fabrication process. The October Project, the IPC and ITRI (Interconnection Technology Research Institute) are actively exploring alternatives to HASL. The documentation relating to the requirements for alternative surface finishes have been well publicized at many industry forums. Many of the requirements, in fact, are obvious. Regardless, the pwb fabrication industry needs to work closely with contract manufacturers and end users to fully appreciate the true impact of technology trends. These trends are significant and include:
The trends listed above are only a snapshot of the many issues that fabricators and assembly companies face. However, those listed are the ones that most closely reflect the trends influencing the solderability of components and the bare board surface. This fact relates mostly to the actual selection and performance of the surface finish. Regardless, the finish must be able to perform under a variety of conditions, consistently and reliably. Back to top TREND TOWARDS MIXED METAL FINISHES In recent years, in order to achieve high-density surface mounting on printed wiring boards, the number of terminals of circuit components have been increasing, and the pitch of the terminals has been significantly reduced. With the trend for increased packaging density has come the use of COB (chip on board), flip chip and TAB (Tape Automated Bonding). In many instances, the surface mounting of such components may be required on pwb’s with copper pads and other features plated with gold, silver, tin or solder. These mixed metal finish boards are becoming very common today and the surface treatment of such circuits will continue to grow in importance. The demand was such that a water soluble surface-treating agent, that was capable of protecting the bare copper from oxidation without leaving a film on the other metals, needed to be developed and implemented. In other words, the need for an OSP that selectively bonds to the copper without adversely affecting other metals such as gold or solder was established. Conventional OSP processes based on long chain alkylimidazole compounds and substituted benzimidazole compounds functioned adequately to protect the bare copper. However, these materials also deposited a significant film on other metals such as gold, tin and solder. This additional film interfered with subsequent operations such as wire bonding, and surface mounting of Quad Flat Packs on solder surfaces. In addition, the contact resistance on the gold increased to unacceptable levels. Thus, when pwbs are fabricated with multiple metal finishes, the metals such as gold or solder would have to be masked to prevent the OSP film formation on their surfaces. In some instances, the coating would have to be removed with alcohol, adding additional labor and cost to the fabrication process. A factor in promoting this film formation on the metal surfaces is the copper contained in many organic solderability formulations. The copper ions form a complex with the active azole ingredient in the OSP chemistry and actually helps to promote film growth. When a copper-solder mixed metal board is processed through such a process, the OSP forms on the solder and has the affect of discoloring the solder, making long term solderability literally impossible to achieve. It has also been determined that the copper ions that are part of the OSP protective film contribute to ionic contamination, a situation being constantly scrutinized by assembly houses and end users. It is desired to keep ionic residues as low as possible. It has been demonstrated that the copper contributes to the staining/darkening of the solder and causes undo build-up of residue on the gold. Therefore, it was imperative to develop an OSP process that would selectively deposit on the bare copper surfaces only, with low residual ionics. However, a film formed on the copper must have sufficient ability to maintain the solderability of the base copper through multiple thermal excursions and with a variety of low activity wave soldering fluxes and pastes. Back to top The Chemistry of the Process For this process, a unique imidazole compound was synthesized as the active ingredient of the OSP. (US. Patent # 5,795,409) This unique compound is solubilized in water and a nominal of amount of acetic acid. Acetic acid was chosen over such other acids as formic. The main reason is that acetic acid is less volatile than formic acid due to its lower vapor pressure (Figure 1.). This attribute contributes to maintaining a very stable pH range, which is necessary in building a uniform and sufficiently thick organic coating for solderability protection. When acids such as formic acid volatilizes, the concentration of the active ingredient can increase, causing the organic material to build the film abnormally thick and in a rather non-uniform fashion. This leads to solderability problems ranging from poor solder paste spread to heavy organic residue being deposited onto other metals and laminate surfaces. 
