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Newbie advice needed for identifying and testing SMD capacitors

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tomizett

Active Member
Hi sky_shrimp and welcome on-board.
As others have said, starting out in electronics by trying to repair leading-edge smart phones is pretty ambitious, so please don't be put off if you don't end up with a working phone at the end of it. That said, I'm impressed by your positive attitude and the eloquence of your posting, as well as by your work.
I've been doing electronics in some form for most of my life, and untill about 5 years ago I regarded surface mount as some kind of back art. Even now I'd not touch an iphone through choice, so if that really is your first attempt with a soldering iron then it's not at all bad!
 

sky_shrimp

New Member
Thanks tomizett!

I won an eBay action for an icloud locked 5c at a very cheap price so I'll use that to salvage the parts I need for this iphone and others. I'll probably need the touch ICs and a few caps from it. The area I burnt with the solder iron has me puzzled due to the tiny traces that were destroyed, revealed after scratching. They were not visible before I scratched but the damage was still there but not seen. I could have just made the pads on the two visible trace ends without knowing about them. There may even be more I'm unaware of.

I'm going to try to melt the same area on the doner board with pure acetone to see if I can reveal were the traces are meant to be. That's if acetone does melt this material. I'll use copper wire cut from solder braid for the repair attempt. The burnt area is inbetween the touch ICs. I'm still stumped as to how I burnt it as I've also removed the other solder blob and a few caps without damaging the pads, let alone the board. The burnt area found under the first blob I removed was the first time I touched an iron to a board so the crater must have been made by me.

Some of the resistors are the size of a few grains of playsand. I tried to test one with smart tweezers. After 10 minutes of trying to line it up, I applied a miniscule amount of pressure to get a reading and the resistor jumped like a flea into the abyss (aka the bedroom carpet). I'm definitley attempting that test in a pot next time.
 
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kubeek

Well-Known Member
Most Helpful Member
I think the key in not destroying traces is not using an iron with 400°C at the tip, there is rarely ever a case where you need that much. Fur such small stuff I would use 320°C, maybe 340-350 if you have large ground planes to fight.
 

sky_shrimp

New Member
Thanks, I'll definitely only go to 350c next time. I'm not sure if my rework station is faulty. It's a Hakko Chinese knock-off called the Huakko 998D. I've seen good reviews for Huakko products though.

It was supposed to be brand new, but the lead of the solder iron has been repaired. They've used a white resin to fill a 2" area where the black rubber lead had cracked. I might have been sent a customer return for all I know. It makes me wonder if I'm really getting the correct heat on the LED temp display. When I turned it on the first time to 300c, I melted some solder in a ceramic pot to get a feel. The flux stopped smoking and I noticed it had dropped to 45c on it's own. I switched if off for a bit and tightened the solder tip a little more. After switching it back on, it never dipped again so it may have just been a loose tip. I can't seem to melt unleaded solder easily but if might just be due to being unexperienced. I need to buy some unleaded solder to melt in a pot so I can get a feel before touching the logic board again.
 

sky_shrimp

New Member
I finally got that bare logic board and had a look under the microscope. I was relived to find the circuitry on the first layer wasn't as complexed as I imagined. I can see that the lighter areas on the board are where the solder mask is covering the copper circuit.

JFPsiHX.jpg


The scratches I made on the ground area part revealed the copper under the solder mask,

MD5zcq7.jpg


I didn't know ground could cover a large area like this. I thought the ground area just lead to a small point from looking at the schematic. I still don't fully understand ground atm.

I plan to first cut two tiny pads from a spare PCB and solder the replacement cap to them first. Then I'll fill the crater with two part epoxy and place the cap/pad assembly on top. It takes 4 to 6 minutes for it to set. Then I'll make the traces with copper wire and solder them on to the board. I hope this isn't too boring as I can see you're all very advanced in this field.

I've found a few other missing resistors that I need to replace. The touch FPC looks damaged on the ends so I'll replace that after. It might just be that connector that is causing the phone to not respond to touch.

tSoLVxm.jpg


I've had quite a few cold beers in the evenings this week and it doesn't mix with this at all. No beers for me during these repair attempts. If it wasn't for Mickster I would have caused more damage chasing my tail o_0

I think I'll get this phone working in the next few days, now that I can see the circuitry without the components.
 
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