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Pad design for SMD Bottomside wave soldering



Pad design for SMD Bottomside wave soldering | 9 May, 2001

Hi, My Bottomside component pads are 1:1 with my topside land sizes/dimensions. I have difficulties catching solders on my bottomside smd chips (SMB Diodes, 0805 RC Chips). What should the bottomside components for wave soldering be greater than my topside si i could be able to catch solder from the bath ? I'm also encountering Solder Icicles or Spikes on my bottomside chips during wave soldering and I suspect that it is caused by my pad design since i have done most of the tweakings.

Please help.

Thanks and regards,

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Pad design for SMD Bottomside wave soldering | 9 May, 2001

First, your primary side component pads should be designed according to SM-782A, "Surface Mount design & Land Pattern Standard". Next if you did that and your secondary side component pads are the same size as your primary side components, your difficulty in soldering secondary side components has nothing to do with pad size and everything to do with your wave soldering process. Taking this from a different perspective, if you increase your secondary side component pad size without changing your wave soldering process, you will have icicles / spikes [and they may even be bigger] on secondary side components than you have now.

Icicles / spikes on secondary side components, SMT or PTH, are caused by insufficient thermal mass. Now in wave soldering, insufficient thermal mass can be caused by: * Conveyor speed too high * Incorrect flux or flux activity * Improper conveyor angle, rough wave exit, or excess dross * Low solder pot temperature * Poor solderability of board and / or components * Large open land areas, large components, etc.

Although, there is a board layout issue that you might consider. As with the required orientation of PTH components, wave soldering of secondary side SMT components requires that the wave "see" both leads of the component at the same time. Additionally, SO devices often require scavenger pads and all that, but most likely all this layout stuff is probably tangential to the icicling / spiking issue on your boards.

Now going back to your "increasing the secondary side SMT component pad size" postulation, let me spin this, just to make a point about product reliability. Secondary side, wave soldered SMT component pads based on SM-782A could be made more reliable by decreasing the pad size. Wave soldering puts a tremendous amount of solder, relative to reflowed connections, on each end of the ceramic chunks that comprise SMT resistors and capacitors. Wave soldering puts more solder on these connections for two reasons: 1 There is more solder available to form the connection in wave soldering than reflow soldering. 2 Components are stood-off the board more in wave soldering than reflow soldering as a result of using adhesives to attach the components to the board.

This is undesirable because components can be damaged over-time caused by the large difference in CTE between ceramic and solder. [There I�ve done my good deed for the day by hopefully fanning at least one person�s anxieties out product reliability. Yes!!!]

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