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The hidden worries of lead-free soldering on circuit boards (3) Interface micro-holes
2021-10-06
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Author:Aure

The hidden worries of lead-free soldering on circuit boards (3) Interface micro-holes



 In the early stage, the author firstly used three major deficiencies such as solder joint voids, solder ring floating, and lead tin whiskers, indicating that lead-free soldering has more and worse poor quality than current lead soldering. In fact, there are more hidden worries that make the reliability of lead-free soldering inevitably compromised. The presentation of these various negative qualities is still waiting for the industry and suppliers to fight for timeliness and intensify their efforts to try to solve some painful and some known and unknown problems, otherwise there will be a gap between upstream and downstream and surrounding supply and demand. , Causing endless disputes and endless troubles.
       1. Formation of mainstream solder
       Lead-free solders have been finalized as SAC305 (S n 9 6.5%, A g 3.0%, C u 0.5%) or SAC405 (S n 9 5.5%, A g 4.0%, C u 0.5%), etc. Nominal (NO mina 1) weight ratio alloys are the mainstream. The industry has done a large number of diversified tests, established examples and consensus on temperature timelines and multiple failure modes available for mass production, and gained some practical confidence. It is almost impossible for upstream and downstream to spend more energy and time on other alloy solders. Japanese companies often use low melting point "tin-zinc-bismuth" alloys (S n 89%, Z n 8.0%, B i 3.0%), or other antimony (S b), handcuffs (G e) or nickel (N i) Various solders; in fact, the major OEM customers in the lower reaches of Europe and the United States rarely agree.
This is the comparison of lead-free SAC and leaded Sn63 (Relow) temperature-time curve (PrOnle)

       Among the two mainstream tin, silver and copper solders, SAC305 has the upper hand. One reason is that the cost of 1% less silver is naturally cheaper. The second is that the long strip Ag3S n in the solder joint has poor IMC. The reduction. The current S A C 3 O 5 alloy formula patent is jointly owned by Senju Metals of Japan and the University of Iowa in the United States. All solder suppliers in various countries must pay the patent fee.
       As for wave soldering, waVeSolderin g, and the spray tin process of PCB, although SAC305 can also be used, for the sake of cost savings, cheaper tin-copper alloys (Sn99.3% by wt, Cu 0.7% bywt) can also be used. But its melting point is 10°C higher than SAC305 and reaches 227°C. Furthermore, it is expected that the tin dip time is much longer than that of S n 6 3/P b 37, so the residence time at the peak temperature of 2 6 5-2 7 0 ℃ needs to be changed from the original lead 3-4 The second is extended to 4-5 seconds.



The hidden worries of lead-free soldering on circuit boards (3) Interface micro-holes


And when the amount of copper in the solder bath of wave soldering or spraying tin increases by 0.2% by wt, its liquefaction temperature will rise by another 6°C, which will increase the damage to the plates and parts. This kind of damage is even worse for large-scale and thick-thick boards, and it often results in a tragic end to the delamination and bursting of the board. Once the large blank board has been tin sprayed at 270°C, it will be more difficult for the subsequent dip welding to be tortured by strong thermal stress for 4-7 seconds. In terms of cost and quality, whether to use 2 2 7 C tin-copper alloy for tin spraying and wave soldering is indeed worth thinking twice.
       2. Interface micro-holes
       (1) The location of the interface micro-hole
       When the solder (So1der) forms a solder joint (So1der Poi nt) on the PCB pad (P ads) base, there is often a lot of occurrence between the solder pad base (referring to copper and nickel) and the main solder of the solder joint. Tiny holes. Such micro-holes with a diameter of less than 40μm at the interface are in fact mostly between copper and tin, and interlaced and coexist with IMC. Naturally, IM C cannot be generated where the microcavity occurs, and its bonding strength must be in short supply.

       (2) The troubles of interface micro-holes
       The micro-holes in the interface are not exactly the same as the holes in the solder joints that are far away from the interface. But for the bonding strength of solder joints, a large number of interface micro-holes are more lethal. These differences can be recognized from the high-quality micro-slice screens, or can be clearly judged from the high-power and high-resolution X-ray inspection equipment. Once the solder joint strength problem occurs, all evidence will be hidden.
The voids formed in the interface can be large or small. Large interface holes over 40μm have a great destructive force on the strength of solder joints.

       (3) The main reason for the micro-holes in the interface
       Practical solderable treatments on the surface of PCB pads include: spray tin, immersion silver, immersion tin, OSP and ENIG. The first four are all based on copper as the IMC base, while nickel immersion gold uses electroless nickel as the welding base to form Ni3Sn4. In fact, for high-density SMT welding, the tin spraying process is often not suitable. The other four surface treatment layers will all have the participation of organic matter. When they are cracked into gas at high temperature and fail to escape in time, of course they have to stay in place to form microcavities. In fact, Gongqu S n, IA g, OSP and other films and ENIG's gold layer, etc., only act as a protective film for the bottom copper and bottom nickel so that they will not oxidize and resist soldering, and they are not involved in welding (except IS n). reaction. Therefore, it is known that the thinner the thickness, the less organic matter will be, and the smaller the chance of interface micro-holes, but the thinner the film will not be able to achieve the function of welding protection.
       In addition to the surface treatment layer itself, the reason for the micro-holes in the interface is to blame. In addition, the flux formula, the profile of the soldering temperature, the cleaning of the pad surface, the water absorption of the solder paste, and the design of the pad will all be the interface. The cause of the microcavity. At present, in order to reduce the assembly height and save costs, some of the original QF P (Quad F1at Package) extension feet Gu11 wing or hook feet J-1 ead, etc., some products will be cancelled, and the outer periphery of the abdominal bottom of the component will be directly designed above Pad, the PCB board surface is also corresponding to the additional pad, directly use the solder paste for face-to-face soldering, especially called Quad F1at NO-1ead (Q FN). This kind of brand-new Q FN solder joint has already had a lot of voids in Central Plains, and lead-free soldering will add fuel to the fire. In addition, the pure tin plating layer on the cut side of the component's ventral bottom will cause the growth of tin whiskers adjacent to each other will be another confidant pain.
The left is the void in the BGA ball foot solder joint application; the right is the void in the QFN full flat solder joint. However, if the center of the larger square pad of the package body is received, it is due to the uneven connection of the thermal paste


       (4) Hypothesis of interface micro-holes
       Since there are too many causes of voids in various solder joints (the ones with lead and lead-free mixtures have more voids), it is not easy to carefully distinguish the interface microvoids. Two main hypotheses are proposed:
       1. The copper surface is contaminated, such as trace residues of green paint.
       2. Excessive roughness or other pollution on the copper surface, or adhesion of moisture, etc.
      
       (5), another Kirkendall Voids
       At the moment when the copper base undergoes high-heat welding, when the dissolution rate of its copper into the liquid tin is uneven, another kind of substitute will be formed between the grown Cu6Sn5IMC and the bottom copper, so that nearby atoms can’t move. (Disp1acement) and the formation of micro-holes, called K-holes. Usually when the welded board is continuously subjected to high temperature impact, these K holes will gradually become larger and larger, which will also bring about the trouble of insufficient solder joint strength. At present, the research on this kind of K hole is not popular yet.
This is lead-free soldering and high temperature aging. The K hole seen from the day surface of the SEM can clearly identify that it exists between the bottom copper and the long thick IMC