The use of via hole mounting of ball grid array (BGA) devices can have advantages and disadvantages in the manufacturing process. Alternatively smaller vias in pads or through hole vias offset from the mounting pad has become quite commonplace in many mobile applications, but again some issues may be experienced unless a full understanding of the process is obtained through production trials. Often a design concept is not fully considered or proven in manufacture. Using stacked vias, full via plugging or pulse plating in printed board manufacture are options for the design engineer, but what is the additional cost and availability?

Figures 1, 2, & 3: Successful via mounting of ball grid array devices on via holes

Disadvantages of BGA via mounting
During wave soldering, secondary reflow of the topside array joints can be experienced. Often it is necessary to mask these positions with a soldering fixture or peelable mask, but you must still be very careful of peak temperature. Many people have seen secondary reflow of fine pitch parts when high solder bath temperatures of 250oC are used for tin/lead. Most conventional wave soldering is conducted between 235-245oC; with lead-free this will increase to 260-265oC. High mass boards can hold the temperature in the board for many seconds after the final wave contact. Sealed nitrogen wave soldering systems also maintain high internal temperatures that in the past have not been considered as significant.

Figures 4-6: Examples of peelable solder mask used to cover vias and act as a shield to temperature rise during wave soldering. Don’t you just love it when the mask just goes into the vias and breaks off when you peel it off after wave soldering?!

In the early days of BGA, many computer manufacturers experienced secondary reflow when they did not profile the board correctly through the wave and monitor topside temperature. The temperature on the topside of the board will depend on the position, size and density of the vias. It will also be altered by the wave contact time. Using temporary screened peelable mask can reduce temperature just like the addition of a blanking plate fitted to a soldering pallet if used. Secondary reflow on the wave can also be seen with lead-free products where lead is a contamination in the joints, again seen where engineers do not control the topside board temperature.

Double-sided BGA assembly can be a problem due to the solder paste reflowing, filling the via and causing a bump on the alternative side of the printed board. This will make second side printing of paste impossible; the stencil would be lifted locally off the pad surface. If you do the BGA assembly on the second pass, any bumps have little impact on further assembly steps.

Figure 7: Solder bumps forming on the via pad on the opposite side of the board while trying to fill vias with solder paste to aid in circuit probing of the vias. The same issue can been seen with the via mounting process.

You cannot use partly filled or plugged vias with solder resist as you will get random voiding in the joints due to blow back of the volatile from the paste. During reflow, the paste slumps into the hole prior to reflow, not allowing all the solvents to escape. During reflow, the gas blows back against the plug in the via. The same issue, but to a lesser extent, is seen with blind via in pad.

Gassing is the same type of fault seen on wave soldering when components like large sockets, turned pins or hairpin mounting of passive components is employed. They are positioned hard down on the PCB surface, which restricts the gas from the soldering process escaping through the top of the hole; it looks like outgassing of the joint, but it is not.

Direct via mounting is not a real benefit for component alignment of the balls in the via holes. If the device is fully depressed into the paste, you will displace the paste around the pads; it will not necessarily flow into the hole. This will then not allow complete via filling to take place; it may also cause shorting between balls or pads.

After reflow, you will tend to find that the standoff height from the board is less than with standard techniques. This is due to the loss of the solder volume to the via. From an inspection point of view using vias and the associated smaller surface pads may reduce the x-ray inspection efficiency. The joint area will all be much smaller than the ball not allowing wetting indicators to be used effectively. If any track is left exposed the solder will not wet out that far to provide an effective indicator.

Figures 8 & 9: X-ray images showing voiding in one ball and the void is just breaking out of the solder.
The second image shows voids that are practically the same size as the balls.
Some balls and pads are missing due to mechanical testing of the assembly.

Double printing and reflow process steps have been used in the paste with vias. First solder paste is printed and reflowed to fill the vias prior to reprinting with paste, placement and reflow. This can work, but may depend on the quality of the through plating. Just like plated through holes, vias can outgas, with moisture coming through the plated walls during normal reflow soldering.

Do not under estimate the energy of the water vapour coming out of a via during soldering. If the plating is thin or porous, it will lift solder mask or cause it to pop when tenting is used on both sides of the board without plugging the hole first. Plugging vias has become popular with design engineers, even if it increases cost. Improvements in reliability have been demonstrated but this process has seen drawbacks. If the filler is not fully levelled it will lead to the solder mask being higher than the pad surface, an issue for fine pitch paste printing. Using the incorrect filler material has resulted in greater expansion of the plug again leading to issues during printing.

Figure 10: Example of via with tented solder resist which has
popped leaving a very high resist bubble above the pad surface.

During BGA rework, it will take longer for complete reflow of all terminations to take place unless correct pre-heating is used. This is due to the direct connection of the solder pads to barrels of copper and solder slugs. This may, however, have been the reason for using direct via mounting technique in the first place. The temperature dissipation will be much higher; however, that was one of the reasons for potentially using this method.

Advantages of via mounting BGA
The process can work fine and all the via can be filled with solder if you use the right size hole and pad, paste and low temperature balls. You do not have to use high temperature balls as has been suggested in the past. It can make track routing easier due to the reduced via pad size, but through vias do take up a lot of space under the BGA.

If the technique is used with large vias, they can be used for positioning thermocouples under the BGA by passing them up through the via. If via filling is considered a problem or blind vias are considered then a pre reflow of paste could be used. This would fill the through hole or blind via prior to final paste printing, BGA placement and reflow. This technique of building does subject the board to one unnecessary reflow cycle that should be avoided with some alternative finishes, like tin and copper Organic Solderable Protector (OSP). If solder-levelled boards are used, then the PCB manufacturer could fill the vias during levelling. This has been done successfully in the past with polyimide boards prior to assembly to reduce via gassing, but it adds to the product cost.

Rework, test and inspection can benefit from the technique as can heat dissipation of the product in operation. In the case of rework, direct access can be made to the solder connection, allowing easy reflow for part removal. A solder fountain generally used for conventional component removal may be used for BGAs.

There are always advantages and disadvantages to any of the choices made in design and manufacture. The important thing is to look at the total solution in producing products at the right price, quality level and performance.

This column appeared in Global SMT & Packaging magazine issue 7.2 - February 2007.

Bob Willis is a process engineer and trainer providing engineering support in conventional and surface mount assembly processes. He organises production lines and seminars for suppliers and also provides seminar and workshops worldwide. Bob has one of the largest collections of training videos, interactive CD-ROMs and training material in the industry. For further information on how Bob may be able to support your staff or answer your process problem contact him via his web site www.ASKbobwillis.com

Keywords : Willis, via hold mounting, wave soldering

Average user rating

   (0 vote)