This process employs a combination of a phenyl imidazole compound mixed in an aqueous solution with acetic acid, a complexing agent and a water soluble iron compound. The combination of these additives permit the uniform coating of the organic film on the copper without building any appreciable amount on the non copper surfaces. After the printed wiring board has been prepped in an acid cleaner, followed by a micro-etch, the OSP coating is applied to the pwb by either dipping, spraying, or flood coating.  Back to top OSP Coating Thickness A common fallacy among OSP users is the thicker the coating, the better the solderability protection. The data does not support that statement. To the contrary, very thick coatings make the removal of the OSP by the action of the flux vehicle more difficult. It has been demonstrated that thicker OSP coatings reduce the spread of solder paste, often leaving bare copper visible. Indeed, additional OSP residue not completely removed by fluxes has contributed to insulation resistance on gold contacts, and poor solder joint formation. Figure 3. Soldering Process 
The nominal thickness of the OSP film produced from this process is between 1500-2500 Angstroms as compared to other commercially available systems producing thicknesses on copper in the range of 4000-7000 Angstroms. The thickness of the coating is determined by the procedure outlined below:
Thickness (µ m) = 0.105 X A1 X V/S Back to top Process Control To achieve the best performance with this OSP Process, several precautions should be taken. These are:
Regardless of whose OSP is ultimately used for production; the pH of the chemistry must be tightly controlled within specific limits. If the pH is allowed to increase, the thickness of the OSP coating will also increase. Higher pH values will cause the azole component of the OSP to crystallize out of the solution and even deposit as a heavy residue on the pwb. Acetic acid based OSP’s are easier to control than the formic acid based OSP’s for reasons discussed earlier. When an OSP process is operating in horizontal conveyorized equipment, it is customary at the exit of the OSP chamber itself to employ a set of squeezing rollers to minimize drag-out of the OSP chemistry, and to assist in the drying of the coating. Depending on how often the line is in use, it is possible for the rollers to receive some build-up of the active ingredient in the OSP. If not maintained, the crystallized material will redeposit onto the boards, creating contamination problems. It is recommended to spray the rollers with water on a periodic basis to aid in OSP removal. If crystals do form, a 5% acetic acid solution applied with a wipe will remove any crystals. However, this OSP Process does not require drying of the OSP coating prior to final rinse. This fact greatly simplifies the operation and improves overall coating uniformity. Water drag-in and micro-etch residues (especially persulfate based etches) are harmful to the performance of an OSP solution. Rinse water drag-in causes the pH to increase. Persulfate micro-etch requires an acid post clean up prior to the boards entering the OSP chamber itself. Hydrogen-peroxide micro-etch is the process of choice as the chemistry is easier to rinse, and does not require an acidic neutralizer. The azole component of the OSP process should be maintained between 85-105% of its optimum concentration. By maintaining this process window, the thickness of the OSP coating will remain within the optimum thickness window. A consistent thickness is critical for ensuring optimum and consistent results through the soldering process. Back to top Implementation of the Process Innovex Solutions, Inc. is both a fabricator of flexible printed circuits, and an assembler of flex circuits into the next higher level of product. In this role they are able to observe first hand both the advantages and disadvantages of various alternative surface finishes on assembly processes. The surface finishes currently in use at Innovex are:
Flex circuits have some key design and manufacturing differences from typical rigid board requirements, which introduce unique problems when processing these circuits through an OSP line. The use of punched or lased openings in coverfilm material creates "pockets" that can trap liquids during processing. Where mounting rigidity or heatsinking requirements are necessary, metallic and/or non-metallic stiffeners are attached to the flex circuit during processing. Any OSP used for these circuits must have good compatibility with a wide variety of stiffener materials, from reinforced and unreinforced polymers to various alloys of stainless steel and aluminum. 
The ideal OSP coating material for the most demanding flex circuit applications, hard disk drives (HDD), would have the following characteristics:
Innovex has been producing parts for three years utilizing a standard OSP process based on a substituted benzimidazole compound. The process has run successfully with certain limitations inherent to the OSP material and the process recommended to apply it. The following are some of the problems and limitations experienced over the last three years with this process: Metal Stiffener Compatibility Because aluminum cannot be run down the line without visible chemical attack, customers requiring OSP have been forced to use stainless steel stiffeners. The aluminum stiffeners could not be applied after the OSP coating operation because of the extended thermal bakes necessary for the stiffener adhesive lamination process, with the resultant degradation of any OSP finish. Gold Discoloration and Contamination Because of customer bonding requirements, most disk drive product gold pads are essentially 24 karat "soft" gold, and these are especially susceptible to staining and discoloration. Gold pads on processed circuits suffered moderate to severe darkening and discoloration from OSP and copper deposition resulting from the OSP conditioning processes. If circuits with gold plated pads were run through OSP, it was necessary to clean each pad with isopropyl alcohol (IPA), being careful not to degrade the OSP on adjacent copper pads. Even this procedure did not restore the gold to its normal color, causing some customers to reject the use of OSP on their parts. Back to top Excessive OSP Deposition The recommendations of the manufacturer of the benzimidazole in use at Innovex; together with the design of their custom line for OSP deposition, insure that the freshly deposited OSP coating be semi-dry before it enters the final rinse. This is to insure that the relatively soft, moist OSP coating is not thinned by the rinse process. This contributes to a significant ongoing problem with the acceptability of the flex circuits; the drying of residual, unrinsed OSP solution on the parts, leaving unwanted deposition of OSP on both the polyimide and exposed adhesive surfaces of the circuit. This also tends to give large variations in OSP thicknesses across the copper surfaces; especially on the pads captured by coverfilm openings, which tend to form shallow cups, trapping OSP solution. The OSP on random regions of copper can sometimes reach thicknesses of tens of microns. The problem is exacerbated by the fact that once these deposits have dried prior to rinsing, they do not dissolve again in the rinse water, but remain in place on the circuit. The excess OSP which is not on copper, but on a polymer surface, or a stiffener may not even be removed by post assembly cleaning; for these regions of the circuit have not been exposed to fluxing action, and a simple aqueous clean may not remove them. Figure 5 shows examples of these undesirable deposits which can form on an unassembled flex circuit, while Figure 6 shows residual OSP from excessive deposition on the underside of a stainless steel stiffener after customer assembly and cleaning. 
