Via Protection Strategy

There has been a lot of discussions on Technet lately about via protection. The first thing to decide is whether it is for functional reasons or simply a manufacturing aid. If it is functional the following can be stated as requirements to achieve the desired end results.

All vias shall be protected. The materials used shall be per 3.2.13 of IPC-6012. The materials and processes shall be qualified in accordance with Customer or end-use environmental requirements prior to use.

There are three types of vias defined in IPC-4761 and a protection strategy. There is a filled and covered via (Type VI-b Via); a filled and capped via (Type VII Via); and a tented and covered via (Type II-b).

The covering of Type VI-b and Type II-b Vias shall be per the application of SMOBC liquid photo imagable solder mask per IPC-SM-840 and in accordance with the requirements of 3.7 of IPC-6012. The Type VII Via may or may not be covered. It is dependent on final surface finish and functional implications (via in pad). The requirement to do this and the proper artwork and data should be supplied. If not it is the fabricators responsibility to furnish it.

The three via types shall be acceptable for use for any via protection strategy requested provided they have been qualified and meet the applicable IST testing requirements.

Deviations to these requirements shall be AABUS.

The following specifies the requirements for a Type VII Via:

Material shall be applied after plating per 3.2.6.8 of IPC-6012. Tensile and elongation shall be per Class 3/A.

The surface of each filled hole must be coplanar without any deviation from other surface lands on either side of the printed board ( PB). Dimples with a maximum depth of 0.0035 in. are allowed.

The epoxy fill surface shall be plated over a minimum of 0.0003 in. per 3.2.6.8 of IPC-6012. Tensile and elongation shall be per Class 3/A.

There shall be no indication of cracking or peeling that would expose any epoxy paste on or under the SMT land area.

Air pockets shall not be within 0.010” of either side of the outer layer PB surface.

The greatest dimension of any air pocket shall not exceed 50% of the hole diameter after plating and shall not contact any copper surface.

Filled holes must be represented on the IST coupon and be able to withstand 350 thermal cycles of -55 to +125 degrees C air to air after 5 preconditioning cycles in accordance with industry practice.

Filled holes shall meet current IST coupon testing requirements.

Material shall be tested and qualified prior to use. Some materials that have been qualified are as follows;

  • San-Ei Kagaku epoxy resin,
  • PHP-900, type IR-6
  • Tatsuta DD paste series AE1125
  • Peters PP2795

The following specifies the requirements for a Type VI-b Via:

Material shall be applied after plating per 3.2.6.8 of IPC-6012. Tensile and elongation shall be per Class 3/A.

The surface of each filled hole must be coplanar without any deviation from other surface lands on either side of the PB. Dimples with a maximum depth of 0.0035 in. are allowed.

Air pockets shall not be within 0.010” of either side of the outer layer PB surface.

The greatest dimension of any air pocket shall not exceed 50% of the hole diameter and shall not contact any copper surface.

Filled holes must be represented on the IST coupon and be able to withstand 350 thermal cycles of -55 to +125 degrees C air to air after 5 preconditioning cycles in accordance with industry practice.

Filled holes shall meet current IST coupon testing requirements.

Same materials and material requirements as above.

The following specifies the requirements for a Type II-b Via:

Two step solder mask process shall be used. The first step is to apply a dry film solder mask over the via holes to be tented. Via holes shall be tented with photo-imageable dry film solder mask on both sides. This is followed by a liquid photo-imageable solder mask for the finished pattern. Proper artwork or data necessary to generate solder mask tooling for the finished pattern will be supplied. The solder mask artwork or data necessary to generate solder mask tooling for tenting the via holes may not be supplied. If it is not supplied, the manufacturer shall create the tooling and the finished pattern of the solder mask tooling shall be similar to the associated pad that it is tenting.

Tented and covered holes must be represented on the IST coupon and be able to withstand 350 thermal cycles of -55 to +125 degrees C air to air after 5 preconditioning cycles in accordance with industry practice.

Tented and covered holes shall meet current IST coupon testing requirements.

IST Testing

IST testing shall be required for all PBs with an aspect ratio of equal to or greater than 7.5.

PBs manufactured using Teflon, Polyimide, or aramid fiber materials need not meet this requirement.

Deviations to these requirements shall be AABUS.

IST Test Process

Each coupon shall be cycled in accordance with the process defined in IPC-TM-650, Method 2.6.26. The predefined resistance change threshold shall be10% for both power and sense circuits. The lot shall be acceptable after successful completion of 350 cycles, starting at room temperature and cycling to 150 degrees C.

Assembly Simulation

All test samples shall achieve five cycles of preconditioning between room temperature and 230 degrees C prior to IST cycling. This is completed on the IST testers preconditioning program.

IST Testing Acceptance

The minimum number of IST cycles when tested in accordance with the IST Test Process shall be 350. If all six coupons to be tested for determination of lot acceptability meet or exceed the minimum number of cycles, the lot will be deemed acceptable. If any failure occurs, which has been confirmed by failure analysis, on any of the six coupons, all coupons on the remaining panels within the lot shall be required to complete IST testing. If the six coupons have ≥ 50% failure for any reason, the lot shall be deemed unacceptable and rejected.

2 Comments

  1. Mordechai Ben Porat
    Posted November 11, 2010 at 1:20 am | Permalink

    Hi Dewey

    What about laser drilled micro via protection?

  2. Jean Ph.Mundorff
    Posted August 17, 2012 at 12:35 am | Permalink

    Have you any indication for requirements of soldermask thickness after tenting ?


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