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I've printed, populated and baked a test PCB with most components that I needed. I left off a GPS chip and associated passives because the GPS chip itself costs more than the rest of the board, and I'm testing unrelated functionality at the moment (and I thought there would be a good chance that I'd have to change something). That said, the stencil was made with the GPS components in mind, and those pads are now covered with solder.

Would simply covering all the pads with flux, populating the components and reflowing again work?

I'm doing all of this by hand, and would prefer to avoid having populate another 100+ BOM lineitem board. The IC itself is an LGA 69 package, and the passives are mostly 0402.

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If you are only doing a few for your development then I would suggest you manually solder the missing parts on, using an iron or a hot air tool. Put on the flux as you said and then reflow just the added components.

Modules sometimes try to use a slightly higher temperature solder so the standard reflow profile will not remelt them, however as mentioned in the comments components will not just drop off unless they are heavy and on the bottom of a hot board.

When wave soldering surface mount components it is usual to use a glue dot to hold the parts in place but with IR reflow this is usually not done as it would need a dot dispenser in addition to the solder stencil process.

Multiple reflow cycles are not recommended but I would certainly think that taking the occasional risk may be worth it on a development board, worst case is it does not work.

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  • \$\begingroup\$ May I ask how is "higher temperature solder" made nowadays? In past, 63% tin, 37% lead is lowest melting point and other mix percentage can be used to adjust for higher temperature. Does nowadays we use 100% tin (no lead) for RoHS and how to manipulate melting point? \$\endgroup\$ – EEd Aug 24 '14 at 16:55
  • \$\begingroup\$ @EEdeveloper Pb-free solders do not have quite as long a history behind them as Pb-Sn. There are some options that may be worth investigating, both lower and higher temperatures have been developed. For prototyping I would use a Lead-Tin solder, it is not going into the retail sales stream so the RoHS requirements do not need to be met. link link link \$\endgroup\$ – KalleMP Sep 2 '14 at 17:16
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Check if your components can tolerate double reflow. Certain modules cannot (GPS, RF modules etc), most discrete components can (Think a double sided, double loaded PCBs).

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  • \$\begingroup\$ Why 'special' about GPS, RF modules? Is that due to being RF board? How double side work? One reflow, then pick and pace on the other side and second reflow? How to prevent components falling out due to gravity on the bottom side? \$\endgroup\$ – EEd Aug 24 '14 at 6:24
  • \$\begingroup\$ @EEdeveloper Surface tension holds small parts on. Well, unless you tap the board or oven when the solder is molten. \$\endgroup\$ – Spehro Pefhany Aug 24 '14 at 7:18
  • \$\begingroup\$ Also, Modules already went through one reflow pass when they were manufactured. The components might not survive another pass. \$\endgroup\$ – Lior Bilia Aug 24 '14 at 11:10
  • \$\begingroup\$ When you say "module" you don't mean discrete ICs, do you? You're referring to PCBs with multiple components on them in addition to ICs that also have all the passives, that are then soldered on top of a master PCB? \$\endgroup\$ – kolosy Aug 24 '14 at 13:06
  • \$\begingroup\$ This is how double side are made, right? Pick and place on one side, reflow, pick and place on the other side, reflow. \$\endgroup\$ – EEd Aug 24 '14 at 16:38

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