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Investigation and improvement of black pads for immersion gold welding with nickel
1. Reasons for the popularity of nickel immersion gold
For multi-layer boards assembled with various precision components, in order to flatten the soldering pads and improve solderability, the strength of the solder joints and subsequent reliability are more certain. About ten years ago, the industry gradually adopted electroless nickel immersion gold on the copper surface. Nickel and Immersion Gold; EN/IG) plating is used as the solderable finishing (Solderable Finishing) of various SMT solder pads. These mass-produced boards include: notebook computer motherboards and communication card boards, mobile phone handset boards, personal digital assistants (PDA) boards, digital camera motherboards and card boards, and high-difficulty boards such as camcorders, And various card boards (Card, referring to small circuit boards) for computer peripheral purposes. According to IPC's TMRC survey, ENIG only accounted for 2% of PCB surface treatment in 1996, but it had grown to 14% in 2000. Based on Taiwan's mass production experience, the unit operating volume (Loading Factor) has reached 1.5ft2/gal (360cm2/L) in a 1000l large nickel tank, and the tank needs to be changed in two or three days when the work is busy. The reason why ENIG is popular with upstream and downstream in these difficult boards is the following four points after in-depth understanding:
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1.1 The surface is flat and good for printing and welding, and the small cushion is unique
When there are more and more slender rectangular, round, or square solder pads on the SMT surface, the denser and smaller the solder pads are, the unevenness of the soldering and spraying treatment pads makes it difficult to print solder paste and difficult to step on the parts. , Which in turn causes the deterioration of the quality of hot air or hot nitrogen welding (Relow). This is quite different from the through-hole wave soldering that was popular more than ten years ago, or the solder paste fusion soldering when the backing surface was large enough. At that time, the tin spray treatment of the pad surface, no matter in terms of solder (Solderablity) or joint strength (Joint Strength), was not what other solderable treatments could expect. A good ENIG can withstand an average of 3 times of high temperature soldering, which is currently not comparable to silver immersion or tin immersion treatment.
     However, the soldering of many mini-BGAs or CSPs on the mobile phone boards and their many micro-pads not only shocks the component manufacturers and assemblers, but the PCB industry is even more discolored (return compensation for poor quality) . At present, the general market CSP on the mobile phone board (this chip-level package product refers to the BGA with the pitch of 0.8mm or less), the pitch of the round pad is only 30mil, and the pad diameter is only 14mil; and the small QFP is rectangular The pad width of the solder pad has been narrowed to only 8mil. How can the precision solder paste printed on such a small pad surface tolerate the unevenness of the previous spray?
      The uniform and flat solderable layer is of course not only ENIG. The former mass-producers still have OSP organic solder mask treatment and immersion tin treatment (Immersion Tin; the scale of mass production has recently appeared, and the after-effect remains to be seen), Nickel immersion palladium gold treatment, even chemical immersion tin or chemical tin-lead treatment. Except for OSP, most of the others are unable to become the climate due to unstable process or too much trouble. In practice, of course, it is not the opponent of nickel immersion gold at all. And the durability and stain resistance of OSP are not as good as nickel immersion gold, and whether the weakly active flux in the no-clean solder paste can remove the aging film of OSP immediately before soldering, and can be smoothly dipped and soldered There are also big problems.
Picture In addition to the above-mentioned general soldering, the pad surface of ENIG can of course also be used as the ball planting base of the FC package board, or the bump pad after the solder paste is formed into a hemisphere.
  1.2 The contact continuity of the cushion surface has always been excellent, no semicolon
       In addition to the welding of parts for the mobile phone board, some pads also need to be turned on by pressing the key. The gold does not rust and is the best candidate for Contact Connection. This method of key pad on the mobile phone board is compatible with the ACF pressing pad on the LCD-TFT module board, whether it is directly laid out on the main board, or another extremely thin double-sided hard board or soft board is used. With the motherboard, the surface of its contact pads must be electroplated with nickel and gold to reduce its contact resistance. Nowadays, under the oversupply of demanding HDI mobile phone boards, the price has already fallen to cheap products. The original formal electroplating treatment has to be downgraded to primary nickel immersion gold.
   What's more, ENIG can also be used as a gold finger for the insertion and removal of some low-cost multilayer boards. Although it is not the right way to comply with the law, it is better than nothing. Many low-cost DRAM long and narrow card boards have no beveled gold fingers, that is, these counterfeit methods are adopted. For all kinds of commodities with high prices, everything has to be kept simple.
