- Aug 01.2018, 10:44:31
Plated through hole(PTH) in PCB fabrication
There is a confusion about whether is conductive or not between two sides circuit board or multilayer circuits as the base material only has copper foil on both sides, and insulating layers is in the middle. What’s more, how to connect with the two side circuit to flow easily for current?
What’s plated through hole(PTH)?
Plated through hole is also called electroless plating copper, and PTH is for short. And plated through hole is an autocatalytic oxidation-reduction reactions, depositing on the non-conductive substrate, it can’t be denied that plated through hole plays a role in bonding between the layers in double-sided board and multilayer board. And plated through hole is used for the ceramic copper plating, resin copper plating, diamond copper plating and so on. It will do the plated through hole process after drilling for two layers or multilayer board.
What does the plated through hole(PTH) work?
PTH plays a role in depositing a thinner electroless copper with chemical process to be the later substrate of copper plating on a drilled and non-conductive base material of hole wall.
How to decompose for PTH?
PTH decomposable process: alkaline cleaning→class2 countercurrent rinsing→roughening(mircoetching)→class2 countercurrent rising→pre-dip→activating→class2 countercurrent rising→desmearing→class2 countercurrent rinsing→PTH→class2 countercurrent rinsing→picking
PTH process apparatus
Desmear, PTH and PP(panel plating) are manufacturing together automatically.
Explaining for PTH process
1. Alkaline cleaning
Remove the board oil , fingerprint, oxide, dust in the hole; adjust the negative charge to positive charge for hole wall so that the colloid palladium does well in being adsorb in the post-process; it should be strict with cleaning based on guidance after unoil and test with PTH backlight.
Removing the oxide of board, and roughening the board, to make sure that there is a good bonding between plated through hole layer and base copper; the new copper foil has a strong active and does well in adsorb the colloid palladium;
It’s mainly in protecting the palladium slot from being polluted for the bath of pretreatment and extending the shelf life, the main ingredients of pre-dip is the same as palladium slot but except the palladium chloride, and pre-dip plays an effective role in wetting the hole wall so that the following activator enter the hole on time to be activating effectively;
The hole wall with positive charge plays an effective role in adsorb the colloidal palladium particle with negative charge to make sure that the following the averageness, continuous and compactness of plated through hole after the polarity of adjusting for pretreatment alkaline cleaning; so unoil and activating is so important for the later plated through hole. The control point of activating: specified time; normal density of stannous ion and chloride; what’s more, it’s also important for pecific gravity, acidity and temperature, so it should be controlled strictly on guidance.
Remove the stannous ion outside the colloidal palladium particle to expose the palladium nucleus so that plays an effective catalysis role in starting the plated through hole, it can be shown that it’s better to use the fluoroboric acid as desmearing.
Bring out the autocatalytic reaction of PTH with activating of palladium nucleus, it can be a catalyst to do the vcatalytic reaction for the new electroless copper and by-product hydrogen to keep the plated through hole. And it can be depositing a plated through hole on the board or hole wall after treating. The bath need to be normal air agitation in the process so that the soluble cupric is changing.
The quality of plated through hole makes a direct effect on the quality of printed cricuit board fabrication, which is the main source process for the via can’t work, poor open and short, and it’s inconvenient to visually test, it has no choice but to screen at random with destructive test for post-process, and it can’t make an effective analysis and monitor, so it must be a batch problem once something happens, and it can’t preclude as testing and have no choice but to scrap, so please operate strictly on guiddance.
The problems and solution for plated through hole
1. Blistering and lamination on the board
There is a poor bonding between panel plating copper and chemical reaction of copper or chemical reaction of copper and base copper.
① it can’t remove the adhesive and oil pollution after rough grinding: pressing and drilling are to strengthen prevention.
② It’s unclean for neutral water rinsing after desmearing , and there is Mn on the board: to check the neutral treating technics parameter and make an adjustment.
③ Water rinsing is insufficient after unoil and there is the surface active agent on the board: to check the density of acid dip, quantity of water rinsing and cleanliness in the slot.
④ Insufficient mircoetching and roughening on the surface of copper: to check and adjust the humidity, density and time.
⑤ The properties of activating agent is aggravating, replacement reaction is on the surface of copper foil: to check the activating treating process, it’s necessary to change the bath.
⑥ Accelerated processing is insufficient, there is Sn on the cooper surface: to check and adjust the accelerated process parameter.
⑦ It keeps for a long time resulting in oxiding on the surface of copper foil before electroless plating copper: to check circular and dripping time.
⑧ It pollutes the electroless plating copper by foreign material, resulting in largening the copper grain and inclusion hydrogen: to check the process and not allow the foreign material to be coming.
2. No copper in PTH
① unbalanced degreaser: analyse and adjust to normal range.
② Insufficient activating: analyse and adjust to normal range.
③ Unbalanced ingredients for PTH cylinder: analyse and adjust to normal range, and it need to reopen the cylinder if necessary.
④ Excessive acceleration: reduce the cccelerated treatment conditions( such as density, temperature, and time).
⑤ Excessive drilling roughness: control the drilling room.
⑥ It’s keeping for a long time after PTH: finish the board within 8 hours after PTH.
⑦ Fill in the foreign material or blister on the board after PTH: to check the panel copper and high speed cycle.
3. Plug hole
② There is the copper particle on PTH cylinder: to check the filtering system and prevent the copper from liquating.
③ Panel copper residue/copper sheet/foreign material: doing the carbon treatment on time, it keeps the orifice plate about 1″to prevent the plate-burning from dropping copper powder.