BGA components height after soldering - Page 1
Forum section: electronics / projects

author: nihial
author_rank: Regular Contributor
timestamp: June 04, 2021, 03:24:05 pm
content: Hi,I'm trying to assess what will be the height of some bga components after soldering but I'm not sur about some thingsFor example, this FPGA :I would expect the sum of A1 and A2 to equal A but it's not the case. I don't know if A represents the height after soldering or not.When I measure the height on a board I'm always just below those values (below the minimum even).The closest to reality I get is taking A2 , diameter of the balls and removing 2/3 for balls reduced height after soldering but I don't know if it's valid or not.I have a similar issue with an intel processorI'm using F2 + F3 + 2/3N, it looks coherent with what I measure but I don't know if it's legit.The reason for this is to optimize heat dissipation with TIM materials, the only parameters I can play with are distance to heatspreader and TIM material thickness, I can't use some spring based heatspreader so I need the most accurate figures as possible.Thanks to anyone reading this

author: ajb
author_rank: Super Contributor
timestamp: June 04, 2021, 04:47:16 pm
content: The drawing clearly shows dimension A as being the before-soldered state, since the dimension line hits the same leader as A1, and not the seating plane line. It's possible that this is an error in the drawing, and A was meant to be on the seating plane, but the A, A1, A2 nomenclature sort of suggests that isn't the case.This is speculation far outside of my area of expertise, but the way that tolerances stack in an assembly or even in a single part can be complicated, and in some cases not all tolerances are independent from one another. In the case of the first drawing, it's possible that the balling process controls dimension A somehow, like if the package is located by the top of the lid instead of the bottom and the ball height is set directly by the processing equipment, which would cause the resulting A1 and A2 dimensions to be interdependent. Or maybe they select the ball size (A1) based on A2 (not necessarily on every part, but maybe on each run after setting up the packaging line to a particular A2) so that A is better controlled, and they're giving you the worst-case scenario for A1 (which affects things like underfill) and A2 but also indicating that you'll never see max or min on both at the same time.I would expect that the final soldered height is highly process-dependent, so any specification of that height will be given with a lot of conditions. If you're contracting out the assembly then it may be best to discuss the need for closely controlled final height with your assembler and see if they can offer any advice or can do anything to help with that, or it may be that you will need to be prepared to adjust your TIM thickness after some sample runs.Quote from: nihial on June 04, 2021, 03:24:05 pmJust to check--how are you measuring this height, and what dimensions are you finding? The seating plane is at the top of the copper (well, the top of the surface treatment), which will be below the top of the solder mask, so you have to account for that. Then you have to account for any errors in surface form or position that may interfere with measurement, like if the board is slightly warped or if the surface you're referencing is not parallel to the surface you're measuring. Measuring to the tens of microns level is not particularly exotic, but is absolutely at the level where you need to be careful about your measurement setup. I think to do this to a sufficient degree of confidence you'd want to set the board up on a surface plate and use a good dial test indicator to take multiple relative measurements of both the part and the board immediately around the part, and you'd need more than one sample board to know how consistent those measurements will be. Or, you know, throw it on aCMMif you happen to have one of those lying around.

author: amyk
author_rank: Super Contributor
timestamp: June 05, 2021, 06:19:27 am
content: Quote from: nihial on June 04, 2021, 03:24:05 pmIf your mounting is that rigid, I would be concerned about thermal stress.

author: ebastler
author_rank: Super Contributor
timestamp: June 05, 2021, 08:50:21 am
content: Quote from: nihial on June 04, 2021, 03:24:05 pmFor thenominalvalues, A = A1+A2 as one would expect.The fact that this relationship does not hold for the minimum or maximum values has nothing to do with measurements before vs. after soldering. It is simply statistics: Since A1 and A2 vary independently, it is unlikely that both assume their minimum value for a given part (or both their maximum). Hence the stated minimum and maximum values for A, which will be 3 sigma values or whatever convention they use, is closer to the mean than the sum of the extreme A1+A2 values.I'm afraid I can't help with your ultimate question of height after soldering. No idea whether there is a spec on this? The outcome must depend on your soldering process, i.e. how much solder paste and flux you apply to the PCB, and maybe also details of the temperature curve etc.?

author: nihial
author_rank: Regular Contributor
timestamp: June 08, 2021, 10:24:47 pm
content: Quote from: amyk on June 05, 2021, 06:19:27 amWe'll see, it's not optimal at all

author: nihial
author_rank: Regular Contributor
timestamp: June 08, 2021, 11:36:12 pm
content: Quote from: ajb on June 04, 2021, 04:47:16 pmThis is possible I guess, I talk to a mechanical engineer, when they have multiple sub assemblies with tolerances, they use statistical analysis and basically, the more sub assemblies you have with independant tolerances, the worst case will have a very low probability of happening so for a given sigma from the mean the min max will be less worse.Quote from: ajb on June 04, 2021, 04:47:16 pmMaybe asking the assembler could be somehow a solution, I will talk with them this isn't a bad idea !Quote from: ajb on June 04, 2021, 04:47:16 pmI'm using a caliper, solder mask is thin, and around the same value as plating.To resume, the measurements I'm doing seem coherent with the hypothesis I took, so for the FPGA I noticed depending on the location of the measurement. Maybe the fpga isn't soldered uniformly or something, but measurements are all between 3mm(rarely under) and 3.3mm.For the intel processor it's on a module, I expect the assembly process is well defined. For that one it looks like on the parts I have it's 2.4mm, I expected lessBut I only have a handfull of samples

author: nihial
author_rank: Regular Contributor
timestamp: June 08, 2021, 11:39:19 pm
content: Quote from: ebastler on June 05, 2021, 08:50:21 amAsked a mechanical engineer he came up with the same explanation so I guess this is probably it. Seems logical in a sense.Found that intel note :https://www.intel.com/content/www/us/en/programmable/support/support-resources/knowledge-base/solutions/rd06022010_580.html#:~:text=You%20can%20expect%20the%20solder,about%200.2mm%20after%20reflow.So yes I guess it depends on some parameters but this gives a rough idea of what someone can expect, I don't know if it's 1/3 of balls diameter or 1/3 of balls height though

author: ebastler
author_rank: Super Contributor
timestamp: June 09, 2021, 06:16:28 am
content: Quote from: nihial on June 08, 2021, 11:39:19 pmSeems very clear to me on the Intel page you linked to?Theheightof the balls is reduced by 1/3.Quote

author: nihial
author_rank: Regular Contributor
timestamp: June 09, 2021, 08:14:15 pm
content: Quote from: ebastler on June 09, 2021, 06:16:28 amI probably need some sleep this was indeed clearly stated