Things to think about when soldering a BGA with leaded paste - Page 1
Forum section: electronics / general

author: Korken
author_rank: Regular Contributor
timestamp: June 15, 2016, 06:30:06 am
content: Hi all!I am about to solder my first BGA st the university's electronics lab, but I noticed that it only has leaded solder paste.Is this a problem? I looked through the datasheet and it does not seem to state the composition of the balls, but it is ROHS, so I expect it to be lead free balls.Is there an industry standard for BGA balls?If I use leaded solder paste, should I use the leaded or lead free profile on the soldering oven?All tips are welcome, thanks!

author: l0wside
author_rank: Regular Contributor
timestamp: June 15, 2016, 11:42:50 am
content: Can you get yourself some BGAs to practice? A lot of suppliers have free samples. Cut the PCB and the BGA through after soldering with a small bladesaw and check the solder joints.Both the BGA balls and your solder should melt, so it would make more sense to use the leadfree profile. Keep in mind that you create an eutectic system, the melting point of the resulting solder joint might be lower than that of both leaded and leadfree solder.Max

author: wraper
author_rank: Unknown
timestamp: June 15, 2016, 11:57:24 am
content: Mixing lead free solder balls with leaded solder paste will give horrible result. Use tacky flux instead if it is available. it's near to impossible getting new parts with leaded finish. Also ROHS means finish certainly is lead-free. However there are exceptions for the internals of the parts.

author: Korken
author_rank: Regular Contributor
timestamp: June 15, 2016, 03:43:20 pm
content: Thanks for the info!We have "tacky" flux, so I tested this method.The BGA has 0.65 mm pitch, so without extra solder it is not possible to see under the BGA, so I cannot say anything about how well it went (even with the Mantis microscope in the lab).I was thinking of de-soldering it to check that all balls had made contact, but I was unable to remove it afterwards (I think it is due to the connection to the internal ground plane with the center balls).So I will simply make another one and mount PSU and see if I can program it.When applying flux, is there any recommendation on amount?I tried to get a thin, even layer this time.Thanks again for the help!!

author: wraper
author_rank: Unknown
timestamp: June 15, 2016, 04:04:10 pm
content: Even with solder paste applied, the gap will remain almost the same. Anyway there must be very tiny amount of solder paste, otherwise pads will short. Yes, thin even layer of flux is the best.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 15, 2016, 04:32:48 pm
content: Inspecting by cutting is done with an ion beam, not with a regular saw.

author: Korken
author_rank: Regular Contributor
timestamp: June 15, 2016, 06:35:31 pm
content: Quote from: wraper on June 15, 2016, 04:04:10 pmOkey, good to know! Thanks!What is your way of spreading it even?I only used the tip on the syringe, but I was thinking of buying a small paint brush to get it more even and easier to apply.

author: wraper
author_rank: Unknown
timestamp: June 15, 2016, 06:43:20 pm
content: You can use some kind of squeegee. Like a piece of old credit/discount card.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: June 15, 2016, 06:53:19 pm
content: Quote from: wraper on June 15, 2016, 11:57:24 amWhat are these horrible results? I usually use leaded solder paste for my prototype boards and have not found any issue with that. The leaded solder paste melts first and cause good wetting both to the lead-free balls and the PCB pads. The best result, however, when reworking, seems to be to use leaded balls and leaded solder paste.

author: wraper
author_rank: Unknown
timestamp: June 15, 2016, 07:29:45 pm
content: Quote from: John_ITIC on June 15, 2016, 06:53:19 pmThen take some PCB with leaded HASL finish (or just cover the pads with a tiny bit of leaded solder), apply lead free solder paste on it and reflow. Then check how solder joints will look like. Using a lot of leaded solder together with tiny amounts of lead free is fine but not the other way.

