Hi Guy, My mistake confusing wt% and volume%. Glen is correct, it should be wt%. Gold embrittlement is associated with AuSn4 which is why I incorrectly assumed alloy content is important. However, after a little research, I found that AuPbx intermetallics are "possible", and besides, the 4% rule applies to Sn/Pb bulk anyways. The percent of solder is 50%, because you stencil print solder as a function of calculated volume, not weight. (Now that I mention it, my formula is further incorrect in that the volume of printed paste is never 100%, rather something like 50-70% depending...man, I'm full of errors!) The 50% comes from the approximation that neatly stacked spheres will outline a shape roughly the same as neatly stacked cubes. The spheres are the solder powder, the space between the spheres is the flux vehicle and the rectangular shape represents the shape of the stencil aperture the solder power will be forced to conform to. The volume of the sphere inside a cube of the same dimension is roughly 50%. O.K., technically its 47.7465...% because of the 3/pi. ([4/3 pi r^3]/8 r^3)=volume of a sphere divided by the volume of a cube. Or, you can back calculate from the density of the 90%wt solder compared to the density of the 10% flux to arrive at approximately the same 50 - 50 ratio of solder powder to flux by volume. But to get the allowable thickness of gold, weight has to be converted to volume to get thickness. Since weight=Volume * density, the density of solder and gold must be added to the equations. V_g = l*w*height_gold*density_gold V_s = l*w*.5*height_solder*density_solder where height_solder is assumed the stencil thickness max_height_gold = 0.04 * stencil_thickness * .5 * density_solder/density_gold Density of gold = 19.3g/cc Density of solder = 8.46g/cc Fortunately the units will cancel each other so we can apply the forbidden mixing of inch and metric units. (My Physics professors would kill me for this.) Solving for height_gold=0.04*.005inches*.5*8.4grams/19.3grams=44uin assuming 100% paste release. Assuming 70% paste release...we're down to 30uin. I've lost my good references of gold dissolution rates into tin at reflow. But the one source I have is 36uin/sec at reflow temp. Since joints are typically molten for several seconds, I'd imagine the gold would dissolve pretty completely...but, I have heard several sources indicate they've found a gold rich region in gold containing solderjoints. Concerning Dave's comment questioning 10 uin of gold being insufficient to protect solderability, I have to agree, if we are talking about electroplated gold. Under the high power magnification of an SEM, holes can be seen in the electroplated gold and the underlying nickel can often be seen. Immersion gold is a completely different story. It's self limiting for a reason... Ryan -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey Sent: Friday, August 19, 2005 2:28 PM To: [log in to unmask] Subject: Re: [TN] max thickness electroplated gold I would describe your equation as a rule of thumb. And I thank you for it. However, I have a couple of questions about the equation. max_height_gold = 0.04 * stencil_thickness * .63 * .5 Why do we care what the percent tin is? Isn't the number 4% in the solder ( and Glen brings up and interesting question)? Are we sure it (the gold contamination) is by volume, or is it by weight? Glenn Why would the percent solder be 50%, our paste is 90% metal . . . by weight? or volume? hummm. Why wouldn't we need to factor in the rate of dissolution of gold into the solder? Since the alloy forming is AuSn(2 and 4). Won't the tin be consumed at the interface? Won't the resulting Pb rich region inhibit dissolution of the gold through the filet? Wouldn't this result in a region rich in gold near the board surface. With a soldering iron we can stir the pot a bit, but in a reflow oven there is none of that. I get 180 uin as a max when the stencil is 0.005 in thick. But, I know better. In fact, I think the 67 number is way to thick as well. My experience tells me the number is much lower than this rule gave us. I think it is even under the 18 uin in the IPC-6012B. I don't think we have a reliable model mathematical model here. But, it's a start. This old thread from the archive is pretty interesting. Does anyone have a copy of Mike Wolverton's paper? Dave, do you still think that we need more than 10 uin of gold to protect solderable basis metals? Or were you only speaking to component lead finish? Date: Fri, 21 Mar 1997 15:42:08 -0800 From [log in to unmask] Fri Mar 21 17:22:15 1997 In-Reply-To: <[log in to unmask]> Reply-To: Mike Wolverton <[log in to unmask]> X-Receipt-From-Agent: true Resent-Message-ID: <"aC16b2.0.3ED.YtlCp"@ipc> Resent-From: [log in to unmask] X-Mailing-List: <[log in to unmask]> archive/latest/11286 X-Loop: [log in to unmask] Precedence: list Resent-Sender: [log in to unmask] Status: O X-Status: Gold embrittlement is well understood, but not widely understood. The good news is, the risk of gold embrittlement has been quantified. It is a function of the solder density and the allowable (maximum) percent gold. The maximum ratio of gold finish thickness to solder alloy thickness can be calculated, to help prevent a problem from excessive gold-tin (i.e., AuSn4) compound. For the equation and its derivation, please refer to a paper of mine, "The Use of Capillary Action Measurements