Soldering SMD with ground pad - FIXED - ALMOST - Page 1
Forum section: electronics / repair

author: NY2KW
author_rank: Regular Contributor
timestamp: November 05, 2019, 10:41:04 pm
content: I am repairing a R&S SM300 Signal Generator. It works great but output limited to -26dBm and it should go up to +13dBm. I was advised that the gain block amp (AGB3303) needed to be replaced. I have limited SMD experience and tried Chipquik and was able to remove the original but it too more heat and time than I expected. I cleaned the pads up (see pictures before and after). The specs for the AGB3303 say to keep the reflow at no more than 260C but with my Hakko FX-888D and regular tip it took a setting of 400C and several seconds to get the old one off despite the flux and Chipquik. I wish I could find a repair shop in my area but I am willing to give it a try if I can get some advice. I watched numerous videos on Youtube and have no problem with 2 pad components but have never dealt with a ground pad.Thanks in advance.Jerry[ Attachment Invalid Or Does Not Exist ]

author: NY2KW
author_rank: Regular Contributor
timestamp: November 05, 2019, 10:42:00 pm
content: Sorry here it the picture

author: DaJMasta
author_rank: Super Contributor
timestamp: November 06, 2019, 12:32:17 am
content: The trouble with this is that to get good RF performance, there's basically always a large, well grounded pad on the package, and that's always well attached to a nearly full-board ground plane for EMI and ground loop reasons (you can see the vias under the soldermask around the pad)For that reason, I don't think an iron alone, even with chipquik, will do the job. I'd get an inexpensive hot air gun and use it as a preheater for the area - maybe set to like 250C and high flow with a wide nozzle - then use your iron to actually make the connection (at a more reasonable temperature) after a preheat period. Still better would be a board preheater, since the ground plane you're soldering to probably runs the whole board, but the entry price for that is higher.An added advantage is that when you have to work on SMD stuff, a hot air gun is almost always preferable because it is SO EASY to desolder with. It may not be a good choice for this kind of part alone, since the temperature required could damage the board, but you can always use the preheat method and use an iron so long as the pin count isn't too bad.I've got a cheap WEP 858D (there are many rebadges, who knows who the OEM actually is) and have had good luck working on an SMIQ06 board RF amp using this method, among many others, even without chipquik.

author: NY2KW
author_rank: Regular Contributor
timestamp: November 06, 2019, 12:56:38 am
content: that was my main concern that I wouldn't get a solid ground plane solder. I have a hot air gun with small tip but the tip is still too big and I will heat up and maybe move the closely spaced adjacent passive components. What temperature can I heat the entire board to without risking collateral damage and for how long? I have a small 5" x5" IR heater top on an old X-tronic soldering station.thanksJerry

author: TheMG
author_rank: Frequent Contributor
timestamp: November 06, 2019, 02:29:16 am
content: What tip are you using with the Hakko? For this job I would use a chisel tip that is at least as wide as the ground pad.Tin the ground pad with a thin even layer of solder, add a bit of flux, hold the component in place with tweezers while simultaneously applying heat to both the pad and the tab of the component until the solder flows and the component sits flush against the PCB. Then solder the other pins.With a suitably sized tip and soldering iron with good thermal capacity it should dump heat onto the pad faster than the ground plane can suck it away, usually.Pre-tinning the pad helps in two ways, by improving transfer of heat from the soldering iron tip, and also ensuring you get a good flow of solder underneath the entire ground tab of the component.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 06, 2019, 02:31:55 am
content: I probably wouldn't go over like 150C or so with a full board style heater - mostly for connectors and stuff - but odds are the board was reflowed at like 230C or more so for a limited time most parts can likely take it. Even just having the hot air as an area heat should help you get to a reasonable soldering temperature using the iron in tandem - even with a small tip it could be ok, but if the tip is removable, just removing it could be an option too.The nice thing about the board preheaters is that the heatsinking effect you're trying to combat is from temperature differential in the material - if you can raise the base temperature of the ground plane 50-100C, the differential between the soldering temperature and the plane is maybe 2/3 what it is unheated, which means the rate of heat loss will be much smaller and that the same iron will be able to maintain a much higher temperature - and since you got it to work without before, you may not actually be that far off.

author: NY2KW
author_rank: Regular Contributor
timestamp: November 06, 2019, 02:58:19 am
content: I bit the bullet and used the hot air at 175C and applied heat to the a 2-3" area around the SOT89 pads for about 3 mins. My Hakko solder tip is chiseled 2mm wide almost same as the ground pad width. I started with the Hakko set for 500F (260C) and pretinned the ground pad and put on a drop of high quality flux. Then added the SOT89 and just couldnt get enough heat to make a good flow... I upped the Hakko temp to 750F (450C) and still not fast enough heat transfer to flow. I have several of the gain block amps - they were fairly inexpensive. I was reluctant to use the IR heater tray because the board has a plastic edge connector receptacle about 10cm away. I could try to heat the board with the edge connector hanging off the IR tray. I just hate playing with this board as it is virtually irreplaceable - rarely see anything but a working SM300 going for >$1000. I think I will play with a full copper bare PCB sort of Manhattan style to see what kind of preheat and soldering tip I need - all very new to me as I have only worked on old tube boatanchors for the past 25 years :-) I have sent out requesting a quote from a few PCB reflow repair centers and will see what kind of quote.

author: NY2KW
author_rank: Regular Contributor
timestamp: November 06, 2019, 05:43:14 pm
content: I got a quote of $175 from a service center in MA. I am going to give it one more try on my own. Question to the experts - am I better off using a hot air gun a few inches above the board to get a general area around the SOT89 bare pads up to 150-200C or heat from below using an IR heat tray? Attached is a picture of the two sided board and as you can see the exact spot behind the SOT89 is covered in a shield can so not sure what is in in such as plastic coils, etc). I was thinking I could use a copper heat spreader on top of the IR tray and place the PCB top up on the copper to get a better wider area of preheat. I have read preheat in IR ovens etc is basically only 1-2 minutes but I guess I need to monitor the temp of the PCB board itself to get the area in question up to at least 150C??Any advice appreciated

author: NY2KW
author_rank: Regular Contributor
timestamp: November 07, 2019, 11:35:54 pm
content: I preheated with hot air pencil for about 2-3 mins at 150C then soldered with larger tip - just barely able to maneuver but got it done. Assembled and working but found other problem which i will post separately.Thanks to allJerry