Picture 1.3 It also has wire bonding capability to replace electroplated nickel and gold
      The surface of many chip mounting boards (Chip on Board; COB) requires wire bonding (Wire Bonding) to thicker (5-20 m) aluminum wires, so that the chip can be directly interconnected with the circuit board, and The packaging cost of the component is waived. In such occasions where the fixation force is not important, the nickel-plated gold will naturally replace the expensive nickel-plated gold. In the early days, some timers, such as electronic watches and electronic meter pens, adopted this "downgrade" approach. However, in recent years, the manufacturing base for such cheap goods has long been transferred to mainland China. Its current practice even cancels ENIG due to cost considerations. It only needs to wipe the copper surface of the cushion with a soft rubber when shipping. The pulling force after wire bonding is even not inferior to that of nickel immersion gold. Under the principle of the supremacy of cost, these cheaper "non-standard" methods will of course replace the elusive "downgrade" policy.
Picture 1.4 Non-oxidizing at high temperature can be used as a heat dissipation surface
      Some high-power components can be equipped with heat-dissipating fins on the back, and some heat-dissipating through holes can also be provided on the plate surface area of ​​the ventral bottom. At this time, the hole ring of the hole wall can be treated with nickel immersion gold, which can help the heat dissipation without oxidation, and can increase the mechanical strength of the hole wall. Like a rivet, the structure of the multilayer board can be more optimized. Many notebook computer boards are used in the receiving area of ​​the CPU, or the soldering base of BGA-type components, or other small card boards that use this comprehensive ENIG heat dissipation hole.
     Figure 9. The above three types of six- or eight-layer boards all use ENIG as the heat dissipation of high-power multi-leg components. Due to the reinforcement of the nickel-gold layer on the copper holes, long-term tasks such as interconnection and heat dissipation or only heat dissipation can be achieved. The picture in the middle shows the TAB-style Pentium-1 320-pin CPU, a ten-layer board with Super Solder pads, and its central square ENIG film area, with a total of 9.8 mil small holes
256, which is of course very expensive. In order to reduce the cost, we simply knocked down all the window lattices and changed them to a larger square door (pictured on the right), which is not only cheaper but also more convenient for heat dissipation. Strange, why did you go?
2. The background of the falling out of favor of nickel immersion gold
      ENIG has the four functions of welding, contact, wire bonding, and heat dissipation. It has always been the darling of various densely assembled boards, and has long become a position that other surface treatments cannot replace. Once upon a time, when the BGA or CSP solder pads on the motherboard of the notebook computer and the later phone handset were too large and small, ENIG gradually experienced poor solderability and insufficient joint strength. The follow-up reliability of the solder joints is low, and even after the solder joints are cracked and separated, there will be various disasters of black pads (Black Pad), which make the producers hate and love, and there is no word to say that there are hardships.
In order to fundamentally improve the quality of ENIG by American industry players (mostly downstream assemblers), the well-known ITRI (Interconnect Technology Research Association) formed a consortium (Consortium) for project research and improvement in August 1997. There are 22 companies in total. Related industry participants (PCB and PCBA industry and potion suppliers), hoping to find out the real cause of the failure under the careful simulation of the special test board (Test Vehicle). However, despite the continuous efforts of everyone in the past five years, not only the gains are limited but the evaluation is also lackluster. In the Proceedings of IPC Show and other journals, more than 20 large-scale papers have been published. It is hard to see the real reason and the solution. After careful reading, it is still confused. The only feasible way is to shorten the service life of bath solutions such as nickel and immersion gold. As for the extent of the life loss, it depends on the grade of the product.
The ENIG used in notebook computer motherboards, three or four years ago, many well-known Taiwanese manufacturers have found that the following occasional solder joint strength is insufficient, and a few solder joints were found within one or two months or even less after soldering. Black Pad problem with cracking and blackened nickel surface. The painful history of his tragic cession of waterloo for compensation is still lingering. The plague dare to say that there is no one without it. Some naive downstream customers and laypersons in the PCB industry initially thought that the black film was caused by the accumulation of carbon content, so they had to mobilize the public to seriously review it, and its boldness was really unsatisfactory.
In fact, this black film is a complex composition of nickel oxide (NixOy). The fundamental reason is that the nickel surface is subjected to excessive oxidation reaction when the nickel surface undergoes the immersion gold substitution reaction (metal atoms dissolve into metal ions and their atomic valence increases. , Known as generalized oxidation), combined with the irregular deposition of very large gold atoms, and the loose and porous rough grains, the formation of the bottom nickel continues to be strongly promoted by the "chemical battery effect" (Galvanic Effect, also known as the Javanni effect) , And continue to oxidize and age, so that the undissolved "nickel rust" continues to accumulate under the gold surface. The aforementioned stupid methods (such as shortening the nickel bath from the original 6 MTOs to the current 4 MTOs) can only alleviate the undesirable symptoms other than normal replacement, and cannot completely eradicate the occurrence and existence of black pads.
