Soldermask on via-holes in case of chemical Nickel-Gold surface finish

Soldermask on via holes

There are 3 ways our customers prepare their layouts with respect to covering via-holes with soldermask:

  • Vias open (not covered by soldermask) on both sides of the PCB
  • Vias closed (covered with soldermask) on both sides of the PCB
  • Vias open from one side and covered from the other side of the PCB

As necessary background information we need to briefly introduce you into the technology of applying soldermask to the boards.


  • First we cover the whole surface of the production panel with soldermask ink and then dry the panel (printing the soldermask)
  • The ink we use is a UV sensitive material. When exposed to UV-light, the ink will harden (exposing the soldermask)
  • Ink that is not exposed remains soft and can be washed away using a 1% alkalic solution (developing the soldermask)

The easiest production method is to have all vias open from both sides. The vias will be clean. They will not contain any contamination nor soldermask. The next picture shows vias free of soldermask. We did not expose the soldermask on the via pads so that it remains soft and is washed away during the developing process.

Another practical production method is where the vias are covered on both sides of the PCB. We expose the soldermask on both sides of the via-pad and via-hole so it will harden and stay on the via-pad and over the via-hole to close it. There is a risk however that (mostly in case of via-holes with a larger diameter) the via-hole is not completely covered and a small opening remains in the middle.

There is a danger that chemicals get stuck in these small openings during the processes that follow after the soldermask application. These chemical can contaminate and affect for instance the chemical Ni/Au process. A further danger exists that chemicals of the Ni/Au process remain in these openings and as they are agressive chemicals they might keep on reacting in the via hole years after the board has been produced causing possible failures in usage of the PCB in its application.

The third case (vias covered from one side and open from the other side of the PCB) is the most problematic in production. This design creates a pocket. We expose the soldermask from one side but not from the other side. This soldermask in the middle of the via-hole will only be half polymerised. During the baking process this material can come out of the hole from the open side and contaminate the copper surface and thus disturb the surface finishing process. The pictures below shows a typical failure.

Vias and Chemical Nickel-Gold (ENIG)

Vias that are not completely covered or not properly filled with soldermask may create “skip pads” in the ENIG process.

Till now we didn”t receive any reasonable explanation from our material suppliers nor did we found one elsewhere that reveals the source of this problem. However supplier advise and long term experience guide us to two possible solutions to avoid the issue:

  • Modify the layout so that all vias are open. Our engineers favour this solution. Sadly this is not always accepted by our customers or the design may not allow it.
  • Apply the soldermask after the ENIG process. This is a costly solution as all copper surfaces are gold-plated and the soldermask adhesion becomes worse.

For closed via-holes we have developed an alternative solution which avoids chemicals getting trapped in the partially closed via-holes during developing of the soldermask or during application of the Ni/Au. Before the coating the entire panel with soldermask we selectively print soldermask into the via holes using a stencil. During a second print run we then cover the whole panel. This way the via-holes are completely filled with soldermask. An even layer of soldermask now covers the via-holes leaving no pockets to hold residual chemicals. We have used this technique for over 6 months, and it has proved successful in dramatically reducing the number of skip pad problems.

The following movies show this process of via filling and soldermask printing.

Setting up the machine:

{flowplayer}http://www.eurocircuits.com/images/stories/Movies/02_set_the_stencil_on_the_frame.flv{/flowplayer}

Printing the soldermask into the via holes:

{flowplayer}http://www.eurocircuits.com/images/stories/Movies/09_printing_closer.flv{/flowplayer}

Result after filling the via holes:

{flowplayer}http://www.eurocircuits.com/images/stories/Movies/10_result_close.flv{/flowplayer}

Cover the panel with soldermask:

{flowplayer}http://www.eurocircuits.com/images/stories/Movies/12_SM_printing_closer.flv{/flowplayer}

Result after printing the soldermask:

Drying the soldermask layer:

{flowplayer}http://www.eurocircuits.com/images/stories/Movies/13_Pre-dry.flv{/flowplayer}

Hand-soldering – point by point or mini-wave technique

Hand-soldering with the same high quality result as reflow or wave soldering?

Are you ready for the challenge ? Let us look at the mini-wave soldering technique.

This picture shows that tools and skills are the basics to achieve a good result in hand-soldering. You agree ?

Hand-soldering is in most cases the last step in the prototype assembly process. Why is it less controlled and more difficult than other steps in the manufacturing. It is something we all know how to do. It”s just heating up a PCB and a component to make a solder joint. Isn”t that simple?

PCB designers, technicians, electronics engineers, we all learned at school how to solder with an iron. It can be 5 or 35 years ago, but we assume that not a lot has changed. A lot did change!

