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SMT packages that can solder inverted?

Dan Gosselin


SMT packages that can solder inverted? | 20 August, 2003

Do any of you guys have a list of parts that cannot survive hanging upside down during reflow soldering. As our board designs become increasingly dense and higher speed, we need to use all available pcb real estate. This means locating parts we historically placed on top and instead placing them on the backside. I remember back in the LCC days that the largest LCC that we could hang upside down without it falling off was a 48 pin ceramic LCC. Our present experience is that most of your standard SOICs, D-Paks, passives will stay in place but I don't know what the threshold is. Any help from you guys (based on experience or info you have from industry) will be appreciated. An equation based on solder contact area vs. component mass would be ideal.


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SMT packages that can solder inverted? | 21 August, 2003

Dan- We here do double sided reflow: we place one side and the other side and reflow. On the second flow the underside of the PCB- we will reduce the bottom side preheat. fortunatly for us the largest component we only place a 40pin PLCC. I would also think Flux and Solder would also need to play in the formula.

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Tom B


SMT packages that can solder inverted? | 21 August, 2003


We, do alot of double reflow. Your right we typically see alot of the standards components, SOT's, SOIC's, Chip stuff. The largest we do with out adding epoxy is some 32pin PLCC's. We have several assemblies were were doing PLCC44's, Sockets, and heavy Crystals, switches, and connectors. Based onmy experience we will usually add two dots of epoxy on diagonals to help hold parts in place. So far this has worked great for us!

Things to look for:

1. Bottom side heat, we sometimes run bottom zones cooler! That depends on the components, some can stand a second peak cycle (Not often though)

2. Definetly ensure that conveyor has not vibration or G shock motion, many a part has fell off due to exerted forces by a conveyor.

3. You can if possible design in larger than normal pads. This will give you more surface area for solder area to hold onto parts.

4. Posibble to use two alloys. A alloy that has plastic range for the bottom side or first side. This will give you a window before the solder becomes full liquidous. Then an eutetic alloy. You can use this for topside of last reflow. You will have to watch the temps of the alloys to ensure no problems. (I ruled the 2 alloys out becuase that means you'll have to stock 2 different pastes, plus monitor the reflow profile!

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Tom B


SMT packages that can solder inverted? | 21 August, 2003

Correction on note 1:

Some can't stand a second peak profile

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SMT packages that can solder inverted? | 22 August, 2003

Search the Archives, Dave F I believe posted the formula not to long ago.


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SMT packages that can solder inverted? | 26 August, 2003

You may check with the TDS of respective "active parts" IC manufacturers for their recommended no. of thermal cycles permissible in the reflow process.

Last I checked 3 cycles for our RF-apps IC was still allowed. For 48pin-QFPs this can be allowed up to 10 cycles max. (believe the no. thermal cycles include any rework soldering exposure?).

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Dan Gosselin


SMT packages that can solder inverted? | 28 August, 2003

Thanks everyone for your help. I have since found a good article I am downloading of of the SMTA website. Dan

Title : EFFECT OF SURFACE TENSION OF SOLDER ON UNDERSIDE DEVICE SUPPORT DURING REFLOW OF A PCBA Author : Mulugeta Abtew Author Company : Sanmina - SCI Corporation Date : 09/22/2002 Conference : SMTA International Abstract : An experimental approach was taken to investigate the capability of liquid solder to hold components in place during inverted reflow in a typical Printed Circuit Board assembly (PCBA) soldering process. Three lead configurations, namely Jlead, Gull Wing and Solder Balls were evaluated. The maximum mass per lead that can be supported by the surface tension of liquid solder during bottom side reflow of a peripheral J-leaded device with 84 leads was determined to be 0.089 grams; for a Gull wing device with 208 leads the maximum supported mass per lead was 0.029 grams while for a 388 pin Plastic Ball Grid Array and a 48 pin CSP devices the maximum supported mass per lead was found to be 0.045 grams and 0.013 grams respectively. Further, it was found that the weight carrying capacity of molten solder is strongly dependent on the size of the solder fillet area on a component lead. Analytical model to estimate the optimum weight of a device that can be supported by liquid solder during inverted reflow is proposed.

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