        The main purpose of this kind of getting more and more afraid and changing the tank early is to reduce the accumulation of H3PO4 in the tank solution, maintain the normal phosphorus content (7-9%) in the plating layer, and maintain better solder resistance and resistance. Oxidation (not exactly the same as corrosion resistance), it can reduce the degree of excessive attack by high temperature gold water, and even delay the subsequent fermentation of Galvanic effect. To put it bluntly, this is just a reluctantly feasible approach. Nowadays, not only the solder pads of various sizes on the mobile phone board are almost entirely dependent on it, even the Flip Chip package carrier board used in the famous CPU has to use ENIG as the solder "bump" (Solder Bump)’s impact. It is said that the life of its nickel bath solution has been shortened to less than 3 MTO, and the nickel pollution content in the gold water should also be reduced to below 500ppm. Even the pure replacement gold plating layer is also used, and the formula is changed to half replacement and half replacement at any cost. Restored composite gold layer (such as Uemura’s product TSB-71) to reduce the occurrence of subsequent black pad disasters.
Due to the large number of in-depth research reports, the pictures cannot be detailed one by one, so you can only choose the ones with perfect content carefully, and briefly describe them according to the author and the company. Readers can directly read them if they are interested in further investigation. A large amount of original data.
3. Summary of the contents of various important research reports
3.1 "RootCause Failure Mechanism" A RootCause Failure Mechanism"
   This article was written by Nicholas Biunno of Hadco. This famous Hadco company was originally the second runner-up in the US PCB rankings, and has been acquired by CEM's leader Sanmina in 1999. This article was originally published in the Proceedings of the IPC 1999 March Exhibition (S-18-5), and later reprinted in the June 1999 issue (No. 6) of the British quarterly publication Future Circuit International, P.133, with 7 pages. This article explains the formation of the black pad in detail, the main points are as follows:
3.1.1 Principle description
The solder joint formed by the ENIG solderable layer is grown on the nickel layer (that is, the IMC forming Ni3Sn4), and the thin layer of immersion gold (2-4 m) will quickly dissolve into the tin body during the soldering process. middle. Therefore, it is known that gold itself does not participate in the organization of the solder joints, and its only function is to protect the nickel layer from rust or passivation (Passivation), otherwise it will not be able to form IMC and cannot be welded firmly. The thicker the gold layer is, the more it will melt into the solder joint, which will cause embrittlement and make the solder joint more unreliable.
   The dissolution of nickel and the deposition of gold occur at the same time as the substitution reaction. Once the interface is sealed by the gold layer and no nickel is soluble, the deposition of the gold layer will also stop. However, since the gold layer is extremely porous, the reaction can still proceed slowly under a less dense structure. In general, the hyperactive of gold water under the influence of certain factors will cause irregular over-oxidation of the local nickel surface. Even after the gold layer is covered, the interface between it and the bottom nickel actually already exists. Some considerable oxides will become annoying black pads after aging and deterioration. This kind of disaster is purely accidental, and the location of its occurrence is unpredictable. It is a kind of hidden danger that is completely unpredictable. The pH value of the nickel bath has the greatest influence on the thickness and topography of the nickel layer, while the operating temperature is the most dominant on the deposition rate of the nickel and gold layers.
3.1.2 Description of the eight replacement modes seen
      Using SEM (Scanning Electron Microscope) with EDS (Energy Spectrometer), AugerSpectroscopy, FIBM (Focus Ion Beam Micropobe) and other instruments to conduct careful surface analysis on ENIG , Which shows that the oxidative dissolution of the nickel layer (nickel atoms) in the gold water, and the negative reduction of gold cyanide ions, and the deposition as the result of the reaction of gold atoms, the article describes with 8 high-magnification photomicrographs The pattern of its replacement process:
        (1) Gold water attacked the crystals on the surface of the nickel layer; only attacked the weak points of the spherical nickel crystal interface. From the side view, it can be seen that the depth is about 1/4 of the crystal ball diameter.
        (2) The gold water still attacks along the nickel crystal interface; it can be seen from the top view that its effect has been deeper, and it exceeds 1/4 of the diameter of the ball.
Picture (3) Jinshui has already attacked the spherulite body of the nickel-phosphorus alloy. Although the depth of the penetration is shallow, the area is very wide.
      (4) The attack of Jinshui has been very deep and wide, and the result of overactive reaction can be seen from the side view. In fact, all of them have invaded the interface and the crystal body.
Picture (5) The spherical crystal of nickel-phosphorus alloy is surrounded by gold water, and the cross section can be seen around it and the bottom interface.