Back to top Ionograph Cleanliness Testing Ionograph testing is routinely used at Innovex for testing lot circuit cleanliness. The Benzimidazole has a component in its molecular structure, which volatizes with time and heat. This component causes parts to have high initial readings on ionograph cleanliness testing, potentially masking other process ionic residues from being observed. Parts coated with OSP must be baked at 70º C for 30 minutes to drive off most of the volatile ionic species before they can be tested on the Ionograph with consistent results. Figure 7 shows a typical example of Ionograph readings on circuits coated with the OSP "as deposited", versus "after baking". 
In early 1998 customer requirements dictated the use of aluminum stiffeners, gold bonding pads, and OSP coated Flipchip bonding pads. As a result of the Flipchip circuit density, and the close proximity of the gold pads to the Flipchip pads, the labor intensive removal of OSP from the gold pads with IPA was not feasible. The decision was made to evaluate the new Phenylimidazole OSP process described in this paper. The key performance indicators that would determine whether the new OSP met the company’s criteria were:
In addition, a " drop in" process was necessary to consider the project successful. The definition of a "drop in" process meant that no equipment changes were required, and no changes in the actual assembly process either in-house or at customer sites would be necessary. The majority of customers dual source product, and the OSP provided on Innovex circuits would have to perform similar to a competitor’s OSP finishes, using established assembly process parameters. Initial testing was performed on a Flipchip test vehicle, to evaluate the solderability performance of the coating. Success with the test vehicle led to actual product testing for multiple product lines; with a variety of surface finishes, stiffener materials, and Flipchip bump metallurgies. Both stainless steel and aluminum stiffeners were processed through the new OSP with no discernible attack on either metallurgy. Gold pads on circuits processed through the new OSP line actually looked cleaner, and more pristine after processing than before processing. Figure 8 shows three gold test coupons; the one on the left has been processed through the standard substituted benzimidazole OSP, the center coupon is "as plated", and the coupon on the right has been processed through the new phenylimidazole OSP. As can be readily seen, the coupon on the right has the brightest, lightest yellow gold appearance.  Figure 9 shows OSP measurements of the thickness of the benzimidazole versus the phenylimidazole over gold on coupons: 
The phenylimidazole does not need to be dried prior to the final rinse, and therefore is not susceptible to extraneous OSP deposition on the noncopper surfaces. Figure 10 shows an example of a circuit processed through this OSP. 
The new OSP does not have any components in the deposit which interfere with an Ionograph cleanliness test, precluding the requirement to bake before testing. Figure 11 compares Ionograph readings between circuits processed with benzimidazole, unbaked, and baked; and phenylimidazole, unbaked. Figure 11. Ionograph Testing of Standard and New OSP's > 
Back to top Assembly Testing Assembly testing began with the Flipchip test vehicles. The assembly processing parameters were identical to those which had been in use for the standard OSP product; the intent was to have a process that would perform equivalently for either OSP. The Flipchip process in use at Innovex is a two step process, with the surface mount devices assembled first with a reflow and clean operation, followed by the Flipchip assembly, with its associated reflow and clean operations. The results were good, the solder wettability and spread was equivalent, with Flipchip standoff height within 5µm of that obtained with the standard OSP. Figure 12 shows a cross section of a Flipchip assembly using the new OSP coating. Figure 12. Example of Flipchip Die Attached with Phenylimidazole 
With these results, the evaluation was expanded to include SMT and Flipchip assembly, reliability testing and solder joint studies on two actual customer products. The new OSP showed no differences from the standard OSP for SMT solderability, Flipchip die bump wetting, and Flipchip underfill flow. Visual inspection was good, with no rejects after assembly. Two parts failed functional test for die related problems, not related to the OSP. The additional tests and results are listed below in Table 1:
The in-house circuit assembly trials demonstrated that there were no process changes necessary to implement this OSP use, and that yields were comparable to those using the standard OSP process. All pre- and post-assembly test and cleanliness results were good. Subsequent assembly testing by HDD customers proved the interchangeability of this new OSP with Benzimidazole type OSP’s on their assembly lines, as well as the increased cleanliness and yields resulting from the lack of extraneous deposition of OSP material. Back to top Summary and Conclusions Innovex’s fabrication division found several benefits by implementing the new OSP process. These are:
The company’s assembly division and its other customers found several benefits including:
The new OSP Process continues to operate in the Chandler facility and will soon be implemented in Innovex’s new operation in Thailand. Acknowledgement We would like to acknowledge the efforts of Hazel Schofield, of International Rectifier (formerly of Innovex, UK) for her work on assembly qualification and testing for the new OSP; and to Koji Saeki of Shikoku Chemical for his technical guidance with the implementation of this OSP. |
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