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February, 2018 Production
Copper Dissolution’s Effect on Time-on-Joint for Selective Soldering
By Stanley Bentley, P.E., Senior Technical Advisor, RapidProto C
opper dissolution was not studied, or even considered, prior to the ad- vent of lead-free soldering. The reason is that the older Sn/Pb solders had a much lower dissolution rate, to the point that it was rarely a fac-
tor in the failure of a solder joint. Until the late 1990s, many in the industry were totally unfamiliar with the term. However, a raft of early mortality of as- semblies, coinciding with the deployment of lead-free soldering, caused an ur- gent need to better understand the problem. A second contributor to the growing awareness of the importance of copper
dissolution was the increase in the use of selective soldering. Selective soldering is not new, but it became very popular with the decrease in through-hole compo- nents and the increase in assemblies with little to no through-hole. The tradition- al wave solder system became too cumbersome for the limited number of through-hole connections that are present in modern SMT assemblies. The combination of lead-free solders and selective soldering brought cop-
per dissolution into the mainstream. The end result was an awareness of a need to control the time allowed for the formation of a solder joint. The indus- try calls this time-on-joint, or TOJ.
TOJ Guidelines We have been told that a reasonable figure for TOJ in selective solder
systems is one to six seconds, depending upon the type of surface finish, the thickness of the through-hole copper, and the clearance between the compo- nent lead and the hole. What is not discussed is where these guidelines come from. The one sec-
ond comes from the pure physics of the formation of the desired tin/copper in- terface (Cu6Sn5) and the thermodynamics of the heat flow required to achieve this intermetallic formation. The recommended upper limit is less clear. The six second (selective solder system) recommendation for TOJ is due
to the rate of copper dissolution that occurs in high-tin solders. SAC305 (Sn, 3 percent Ag, 0.05 percent Cu) is by far the most aggressive. Studies have shown that there is an almost linear relationship between the amount of cop- per dissolved by the liquidus solder and the time the liquidus solder is in in- timate contact with the copper pad and plated through-hole. The contact time with the plated through-hole is important, but more
critical is the refresh rate of the virgin solder. This is where the selective sol- der system creates difficulty. The solder nozzle is continuously refreshed by virgin solder from the solder pot. In a stagnant system, with no new solder, the dissolved copper in the solder will quickly bring the process to a steady state. In theory, a selective solder system never approaches a steady state and therefore always has hot virgin solder in contact with the joint.
Variables that Affect Dissolution An absolute dissolution rate is difficult to define because there are many
variables that affect the dissolution rate. The most common are: l
slow the dissolution process, such as antimony, bismuth, nickel, and germa- nium. SAC305 (SnAg3Cu.5) is the most aggressive. Sn/Pb (Sn63Pb37) is the least aggressive, while SN100C (Sn99.3Cu.7) is in the middle.
l The temperature of the solder. A 15°C increase in the solder temperature
can double the dissolution rate. Pot temperatures typically range from 509 to 572°F (265 to 300°C).
l The flow rate of the virgin solder. Typically this is defined as pump rpm,
as no effective flow measurements are present on selective solder systems. An additional factor in TOJ maximum time is the thickness of the cop-
per plating in the through-hole. The most aggressive attack occurs in the knee of the plated hole. When the standard for plated copper thickness was 1 mil (25.4 µm) in the hole, the knee was usually thicker and therefore could with- stand an attack on the copper. However, with an IPC 6012 Class 2 definition of acceptable copper being
0.711 mil (18.1 µm), the margin for error has been significantly reduced. All of these factors create the difficulty in determining a precise copper dissolu- tion rate. Hence the use of a nominal time of six seconds. If the TOJ is longer than the time required to completely dissolve the knee of the plated hole, then the operator quickly discovers that additional time heating the solder joint will not increase the percentage of solder in the plated barrel. It is even possible for solder to migrate back down the plated hole toward
the virgin solder or heat source, resulting in even less hole fill. This is thought to be due to the formation of an undesirable intermetallic (Sn3Cu) that is gen- erally unsolderable. A reasonable rule-of-thumb for the rate of copper dissolution for SAC305
is 100 µin. (0.0254 µm) per second of contact. This means that a selective sol- der system with an IPC 6012 Class 2 PCB, plated with the minimum through- hole copper, can dissolve all of the copper in the plated knee in about 7 sec- onds. Therefore, the recommendation for TOJ for a selective solder system is
between one and six seconds. Contact: RapidProto, 8116 Zionsville Road, Indianapolis, IN 46268 % 317-975-2060 fax: 317-663-0729 E-mail:
www.rapidproto.com r
The type of solder and the tin content, as well as other elements known to
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