for Solderability Improvement," Soldering and Surface Mount Technology, No. 9, Oct., 1991, Wela Publications, Ayr, Scotland, Table 5. It is interesting to note, The Embrittlement culprit, AuSn4 is 29 weight percent gold. So even if one had 100% tin as the solder alloy and 10% gold were dissolved into the solder, then about a third of the solder joint would be rocks (i.e., compound) and the remainder would be soft solder. Do not count on the rocks to bond to the solderable surface. Hence most recommendations I have seen have been in the 3 to 5 weight % gold range for the maximum allowable percent. Gold is dissolved by molten tin based solder, but the gold does not go into solution (i.e., the result is not a solid solution). For more details on the mechanism and explanation, please refer to a paper of mine: "Understanding Gold Embrittlement in Surface-Mount Soldering," ISHM's 1st Joint Technology Conference, San Diego, March, 1990 (//WWW.ISHM.EE.VT.EDU). Incidentally, ISHM changed its name to IMAPS, (703) 758-1060. The above represents my own view, and not necessarily that of my employer. --------------------------------------------------- Mike Wolverton Texas Instruments [log in to unmask] ------------------ Original text From: "ddhillma" <[log in to unmask]>, on 3/20/97 5:50 PM: To: [log in to unmask] Cc: [log in to unmask] Hi Tom - The words gold embrittlement really mean job security for metallurgists! You would think that as well researched and investigated the gold embrittlement mechanics of failure has been over the last 20 years we would stop making the same mistakes. Two rules of thumb to follow: a) Is there enough solder volume to allow the gold to uniformly go into solution? b) Is the soldering operation of sufficient temperature and time to allow (a) to occur? The thickness of the gold finish isn't the only deciding factor of having a solder joint embrittled by gold/tin intermetallics - the process is equally important. Also, you can avoid the whole gold embrittlement issue by keeping the gold below 10 microinches but you then have to deal with poor solderability on the components if this thin gold plating is of poor quality (and plating a quality gold finish below 20 microinches requires good control and knowledge of one's plating system). Dave Hillman Rockwell Collins [log in to unmask] -----Original Message----- From: TechNet [mailto:[log in to unmask]]On Behalf Of Ryan Grant Sent: Friday, August 19, 2005 1:25 PM To: [log in to unmask] Subject: Re: [TN] max thickness electroplated gold Hi Guy, Its not a rule of thumb, its science... Gold embrittlement happens at 4% gold per volume of tin for eutectic Tin/Lead solder. Clearly this rule isn't a sharp "edge" so I'd recommend a safety margin. Now lets do the math... Assume the component pad size is a 1:1 ratio to printed pad size. Volume of the gold (V_g) = l*w*h_g (length times width times gold thickness) Volume of the solder (V_s) = l*w*h_s*.5*t_r where .5=close approximation of solder powder volume to printed paste volume and where t_r= tin ratio of the alloy. Since we are looking for the ratio where V_g/V_s = 4% as the drop dead ratio, and Since we assume component pad size is 1:1 to printed pad size, l*w cancels out. We now have h_g/(h_s*.5*t_r) = 4% Solve for h_g = .04*h_s*.5*t_r If we assume a 5 mil stencil h_g = t_r*.0001 Assuming Tin/Lead solder at 63/37 h_g = 0.000063 = 63uin That 63uin includes both component and PCB pad gold thickness. That said, your 50 to 100 micro inches is bad news... By the way, if the component is BGA, it comes loaded with a lot more solder, thus the calculation will need to include the volume of the BGA ball. If you are using Lead-Free solder, this rule doesn't apply and transferring principles becomes complicated. The 4% rule is known to apply to silver as well, but you will notice Lead-free alloys are already at a 4% ratio of silver to tin but do not exhibit the same embrittlement failure implied with Tin/Lead embrittlement. The specific point that gold, or silver, embrittles a Lead-Free solder joint has not been studied and published, as far as I know. (If anyone knows otherwise, please let me know the reference). Good luck, Ryan Grant -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey Sent: Thursday, August 18, 2005 2:38 PM To: [log in to unmask] Subject: [TN] max thickness electroplated gold Many of our customers are using the old MIL-STD-275 to specify gold thickness for Hard Gold. They are ordering this because they are doing both wafer and package testing at the die and device level. So, we are seeing 50 to 100 uin of gold over nickel. It hasn't been much of an issue because we build most of the boards by hand soldering methods. We are starting to see components in the parts list for these assemblies that cannot be installed with hand soldering methods. When we solder with printed paste and reflow methods we see dull, grainy joint that are clearly contaminated with gold. When we hand solder this condition is not evident. (I believe the condition may actually be worse because the dissolved gold is more concentrated near the bottom of the fillet. But, that is another discussion). Does any one have a rule of thumb for the maximum thickness of electroplated gold on land patterns intended to receive SMT components . . . say, 0402 passives or MLFs? --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------