I agree pad size is very important. I've worked these issues several times and know there are several things to check.
1. The flux application must be correct and no skips.
2. The temperature profile must not be too hot and prematurely burn off all the flux before it gets to the chip wave especially for No-clean fluxes.
3. Many people slow the conveyor speed to achieve more contact time to reduce the skips without reducing the temperatures of the preheaters.
4. Many companies don't allow the time needed to PM the chip wave as soon as gaps appear across the chip wave. There are two types of chip waves from one vendor. The older type had the oscillating plate with holes which tend to plug up from dross under the plate. The newer rotary chips waves are better but still need maintenance and there are three metal connecting pieces across the slot to keep the curve plate parallel but these interrupt a continuous flow. This effect can be minimized with proper wave height so that the solder touches the board and flattens out widening the coverage.
5. What happens frequently is that the operator will lower the wave heights if bridges occur because they are more noticeable than coverage to a test point and the pressure is on to get the boards through the wave process.
6. Another factor is when t here is a shadow effect from taller SMT parts epoxied to the bottom of the PCA.
Another factor is shadowing or contamination from liquid solder mask, used to cover enig mounting holes or mounting holes, for parts to be assembled after wave solder. If any of the mask ran onto the test points which extends to PCBs solder mask then it will not solder. These test points will solder with a solder iron, because there is a very thin layer, basically contamination, which is wiped off by the tip of the solder iron tip.
John Goulet
MFG/Process Engineer
----- Original Message -----
From: "Amarjot Singh" <[log in to unmask]>
To: [log in to unmask]
Sent: Thursday, January 21, 2010 3:47:17 PM GMT -05:00 US/Canada Eastern
Subject: Re: [TN] Skipping pads in wave solder
Hi John:
I agree with your comment regarding the size of the pad. The pad size is
between 1 mm to 1.17mm. There is a function in the wave solder machine
called chip wave. This is used if there are surface mounts on the bottom
side. When this function on, I see improvement in soldering of test pads.
Now, some of the test pads are connected to vias as these are test points
to test ICs on component side and there are no thermal connections on the
inner layers. Based on your experience, what is the minimum size pad that
can be soldered without any issues. Just to clarify, these are ENIG boards.
Thanks,
Amarjot Singh
-----Original Message-----
From: John Burke [mailto:[log in to unmask]]
Sent: Thursday, January 21, 2010 3:25 PM
To: 'TechNet E-Mail Forum'; 'Amarjot Singh'
Subject: RE: [TN] Skipping pads in wave solder
You need more Z axis wave energy by the sound of it, I have had this problem
in the past (first occurrence 17 years ago) where the solder mask is higher
than the pads and the solder does not touch down on the copper as the system
is at that point airtight if there is no via hole in the test pad.
Increasing wave agitation will fix this or you could consider making the
test points bigge or putting via holes in them which is generally not an
option.
John Burke
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Amarjot Singh
Sent: Thursday, January 21, 2010 11:59 AM
To: [log in to unmask]
Subject: Re: [TN] Skipping pads in wave solder
Hi Gang:
My customer is having a problem in wave soldering. The board has surface
mount and through hole components on top side and there are some test points
on the bottom side. While passing through wave solder, the holes are filling
nicely but solder is not being coated on some of the test points. Some test
points are in the ground plane (solder mask defined) and the others is
isolated but sorrounded by ground plane. I can cover those pads using solder
iron without any problems.
When performing the similar process at some other assembly house in wave
solder, I noticed the same problem. I played with higher pre-heat
temperatures, slower and faster speed but same result. Any thought on this
issue?
Thanks,
Amarjot
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