I would like to add two items to Pratap's response on hole filling;
Detection of cracked barrel plating - On finished FR4 boards when some
of the holes have not filled completely, cracked barrels can be confirmed
easily by viewing the top of the board with a low power microscope and back
lighting the bottom of the board with a strong light source. The flexible
fiber optic light sources work very well. Cracked barrels or any voids in
the hole plating will have a green glow inside the hole. For holes that
have no solder (for what ever reason), place a white sheet of paper on the
bottom side of the board. This eliminates the reflective glare inside the
hole making it easier to see surface defects or green glowing plating voids.
Causes of non-filling - In addition to the board related items included
on the "Why a PTH is not filling?" list, one must consider the wave setup.
All the parameters (preheat, wave temp, angle, time, etc.) must be properly
controlled to insure complete filling.
Norm Dill
----------
From: Pratap Singh
To: DAVY.J.G-
Cc: [log in to unmask]; JOHNSON.R.C-; SEAY.K.P-; PEARSON.R.C-; ANKROM.M.J-;
MENGERS.W.D-
Subject: Re: ASSY: IR Reflow of Thru-Hole Components
Date: Monday, January 13, 1997 11:38PM
There seems to be a lot of confusion about "PLATED-THROUGH-HOLE" fill
requirements. The PTH solder joint is an interconnection that provides an
electrical path between (a) Component Lead and (b) PCB circuit. To
accomplish this solder is deposited either through:
1. Wave solder process
2. Paste print and reflow
3. Manual soldering with an iron
4. Solder fountain (similar to item #1)
Now the questions to be answered are:
a)When a joint is reliable?
b)When is the optimum process running?
c)When a hole does not fill, what is the reason?
Q(a) Under nominal conditions and normal hole quality, for a reliable
joint, it DOES NOT have to be filled 100 %.
Q(b) Since solder has affinity for clean copper surface at the soldering
temperature ( 200 deg. C or so), a copper barrel (ie PTH) should fill in
the presence of flux, solder and right temperature.
A normal process should, therefore, provide a hole filled with solder. In
the presence of a lead, hole filling is a little easier. So a PTH with a
lead, flux, right temperature and solder SHOULD BE FILLED. Right!
If the PTH does not fill, something is wrong either with the process
parameters or the PTH (and PCB) itself. An OPTIMUM runing process with
properly made PTH in a PCB should provide FILLED holes. A FILLED HOLE IS
AN EXCELLENT INDICATOR OF THE OPTIMUM PROCESS and OPTIMUM QUALITY PCB.
Q(c) Why a PTH is not filling?
Reasons are: -Outgassing, PCB moisture
-Poor copper, thin cracked plating with voids
-Laminate voids
-Poor Solderability (even Lonco flux does not help)
So when a hole does not fill, how does one ID the causes?
The main tool is the xsectioning and visual examination along with EDX or
ESCA etc to see what chemical species are on the copper surface etc.
When the hole does not fill due to BARREL COPPER CRACKS, the RELIABILITY
of the interconnection becomes suspectExcept in rare cases, solder does
not jump across the cracked barrel surface. A crack stops the solder dead
in its track.
Let say the PTH is half filled and there is solid connection between
bottom half of the barrel, and the lead. Crack in the barrel is just
above the 50% point. However, the internal siganl trace enters in the
top 50 % of the barrel above the crack. This joint is a failed joint.
A simialr scene can be repeated with a 75 % filled hole etc.
What is one to deduce from this? An UNFILLED hole is an indicator of a
CRACKED BARREL and demands investigation of the incoming PCB quality.
On the other hand a perfect barrel with a lead in it could be filled with
25% or 50% with solder and will work reliably.
Question is how does one know why the hole is not filled? If one accepts
the premise that full hole fill is not required for reliability, then one
may be shipping PCB with cracked barrels. May be a small percentage (x%)
of cracked open barrels are acceptable in the age of 5ppm defect rate.
My net is that when a PTH is filled (to the brim) as a quality person, I
know that both process and product are OK and running well. When there is
a series of unfilled PTHs on PCB it is time to look deeper into the
hole and find the cause. If the reason is cracked barrels change your PCB
supplier.
(Note: reworking the PTH with poor copper makes it worse)
-----
Pratap Singh
Tel./Fax: 512-255-6820
e-mail: [log in to unmask]
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