author: elecman14
author_rank: Unknown
timestamp: June 15, 2016, 07:53:41 pm
content: If you are having trouble looking at the balls under a microscope you may want to build/buy a BGA inspection mirror. Something like thishttp://www.techni-tool.com/550SO840If you are really serious about validating your process you could look into daisy chain BGA parts. The idea is the BGA balls are inner connected. If the correct PCB is used a resistance measurement would show if the BGA is making good electrical connection or not. This will not detect all BGA defects but gives a good piece of mind when validating the process. I do not have any affiliation to them but I have used Topline parts in the past to do this with good luck.The other options you have for validation are mechanical cross section or using an X-Ray machine. The mechanical cross section is usually done by potting a section of the PCB mounted BGA in epoxy and then wet sanding away till the balls are visible. Getting epoxy under the BGA can be tricky and this method is destructive.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: June 15, 2016, 08:49:36 pm
content: Quote from: wraper on June 15, 2016, 07:29:45 pmMy boards all use ENIG plating I can't try HASL finish results. In my case, the lead-free solder sphere volume would be several times the volume of the leaded solder paste applied to the pad so would be fine, then. BGAs should not use HASL board finish anyways since impossible to get the same thickness, causing solder wetting problems.https://www.google.com/?gws_rd=ssl#q=BGA+HASL+coplanarityTo answer the OP, if you have lead-free balls then you should use the lead-free (higher) temperature profile. This is important so that all solder used actually melts! Essentially, run it up to 260 degC. Note that for professional boards, you should use a high TG (180 degC) FR-4 material but for school projects you can get away with standard 150 degC TG FR4.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: June 15, 2016, 11:32:57 pm
content: I've also used leaded paste with lead free BGA's for prototypes without any problems, I'd suggest trying it for yourself. Lead free rework always looks terrible.

author: kony
author_rank: Regular Contributor
timestamp: June 17, 2016, 05:33:42 pm
content: Quote from: Jeroen3 on June 15, 2016, 04:32:48 pmOnly when you are spoiled with free access to such tools.Normally you just saw it, pot it into PMMA or other transparent polymer, then progresivelly grind and polish it. Either manually on sandpaper or on laboratory grinder/lapper.

author: Capa-Alpha
author_rank: Contributor
timestamp: July 15, 2016, 07:45:04 pm
content: Quote from: John_ITIC on June 15, 2016, 08:49:36 pm

author: Korken
author_rank: Regular Contributor
timestamp: July 31, 2016, 07:27:35 pm
content: Thanks for all the tips!I went with just fluxing the pads using a tiny paint brush and tacky solder flux, and it worked great.

author: Monkeh
author_rank: Super Contributor
timestamp: July 31, 2016, 07:48:04 pm
content: Quote from: John_ITIC on June 15, 2016, 08:49:36 pmYou have it backwards. Small amounts of lead added to lead-free create a low-lead alloy, typically with very poor qualities. The other way around (primarily tin/lead alloy with small amounts of added tin from the lead-free) does not substantially change the properties of the alloy.

author: rs20
author_rank: Super Contributor
timestamp: July 31, 2016, 08:24:27 pm
content: Quote from: Korken on July 31, 2016, 07:27:35 pmInspiring, great work! I'm starting to wonder if my fears of BGAs are misplaced. The idea that the solder volume is already controlled for you is quite appealing. Thanks for closing the loop!

author: Korken
author_rank: Regular Contributor
timestamp: July 31, 2016, 09:34:51 pm
content: Quote from: rs20 on July 31, 2016, 08:24:27 pmThanks!Personally, I have always been worried about BGAs - but after this, and how easy it went I think I will increase my usage.There are many nice ICs in BGAs only.

author: Monkeh
author_rank: Super Contributor
timestamp: July 31, 2016, 09:38:25 pm
content: Quote from: Korken on July 31, 2016, 09:34:51 pmBGA doesn't have to be horrific, the really large ones are trouble, but smallish packages aren't so bad. I did a DDR3 package recently and it was no trouble.Everyone was horrified at QFNs at one point - turns out, they're one of the easiest packages to work with.

author: technotronix
author_rank: Regular Contributor
timestamp: August 05, 2016, 07:08:12 am
content: Also you can Soldering Lead-free Components with Tin-lead Solder but it is better to completely melt the Pb-free ball and have a homogenous joint.And the pick temperature should around 230-235C.