Do we still take our car to a service where they have only mechanical hand-tools to fix it ? They do have a lot of tools these days to do a good repair job.

 


Soldering connections – Solderjoint

A good solderjoint is an electrical and mechanical connection which in the best condition is made in one shot with a temperature as low as possible, and as quick as possible. This rule is still valid, even when solder-alloys have changed from Sn63/Pb37 to the leadfree SAC305, SN100,.. or whatever the alloy used.

A good iron and the right skills makes the perfect start. What is the temperature of your iron today? How is the geometry and condition of your tip? What solder-materials do you use? Are you soldering leadfee assemblies?

 

 

Today we show you a technique to solder a SOIC-16 (Small outline Integrated Circuit 16 I/O) or PQFP-100 with Gull Wing leads. The quality we like to match is the same or even better than in a full automated production line. We all know how to solder this point by point, but do we master the Mini-wave technique?

 

 

 

 

See more details from the SOIC in the package outline

What do we need :

  • Soldering station (iron)
  • Soldertip
  • Solderwire
  • Flux past / Flux pen
  • Tweezer
  • Cleaning product
  • EPA zone ( ESD protected area)

How do we proceed:

  • Set the temperature as low as possible considering the pcb design ( layers, copper mass )
  • Insert the tip into the soldering iron
  • Heat-up the tip and check the tip conditions: the solder should flow (spread evenly) over the plated tip area.
    • If this is not the case, clean the tip surface ( to remove oxidation)
    • if this does not help, replace the tip with a new one.
  • Place the SOIC 16 on the PCB and attach 2 or 4 corners to hold the component in place ( Apply flux to the pads before placing the SOIC on the board)
  • Add flux paste or flux to all leads/pads on the SOIC16
  • Clean the solder-tip on a wet sponge or brass tip cleaner
  • Add solder to the tip-end (miniwave or conical tip)
    • Point by point soldering: {flowplayer}http://www.eurocircuits.com/images/stories/Movies/point-by-point-soldering.flv{/flowplayer}
      • Solder all leads individually – point by point by adding the right amount of solder. The solder is provided by hand using a fine solderwire.
    • Mini-wave soldering: {flowplayer}http://www.eurocircuits.com/images/stories/Movies/mini-wave-soldering.flv{/flowplayer}
      • Place the mini-wave tip ( parallel to the pads) on the first pin of the PQFP/SOIC and move along the pins at a constant speed. Solder all leads in less then 5-10 seconds.
      • Do the same thing again on the other sides of the PQFP/SOIC
  • Clean all Flux-residues with a cleaning solvent and ESD-safe trigger grip
  • Check all solderjoints/connections with a microscope/videomicroscope/magnifier.


The difference between both techniques is that the point to point technique takes much more time than the mini-wave technique. With the point to point technique it is also more difficult to have an even quantity of solder on all the joints.

 

Through hole component soldering with the eC-reflow mate

PIP (Pin in Paste) technology for soldering trough hole components

PIP is a technology for assembling through hole components using a conventional reflow soldering process. The process is also known as THTR (Through Hole Technology Reflow).

Most PCB”s that contain SMD components usually also contain some through hole components, such as connectors, switches, capacitors and so on . The principle of PIP is that through hole components are placed into PTH holes with SMT solder past and then reflow soldered with the other SMT components together.

We judge this can be a technology of interest for electronics developers that decide to assemble their prototypes themselves.

The next figure shows the process sequence we advise :

Important parameters for this process are hole and pin sizes, boards thickness, thickness and opening of the stencil , used paste printing technique and used paste.

It is obvious that only components that can withstand the reflow soldering temperatures can be soldered this way.

Most datasheets for PIP connectors also contain useful information such as the recommended stencil design.

Some hints based on our experience to give you the best results:


  • Reduce the hole size as small as possible for the component pin to be soldered
  • Avoid big annular rings
  • Do not put via holes in areas where solderpaste needs to be printed
  • Position the squeegee at an angle of 45° to the stencil to improve the pressure of the paste
  • Increase the size of stencil apertures to overlap on the area around the PTH hole (overprint) – when the solder paste melts, it will flow into the holes.

Image of the bare bottom side of the PCB after printing the solderpaste on the top side :

Cross-section of the component pin after soldering with PIP technology :

Advantages of the PIP technology

  • You can spare one step in the assembly process, this reduces cost as well as time.
  • All components are processed within one SMT solder process.
  • Good wetting and less risk for solder bridges
  • Connectors suitable for PIP generally require less board space, and are easier to repair then SMT connectors.

The Pin in Paste technology is very useful, because you can save time and manpower. We think this technology makes it easier for electronics developers to assemble prototypes in-house in a reliable, quick and affordable way.

More information about the equipment used in the test is available in our section on SMD reflow equipment